7/9/26 AIM
Paragraph Page
Appendix 1. Bird/Other Wildlife Strike Report .................. Appendix 1−1
Appendix 2. V olcanic Activity Reporting Form (V AR) ............ Appendix 2−1
Appendix 3. Abbreviations/Acronyms ......................... Appendix 3−1
Appendix 4. FAA Form 7233−4 − International Flight Plan ........ Appendix 4−1
Appendix 5. FAA Form 7233−1 − Flight Plan ................... Appendix 5−1
PILOT/CONTROLLER GLOSSARY ......................... PCG−1
INDEX ................................................. I−1
Table of Contents xiii
2/20/25 AIM
Chapter 1. Air Navigation
Section 1. Navigation Aids
1−1−1. General
a. Various types of air navigation aids are in use today, each serving a special purpose. These aids have varied
owners and operators, namely: the Federal Aviation Administration (FAA), the military services, private
organizations, individual states and foreign governments. The FAA has the statutory authority to establish,
operate, maintain air navigation facilities and to prescribe standards for the operation of any of these aids which
are used for instrument flight in federally controlled airspace. These aids are tabulated in the Chart Supplement.
b. Pilots should be aware of the possibility of momentary erroneous indications on cockpit displays when the
primary signal generator for a ground −based navigational transmitter (for example, a glideslope, VOR, or
nondirectional beacon) is inoperative. Pilots should disregard any navigation indication, regardless of its
apparent validity, if the particular transmitter was identified by NOTAM or otherwise as unusable or inoperative.
1−1−2. Nondirectional Radio Beacon (NDB)
a. A low or medium frequency radio beacon transmits nondirectional signals whereby the pilot of an aircraft
properly equipped can determine bearings and “home” on the station. These facilities normally operate in a
frequency band of 190 to 535 kilohertz (kHz), according to ICAO Annex 10 the frequency range for NDBs is
between 190 and 1750 kHz, and transmit a continuous carrier with either 400 or 1020 hertz (Hz) modulation.
All radio beacons except the compass locators transmit a continuous three−letter identification in code except
during voice transmissions.
b. When a radio beacon is used in conjunction with the Instrument Landing System markers, it is called a
Compass Locator.
c. V oice transmissions are made on radio beacons unless the letter “W” (without voice) is included in the class
designator (HW).
d. Radio beacons are subject to disturbances that ma y result in erroneous bearing information. Such
disturbances result from such factors as lightning, precipitation static, etc. At night, radio beacons are vulnerable
to interference from distant stations. Nearly all disturbances which affect the Automatic Direction Finder (ADF)
bearing also affect the facility’s identification. Noisy identification usually occurs when the ADF needle is
erratic. V oice, music or erroneous identification may be heard when a steady false bearing is being displayed.
Since ADF receivers do not have a “flag” to warn the pilot when erroneous bearing information is being
displayed, the pilot should continuously monitor the NDB’s identification.
1−1−3. VHF Omni −directional Range (VOR)
a. VORs operate within the 108.0 to 117.95 MHz frequency band and have a power output necessary to
provide coverage within their assigned operational service volume. They are subject to line−of−sight restrictions,
and the range varies proportionally to the altitude of the receiving equipment.
NOTE−
Normal service ranges for the various classes of VORs are given in Navigational Aid (NAVAID) Service Volumes, Paragraph
1−1−8.
b. Most VORs are equipped for voice transmission on the VOR frequency. VORs without voice capability
are indicated by the letter “W” (without voice) included in the class designator (VORW).
c. The only positive method of identifying a VOR is by its Morse Code identification or by the recorded
automatic voice identification which is always indicated by use of the word “VOR” following the range’s name.
Navigation Aids 1−1−1
AIM 2/20/25
Reliance on determining the identification of an omnirange should never be placed on listening to voice
transmissions by the Flight Service Station (FSS) (or approach control facility) involved. Many FSSs remotely
operate several omniranges with different names. In some cases, none of the VORs have the name of the “parent”
FSS. During periods of maintenance, the facility may radiate a T−E−S−T code (- -) or the code may
be removed. Some VOR equipment decodes the identifier and displays it to the pilot for verification to charts,
while other equipment simply displays the expected identifier from a database to aid in verification to the audio
tones. You should be familiar with your equipment and use it appropriately. If your equipment automatically
decodes the identifier, it is not necessary to listen to the audio identification.
d. V oice identification has been added to numerous VORs. The transmission consists of a voice
announcement, “AIRVILLE VOR” alternating with the usual Morse Code identification.
e. The effectiveness of the VOR depends upon proper use and adjustment of both ground and airborne
equipment.
1. Accuracy. The accuracy of course alignment of the VOR is excellent, being generally plus or minus 1
degree.
2. Roughness. On some VORs, minor course roughness may be observed, evidenced by course needle or
brief flag alarm activity (some receivers are more susceptible to these irregularities than others). At a few stations,
usually in mountainous terrain, the pilot may occasionally observe a brief course needle oscillation, similar to
the indication of “approaching station.” Pilots flying over unfamiliar routes are cautioned to be on the alert for
these vagaries, and in particular, to use the “to/from” indicator to determine positive station passage.
(a) Certain propeller revolutions per minute (RPM) settings or helicopter rotor speeds can cause the VOR
Course Deviation Indicator to fluctuate as much as plus or minus six degrees. Slight changes to the RPM setting
will normally smooth out this roughness. Pilots are urged to check for this modulation phenomenon prior to
reporting a VOR station or aircraft equipment for unsatisfactory operation.
f. The VOR Minimum Operational Network (MON). As flight procedures and route structure based on
VORs are gradually being replaced with Performance −Based Navigation (PBN) procedures, the FAA is
removing selected VORs from service. PBN procedures are primarily enabled by GPS and its augmentation
systems, collectively referred to as Global Navigation Satellite System (GNSS). Aircraft that carry DME/DME
equipment can also use RNA V which provides a backup to continue flying PBN during a GNSS disruption. For
those aircraft that do not carry DME/DME, the FAA is retaining a limited network of VORs, called the VOR
MON, to provide a basic conventional navigation service for operators to use if GNSS becomes unavailable.
During a GNSS disruption, the MON will enable aircraft to navigate through the affected area or to a safe landing
at a MON airport without reliance on GNSS. Navigation using the MON will not be as efficient as the new PBN
route structure, but use of the MON will provide nearly continuous VOR signal coverage at 5,000 feet AGL
across the NAS, outside of the Western U.S. Mountainous Area (WUSMA).
NOTE−
There is no plan to change the NAVAID and route structure in the WUSMA.
The VOR MON has been retained principally for IFR aircraft that are not equipped with DME/DME avionics.
However, VFR aircraft may use the MON as desired. Aircraft equipped with DME/DME navigation systems
would, in most cases, use DME/DME to continue flight using RNA V to their destination. However, these aircraft
may, of course, use the MON.
1. Distance to a MON airport. The VOR MON will ensure that regardless of an aircraft’s position in the
contiguous United States (CONUS), a MON airport (equipped with legacy ILS or VOR approaches) will be
within 100 nautical miles. These airports are referred to as “MON airports” and will have an ILS approach or
a VOR approach if an ILS is not available. VORs to support these approaches will be retained in the VOR MON.
MON airports are charted on low−altitude en route charts and are contained in the Chart Supplement U.S. and
other appropriate publications.
NOTE−
Any suitable airport can be used to land in the event of a VOR outage. For example, an airport with a DME−required ILS
1−1−2 Navigation Aids
2/20/25 AIM
approach may be available and could be used by aircraft that are equipped with DME. The intent of the MON airport is to
provide an approach that can be used by aircraft without ADF or DME when radar may not be available.
2. Navigating to an airport. The VOR MON will retain sufficient VORs and increase VOR service volume
to ensure that pilots will have nearly continuous signal reception of a VOR when flying at 5,000 feet AGL. A
key concept of the MON is to ensure that an aircraft will always be within 100 NM of anairport with an instrument
approach that is not dependent on GPS. (See paragraph 1−1−8.) If the pilot encounters a GPS outage, the pilot
will be able to proceed via VOR−to−VOR navigation at 5,000 feet AGL through the GPS outage area or to a safe
landing at a MON airport or another suitable airport, as appropriate. Nearly all VORs inside of the WUSMA and
outside the CONUS are being retained. In these areas, pilots use the existing (Victor and Jet) route structure and
VORs to proceed through a GPS outage or to a landing.
3. Using the VOR MON.
(a) In the case of a planned GPS outage (for example, one that is in a published NOTAM), pilots may
plan to fly through the outage using the MON as appropriate and as cleared by ATC. Similarly, aircraft not
equipped with GPS may plan to fly and land using the MON, as appropriate and as cleared by ATC.
NOTE−
In many cases, flying using the MON may involve a more circuitous route than flying GPS−enabled RNAV .
(b) In the case of an unscheduled GPS outage, pilots and ATC will need to coordinate the best outcome
for all aircraft. It is possible that a GPS outage could be disruptive, causing high workload and demand for ATC
service. Generally, the VOR MON concept will enable pilots to navigate through the GPS outage or land at a
MON airport or at another airport that may have an appropriate approach or may be in visual conditions.
(1) The VOR MON is a reversionary service provided by the FAA for use by aircraft that are unable
to continue RNA V during a GPS disruption. The FAA has not mandated that preflight or inflight planning include
provisions for GPS− or WAAS−equipped aircraft to carry sufficient fuel to proceed to a MON airport in case of
an unforeseen GPS outage. Specifically, flying to a MON airport as a filed alternate will not be explicitly
required. Of course, consideration for the possibility of a GPS outage is prudent during flight planning as is
maintaining proficiency with VOR navigation.
(2) Also, in case of a GPS outage, pilots may coordinate with ATC and elect to continue through the
outage or land. The VOR MON is designed to ensure that an aircraft is within 100 NM of an airport, but pilots
may decide to proceed to any appropriate airport where a landing can be made. WAAS users flying under part
91 are not required to carry VOR avionics. These users do not have the ability or requirement to use the VOR
MON. Prudent flight planning, by these WAAS−only aircraft, should consider the possibility of a GPS outage.
NOTE−
The F AA recognizes that non−GPS−based approaches will be reduced when VORs are eliminated, and that most airports
with an instrument approach may only have GPS− or WAAS−based approaches. Pilots flying GPS− or WAAS−equipped
aircraft that also have VOR/ILS avionics should be diligent to maintain proficiency in VOR and ILS approaches in the event
of a GPS outage.
1−1−4. VOR Receiver Check
a. The FAA VOR test facility (VOT) transmits a test signal which provides users a convenient means to
determine the operational status and accuracy of a VOR receiver while on the ground where a VOT is located.
The airborne use of VOT is permitted; however, its use is strictly limited to those areas/altitudes specifically
authorized in the Chart Supplement or appropriate supplement.
b. To use the VOT service, tune in the VOT frequency on your VOR receiver. With the Course Deviation
Indicator (CDI) centered, the omni−bearing selector should read 0 degrees with the to/from indication showing
“from” or the omni−bearing selector should read 180 degrees with the to/from indication showing “to.” Should
the VOR receiver operate an RMI (Radio Magnetic Indicator), it will indicate 180 degrees on any omni−bearing
selector (OBS) setting. Two means of identification are used. One is a series of dots and the other is a continuous
tone. Information concerning an individual test signal can be obtained from the local FSS.
Navigation Aids 1−1−3
AIM 2/20/25
c. Periodic VOR receiver calibration is most important. If a receiver’s Automatic Gain Control or modulation
circuit deteriorates, it is possible for it to display acceptable accuracy and sensitivity close into the VOR or VOT
and display out−of−tolerance readings when located at greater distances where weaker signal areas exist. The
likelihood of this deterioration varies between receivers, and is generally considered a function of time. The best
assurance of having an accurate receiver is periodic calibration. Yearly intervals are recommended at which time
an authorized repair facility should recalibrate the receiver to the manufacturer’s specifications.
d. Federal Aviation Regulations (14 CFR section 91.171) provides for certain VOR equipment accuracy
checks prior to flight under instrument flight rules. To comply with this requirement and to ensure satisfactory
operation of the airborne system, the FAA has provided pilots with the following means of checking VOR
receiver accuracy:
1. VOT or a radiated test signal from an appropriately rated radio repair station.
2. Certified airborne checkpoints and airways.
3. Certified checkpoints on the airport surface.
4. If an airborne checkpoint is not available, select an established VOR airway. Select a prominent ground
point, preferably more than 20 NM from the VOR ground facility and maneuver the aircraft directly over the
point at a reasonably low altitude above terrain and obstructions.
e. A radiated VOT from an appropriately rated radio repair station serves the same purpose as an FAA VOR
signal and the check is made in much the same manner as a VOT with the following differences:
1. The frequency normally approved by the Federal Communications Commission is 108.0 MHz.
2. Repair stations are not permitted to radiate the VOR test signal continuously; consequently, the owner
or operator must make arrangements with the repair station to have the test signal transmitted. This service is
not provided by all radio repair stations. The aircraft owner or operator must determine which repair station in
the local area provides this service. A representative of the repair station must make an entry into the aircraft
logbook or other permanent record certifying to the radial accuracy and the date of transmission. The owner,
operator or representative of the repair station may accomplish the necessary checks in the aircraft and make a
logbook entry stating the results. It is necessary to verify which test radial is being transmitted and whether you
should get a “to” or “from” indication.
f. Airborne and ground check points consist of certified radials that should be received at specific points on
the airport surface or over specific landmarks while airborne in the immediate vicinity of the airport.
1. Should an error in excess of plus or minus 4 degrees be indicated through use of a ground check, or plus
or minus 6 degrees using the airborne check, Instrument Flight Rules (IFR) flight must not be attempted without
first correcting the source of the error.
CAUTION−
No correction other than the correction card figures supplied by the manufacturer should be applied in making these
VOR receiver checks.
2. Locations of airborne check points, ground check points and VOTs are published in the Chart
Supplement.
3. If a dual system VOR (units independent of each other except for the antenna) is installed in the aircraft,
one system may be checked against the other. Turn both systems to the same VOR ground facility and note the
indicated bearing to that station. The maximum permissible variations between the two indicated bearings is 4
degrees.
1−1−5. Tactical Air Navigation (TACAN)
a. For reasons peculiar to military or naval operations (unusual siting conditions, the pitching and rolling of
a naval vessel, etc.) the civil VOR/Distance Measur ing Equipment (DME) system of air navigation was
1−1−4 Navigation Aids
2/20/25 AIM
considered unsuitable for military or naval use. A new navigational system, TACAN, was therefore developed
by the military and naval forces to more readily lend itself to military and naval requirements. As a result, the
FAA has integrated TACAN facilities with the civil VOR/DME program. Although the theoretical, or technical
principles of operation of TACAN equipment are quite different from those of VOR/DME facilities, the end
result, as far as the navigating pilot is concerned, is the same. These integrated facilities are called VORTACs.
b. TACAN ground equipment consists of either a fixed or mobile transmitting unit. The airborne unit in
conjunction with the ground unit reduces the transmitted signal to a visual presentation of both azimuth and
distance information. TACAN is a pulse system and operates in the Ultrahigh Frequency (UHF) band of
frequencies. Its use requires TACAN airborne equipment and does not operate through conventional VOR
equipment.
1−1−6. VHF Omni −directional Range/Tactical Air Navigation (VORTAC)
a. A VORTAC is a facility consisting of two components, VOR and TACAN, which provides three individual
services: VOR azimuth, TACAN azimuth and TACAN distance (DME) at one site. Although consisting of more
than one component, incorporating more than one operating frequency, and using more than one antenna system,
a VORTAC is considered to be a unified navigational aid. Both components of a VORTAC are envisioned as
operating simultaneously and providing the three services at all times.
b. Transmitted signals of VOR and TACAN are each identified by three −letter code transmission and are
interlocked so that pilots using VOR azimuth with TACAN distance can be assured that both signals being
received are definitely from the same ground station. The frequency channels of the VOR and the TACAN at
each VORTAC facility are “paired” in accordance with a national plan to simplify airborne operation.
1−1−7. Distance Measuring Equipment (DME)
a. In the operation of DME, paired pulses at a specific spacing are sent out from the aircraft (this is the
interrogation) and are received at the ground station. The ground station (transponder) then transmits paired
pulses back to the aircraft at the same pulse spacing but on a different frequency. The time required for the round
trip of this signal exchange is measured in the airborne DME unit and is translated into distance (nautical miles)
from the aircraft to the ground station.
b. Operating on the line−of−sight principle, DME furnishes distance information with a very high degree of
accuracy. Reliable signals may be received at distances up to 199 NM at line−of−sight altitude with an accuracy
of better than 1/2 mile or 3 percent of the distance, whichever is greater. Distance information received from DME
equipment is SLANT RANGE distance and not actual horizontal distance.
c. Operating frequency range of a DME according to ICAO Annex 10 is from 960 MHz to 1215 MHz. Aircraft
equipped with TACAN equipment will receive distance information from a VORTAC automatically, while
aircraft equipped with VOR must have a separate DME airborne unit.
d. VOR/DME, VORTAC, Instrument Landing System (ILS)/DME, and localizer (LOC)/DME navigation
facilities established by the FAA provide course and distance information from collocated components under a
frequency pairing plan. Aircraft receiving equipment which provides for automatic DME selection assures
reception of azimuth and distance information from a common source when designated VOR/DME, VORTAC,
ILS/DME, and LOC/DME are selected.
e. Due to the limited number of available frequencies, assignment of paired frequencies is required for certain
military noncollocated VOR and TACAN facilities which serve the same area but which may be separated by
distances up to a few miles.
f. VOR/DME, VORTAC, ILS/DME, and LOC/DME facilities are identified by synchronized identifications
which are transmitted on a time share basis. The VOR or localizer portion of the facility is identified by a coded
tone modulated at 1020 Hz or a combination of code and voice. The TACAN or DME is identified by a coded
tone modulated at 1350 Hz. The DME or TACAN coded identification is transmitted one time for each three or
Navigation Aids 1−1−5
AIM 2/20/253/15/077110.65R CHG 2AIM 8/7/25
four times that the VOR or localizer coded identification is transmitted. When either the VOR or the DME is
inoperative, it is important to recognize which identifier is retained for the operative facility. A single coded
identification with a repetition interval of approximately 30 seconds indicates that the DME is operative.
g. Aircraft equipment which provides for automatic DME selection assures reception of azimuth and distance
information from a common source when designated VOR/DME, VORTAC and ILS/DME navigation facilities
are selected. Pilots are cautioned to disregard any distance displays from automatically selected DME equipment
when VOR or ILS facilities, which do not have the DME feature installed, are being used for position
determination.
1−1−8. NAVAID Service Volumes
a. The FAA publishes Standard Service V olumes (SSVs) for most NA VAIDs. The SSV is a three−dimensional
volume within which the FAA ensures that a signal can be received with adequate signal strength and course
quality, and is free from interference from other NA V AIDs on similar frequencies (e.g., co− channel or
adjacent−channel interference). However, the SSV signal protection does not include potential blockage from
terrain or obstructions. The SSV is principally intended for off−route navigation, such as proceeding direct to
or from a VOR when not on a published instrument procedure or route. Navigation on published instrument
procedures (e.g., approaches or departures) or routes (e.g., Victor routes) may use NA V AIDs outside of the SSV ,
when Extended Service V olume (ESV) is approved, since adequate signal strength, course quality, and freedom
from interference are verified by the FAA prior to the publishing of the instrument procedure or route.
NOTE−
A conical area directly above the NAVAID is generally not usable for navigation.
b. A NA VAID will have service volume restrictions if it does not conform to signal strength and course quality
standards throughout the published SSV . Service volume restrictions are first published in Notices to Airmen
(NOTAMs) and then with the alphabetical listing of the NA V AIDs in the Chart Supplement. Service volume
restrictions do not generally apply to published instrument procedures or routes unless published in NOTAMs
for the affected instrument procedure or route.
c. VOR/DME/TACAN Standard Service V olumes (SSV).
1. The three original SSVs are shown in FIG 1−1−1 and are designated with three classes of NA V AIDs:
Terminal (T), Low (L), and High (H). The usable distance of the NA V AID depends on the altitude Above the
Transmitter Height (ATH) for each class. The lower edge of the usable distance when below 1,000 feet ATH is
shown in FIG 1−1−2 for Terminal NA V AIDs and in FIG 1−1−3 for Low and High NA V AIDs.
1−1−6 Navigation Aids
2/20/25 AIM
FIG 1−1−1
Original Standard Service Volumes
FIG 1−1−2
Lower Edge of the Terminal Service Volume (in altitude ATH)
Navigation Aids 1−1−7
AIM 2/20/25
FIG 1−1−3
Lower Edge of Low and High Service Volumes (in altitude ATH)
2. With the progression of navigation capabilities to Performance Based Navigation (PBN), additional
capabilities for off−route navigation are necessary. For example, the VOR MON (See paragraph 1 −1−3 f.)
requires the use of VORs at 5,000 feet AGL, which is beyond the original SSV ranges. Additionally, PBN
procedures using DME require extended ranges. As a result, the FAA created four additional SSVs. Two of the
new SSVs are associated with VORs: VOR Low (VL) and VOR High (VH), as shown in FIG 1−1−4. The other
two new SSVs are associated with DME: DME Low (DL) and DME High (DH), as shown in FIG 1−1−5. The
SSV at altitudes below 1,000 feet for the VL and VH are the same as FIG 1−1−3. The SSVs at altitudes below
12,900 feet for the DL and DH SSVs correspond to a conservative estimate of the DME radio line of sight (RLOS)
coverage at each altitude (not including possible terrain blockage).
1−1−8 Navigation Aids
2/20/25 AIM
FIG 1−1−4
New VOR Service Volumes
FIG 1−1−5
New DME Service Volumes
Navigation Aids 1−1−9
AIM 2/20/25
NOTE−
1. In the past, NAVAIDs at one location typically all had the same SSV . For example, a VORTAC typically had a High (H)
SSV for the VOR, the TACAN azimuth, and the TACAN DME, or a Low (L) or Terminal (T) SSV for all three. A VOR/DME
typically had a High (H), Low (L), or Terminal (T) for both the VOR and the DME. A common SSV may no longer be the
case at all locations. A VOR/DME, for example, could have an SSV of VL for the VOR and DH for the DME, or other
combinations.
2. The TACAN azimuth will only be classified as T, L, or H.
3. TBL 1−1−1 is a tabular summary of the VOR, DME, and TACAN NA VAID SSVs, not including altitudes
below 1,000 feet ATH for VOR and TACAN Azimuth, and not including ranges for altitudes below 12,900 feet
for TACAN and DME.
TBL 1−1−1
VOR/DME/TACAN Standard Service Volumes
SSV Designator Altitude and Range Boundaries
T (Terminal) From 1,000 feet ATH up to and including 12,000 feet ATH at radial distances out
to 25 NM.
L (Low Altitude) From 1,000 feet ATH up to and including 18,000 feet ATH at radial distances out
to 40 NM.
H (High Altitude) From 1,000 feet ATH up to and including 14,500 feet ATH at radial distances out
to 40 NM. From 14,500 ATH up to and including 60,000 feet at radial distances
out to 100 NM. From 18,000 feet ATH up to and including 45,000 feet ATH at ra-
dial distances out to 130 NM.
VL (VOR Low) From 1,000 feet ATH up to but not including 5,000 feet ATH at radial distances
out to 40 NM. From 5,000 feet ATH up to but not including 18,000 feet ATH at
radial distances out to 70 NM.
VH (VOR High) From 1,000 feet ATH up to but not including 5,000 feet ATH at radial distances
out to 40 NM. From 5,000 feet ATH up to but not including 14,500 feet ATH at
radial distances out to 70 NM. From 14,500 ATH up to and including 60,000 feet
at radial distances out to 100 NM. From 18,000 feet ATH up to and including
45,000 feet ATH at radial distances out to 130 NM.
DL (DME Low) For altitudes up to 12,900 feet ATH at a radial distance corresponding to the LOS
to the NA V AID. From 12,900 feet ATH up to but not including 18,000 feet ATH at
radial distances out to 130 NM
DH (DME High) For altitudes up to 12,900 feet ATH at a radial distance corresponding to the LOS
to the NA V AID. From 12,900 ATH up to and including 60,000 feet at radial dis-
tances out to 100 NM. From 12,900 feet ATH up to and including 45,000 feet ATH
at radial distances out to 130 NM.
d. Nondirectional Radio Beacon (NDB) SSVs. NDBs are classified according to their intended use. The
ranges of NDB service volumes are shown in TBL 1−1−2. The distance (radius) is the same at all altitudes for
each class.
1−1−10 Navigation Aids
AIM2/20/258/7/25 AIM
TBL 1−1−2
NDB Service Volumes
Class Distance (Radius) (NM)
Compass Locator 15
MH 25
H 50*
HH 75
*Service ranges of individual facilities may be less than 50 nautical miles (NM). Restrictions to service
volumes are first published as a Notice to Airmen and then with the alphabetical listing of the NAVAID in
the Chart Supplement.
1−1−9. Instrument Landing System (ILS)
a. General
1. The ILS is designed to provide an approach path for exact alignment and descent of an aircraft on final
approach to a runway.
2. The basic components of an ILS are the localizer, glide slope, and Outer Marker (OM) and, when installed
for use with Category II or Category III instrument approach procedures, an Inner Marker (IM).
3. The system may be divided functionally into three parts:
(a) Guidance information: localizer, glide slope.
(b) Range information: marker beacon, DME.
(c) Visual information: approach lights, touchdown and centerline lights, runway lights.
4. The following means may be used to substitute for the OM:
(a) Compass locator; or
(b) Precision Approach Radar (PAR); or
(c) Airport Surveillance Radar (ASR); or
(d) Distance Measuring Equipment (DME), Very High Frequency Omni−directional Range (VOR), or
Nondirectional beacon fixes authorized in the Standard Instrument Approach Procedure; or
(e) Very High Frequency Omni−directional Radio Range (VOR); or
(f) Nondirectional beacon fixes authorized in the Standard Instrument Approach Procedure; or
(g) A suitable RNA V system with Global Positioning System (GPS), capable of fix identification on a
Standard Instrument Approach Procedure.
5. Where a complete ILS system is installed on each end of a runway; (i.e., the approach end of Runway
4 and the approach end of Runway 22) the ILS systems are not in service simultaneously.
b. Localizer
1. The localizer transmitter operates on one of 40 ILS channels within the frequency range of 108.10 to
111.95 MHz. Signals provide the pilot with course guidance to the runway centerline.
2. The approach course of the localizer is called the front course and is used with other functional parts, e.g.,
glide slope, marker beacons, etc. The localizer signal is transmitted at the far end of the runway. It is adjusted
for a course width of (full scale fly−left to a full scale fly−right) of 700 feet at the runway threshold.
3. The course line along the extended centerline of a runway, in the opposite direction to the front course
is called the back course.
Navigation Aids 1−1−11
35°
35°
10°
10°
AIM 2/20/25
NM
CAUTION−
Unless the aircraft’s ILS equipment includes reverse sensing capability, when flying inbound on the back course it is
necessary to steer the aircraft in the direction opposite the needle deflection when making corrections from off−course
to on−course. This “flying away from the needle” is also required when flying outbound on the front course of the
localizer. Do not use back course signals for approach unless a back course approach procedure is published for that
particular runway and the approach is authorized by ATC.
(b) From 10 to 35 degrees either side of the course along a radius of 10 NM. (See FIG 1−1−6.)
(a) To 10 degrees either side of the course along a radius of 18 NM from the antenna; and
5. The localizer provides course guidance throughout the descent path to the runway threshold from a
distance of 18 NM from the antenna between an altitude of 1,000 feet above the highest terrain along the course
line and 4,500 feet above the elevation of the antenna site. Proper off−course indications are provided throughout
the following angular areas of the operational service volume:
4. Identification is in International Morse Code and consists of a three−letter identifier preceded by the letter
I () transmitted on the localizer frequency.
18 NM
Limits of Localizer Coverage
FIG 1−1−6
10 NM
EXAMPLE−
I−DIA
NM
NORMAL LIMITS OF LOCALIZERNORMAL LIMITS OF LOCALIZER
COVERAGE: THE SAME AREACOVERAGE: THE SAME AREA
APPLIES TO A BACK COURSEAPPLIES TO A BACK COURSE
RUNWAYRUNWAY
LOCALIZERLOCALIZER
ANTENNAANTENNA
WHEN PROVIDED.WHEN PROVIDED.
6. Unreliable signals may be received outside of these areas. ATC may clear aircraft on procedures beyond
the service volume when the controller initiates the action or when the pilot requests, and radar monitoring is
provided.
7. The areas described in paragraph 1 −1−9 b5 and depicted in FIG 1−1−6 represent a Standard Service
V olume (SSV) localizer. All charted procedures with localizer coverage beyond the 18 NM SSV have been
through the approval process for Expanded Service V olume (ESV), and have been validated by flight inspection.
(See FIG 1−1−7.)
1−1−12 Navigation Aids
2/20/25 AIM
FIG 1−1−7
ILS Expanded Service Volume
c. Localizer Type Directional Aid (LDA)
1. The LDA is of comparable use and accuracy to a localizer but is not part of a complete ILS. The LDA
course usually provides a more precise approach course than the similar Simplified Directional Facility (SDF)
installation, which may have a course width of 6 or 12 degrees.
2. The LDA is not aligned with the runway. Straight−in minimums may be published where alignment does
not exceed 30 degrees between the course and runway. Circling minimums only are published where this
alignment exceeds 30 degrees.
3. A very limited number of LDA approaches also incorporate a glideslope. These are annotated in the plan
view of the instrument approach chart with a note, “LDA/Glideslope.” These procedures fall under a newly
defined category of approaches called Approach with Vertical Guidance (APV) described in paragraph 5−4−5,
Instrument Approach Procedure Charts, subparagraph a7(b), Approach with Vertical Guidance (APV). LDA
minima for with and without glideslope is provided and annotated on the minima lines of the approach chart as
S−LDA/GS and S−LDA. Because the final approach course is not aligned with the runway centerline, additional
maneuvering will be required compared to an ILS approach.
d. Glide Slope/Glide Path
1. The UHF glide slope transmitter, operating on one of the 40 ILS channels within the frequency range
329.15 MHz, to 335.00 MHz radiates its signals in the direction of the localizer front course. The term “glide
path” means that portion of the glide slope that intersects the localizer.
Navigation Aids 1−1−13
AIM 2/20/25
CAUTION−
False glide slope signals may exist in the area of the localizer back course approach which can cause the glide slope flag
alarm to disappear and present unreliable glide slope information. Disregard all glide slope signal indications when
making a localizer back course approach unless a glide slope is specified on the approach and landing chart.
2. The glide slope transmitter is located between 750 feet and 1,250 feet from the approach end of the
runway (down the runway) and offset 250 to 650 feet from the runway centerline. It transmits a glide path beam
1.4 degrees wide (vertically). The signal provides descent information for navigation down to the lowest
authorized decision height (DH) specified in the approved ILS approach procedure. The glidepath may not be
suitable for navigation below the lowest authorized DH and any reference to glidepath indications below that
height must be supplemented by visual reference to the runway environment. Glidepaths with no published DH
are usable to runway threshold.
3. The glide path projection angle is normally adjusted to 3 degrees above horizontal so that it intersects
the MM at about 200 feet and the OM at about 1,400 feet above the runway elevation. The glide slope is normally
usable to the distance of 10 NM. However, at some locations, the glide slope has been certified for an extended
service volume which exceeds 10 NM.
4. Pilots must be alert when approaching the glidepath interception. False courses and reverse sensing will
occur at angles considerably greater than the published path.
5. Make every effort to remain on the indicated glide path.
CAUTION−
Avoid flying below the glide path to assure obstacle/terrain clearance is maintained.
6. The published glide slope threshold crossing height (TCH) DOES NOT represent the height of the actual
glide path on−course indication above the runway threshold. It is used as a reference for planning purposes which
represents the height above the runway threshold that an aircraft’s glide slope antenna should be, if that aircraft
remains on a trajectory formed by the four−mile−to−middle marker glidepath segment.
7. Pilots must be aware of the vertical height between the aircraft’s glide slope antenna and the main gear
in the landing configuration and, at the DH, plan to adjust the descent angle accordingly if the published TCH
indicates the wheel crossing height over the runway threshold may not be satisfactory. Tests indicate a
comfortable wheel crossing height is approximately 20 to 30 feet, depending on the type of aircraft.
NOTE−
The TCH for a runway is established based on several factors including the largest aircraft category that normally uses the
runway, how airport layout affects the glide slope antenna placement, and terrain. A higher than optimum TCH, with the
same glide path angle, may cause the aircraft to touch down further from the threshold if the trajectory of the approach is
maintained until the flare. Pilots should consider the effect of a high TCH on the runway available for stopping the aircraft.
e. Distance Measuring Equipment (DME)
1. When installed with the ILS and specified in the approach procedure, DME may be used:
(a) In lieu of the OM;
(b) As a back course (BC) final approach fix (FAF); and
(c) To establish other fixes on the localizer course.
2. In some cases, DME from a separate facility may be used within Terminal Instrument Procedures
(TERPS) limitations:
(a) To provide ARC initial approach segments;
(b) As a FAF for BC approaches; and
(c) As a substitute for the OM.
f. Marker Beacon
1−1−14 Navigation Aids
2/20/25 AIM
1. ILS marker beacons have a rated power output of 3 watts or less and an antenna array designed to produce
an elliptical pattern with dimensions, at 1,000 feet above the antenna, of approximately 2,400 feet in width and
4,200 feet in length. Airborne marker beacon receivers with a selective sensitivity feature should always be
operated in the “low” sensitivity position for proper reception of ILS marker beacons.
2. ILS systems may have an associated OM. An MM is no longer required. Locations with a Category II
ILS also have an Inner Marker (IM). Due to advances in both ground navigation equipment and airborne avionics,
as well as the numerous means that may be used as a substitute for a marker beacon, the current requirements
for the use of marker beacons are:
(a) An OM or suitable substitute identifies the Final Approach Fix (FAF) for nonprecision approach
(NPA) operations (for example, localizer only); and
(b) The MM indicates a position approximately 3,500 feet from the landing threshold. This is also the
position where an aircraft on the glide path will be at an altitude of approximately 200 feet above the elevation
of the touchdown zone. A MM is no longer operationally required. There are some MMs still in use, but there
are no MMs being installed at new ILS sites by the FAA; and
(c) An IM, where installed, indicates the point at which an aircraft is at decision height on the glide path
during a Category II ILS approach. An IM is only required for CAT II operations that do not have a published
radio altitude (RA) minimum.
TBL 1−1−3
Marker Passage Indications
Marker Code Light
OM BLUE
MM AMBER
IM WHITE
BC WHITE
3. A back course marker normally indicates the ILS back course final approach fix where approach descent
is commenced.
g. Compass Locator
1. Compass locator transmitters are often situated at the MM and OM sites. The transmitters have a power
of less than 25 watts, a range of at least 15 miles and operate between 190 and 535 kHz. At some locations, higher
powered radio beacons, up to 400 watts, are used as OM compass locators.
2. Compass locators transmit two letter identification groups. The outer locator transmits the first two letters
of the localizer identification group, and the middle locator transmits the last two letters of the localizer
identification group.
h. ILS Frequency (See TBL 1−1−4.)
Navigation Aids 1−1−15
AIM 2/20/25
TBL 1−1−4
Frequency Pairs Allocated for ILS
Localizer MHz Glide Slope
108.10 334.70
108.15 334.55
108.3 334.10
108.35 333.95
108.5 329.90
108.55 329.75
108.7 330.50
108.75 330.35
108.9 329.30
108.95 329.15
109.1 331.40
109.15 331.25
109.3 332.00
109.35 331.85
109.50 332.60
109.55 332.45
109.70 333.20
109.75 333.05
109.90 333.80
109.95 333.65
Localizer MHz Glide Slope
110.1 334.40
110.15 334.25
110.3 335.00
110.35 334.85
110.5 329.60
110.55 329.45
110.70 330.20
110.75 330.05
110.90 330.80
110.95 330.65
111.10 331.70
111.15 331.55
111.30 332.30
111.35 332.15
111.50 332.9
111.55 332.75
111.70 333.5
111.75 333.35
111.90 331.1
111.95 330.95
i. ILS Minimums
1. The lowest authorized ILS minimums, with all required ground and airborne systems components
operative, are:
(a) Category I. Decision Height (DH) 200 feet and Runway Visual Range (RVR) 2,400 feet (with
touchdown zone and centerline lighting, RVR 1,800 feet), or (with Autopilot or FD or HUD, RVR 1,800 feet);
(b) Special Authorization Category I. DH 150 feet and Runway Visual Range (RVR) 1,400 feet, HUD
to DH;
(c) Category II. DH 100 feet and RVR 1,200 feet (with autoland or HUD to touchdown and noted on
authorization, RVR 1,000 feet);
(d) Special Authorization Category II with Reduced Lighting. DH 100 feet and RVR 1,200 feet with
autoland or HUD to touchdown and noted on authorization (touchdown zone, centerline lighting, and ALSF−2
are not required);
(e) Category IIIa. No DH or DH below 100 feet and RVR not less than 700 feet;
(f) Category IIIb. No DH or DH below 50 feet and RVR less than 700 feet but not less than 150 feet;
and
(g) Category IIIc. No DH and no RVR limitation.
NOTE−
Special authorization and equipment required for Categories II and III.
j. Inoperative ILS Components
1. Inoperative localizer. When the localizer fails, an ILS approach is not authorized.
2. Inoperative glide slope. When the glide slope fails, the ILS reverts to a non −precision localizer
approach.
REFERENCE−
See the inoperative component table in the U.S. Government Terminal Procedures Publication (TPP), for adjustments to minimums due to inoperative
airborne or ground system equipment.
1−1−16 Navigation Aids
2/20/25 AIM
k. ILS Course and Glideslope Distortion
1. All pilots should be aware that ILS installations are subject to signal interference by surface vehicles and
aircraft (either on the ground or airborne). ILS CRITICAL AREAS are established near each localizer and glide
slope antenna. Pilots should be aware of the level of critical area protection they can expect in various weather
conditions and understand that signal disturbances may occur as a result of normal airport operations irrespective
of the official weather observation.
2. ATC is not always required to issue control instructions to avoid interfering operations within ILS critical
areas at controlled airports during the hours the Airport Traffic Control Tower (ATCT) is in operation. ATC
responsibilities vary depending on the official weather observation and are described as follows:
(a) Weather Conditions. Official weather observation indicates a ceiling of 800 feet or higher and
visibility 2 miles or greater, no localizer or glideslope critical area protection is provided by ATC unless
specifically requested by the flight crew.
(b) Weather Conditions. Official weather observation indicates a ceiling of less than 800 feet or
visibility less than 2 miles.
(1) Holding. Aircraft holding below 5,000 feet between the outer marker and the airport may cause
localizer signal variations for aircraft conducting the ILS approach. Accordingly, such holding will not be
authorized by ATC.
(2) Localizer Critical Area. When an arriving aircraft is inside the outer marker (OM) or the fix used
in lieu of the OM, vehicles and aircraft will not be authorized in or over the precision approach critical area
except:
[a] A preceding arriving aircraft on the same or another runway may pass over or through the
localizer critical area, and;
[b] A preceding departing aircraft or missed approach on the same or another runway may pass
through or over the localizer critical area.
(3) Glide Slope Critical Area. ATC will not authorize vehicles or aircraft operations in or over the
glideslope critical area when an arriving aircraft is inside the outer marker (OM), or the fix used in lieu of the
OM, unless the arriving aircraft has reported the runway in sight and is circling or side-stepping to land on another
runway.
(c) Weather Conditions. Official weather observation indicates a ceiling less than 200 feet or runway
visual range (RVR) less than 2000 feet.
(1) Localizer Critical Area. In addition to the critical area protection described in 1−1−9k2(b) above,
when an arriving aircraft is inside the middle marker (MM), or in the absence of a MM, ½ mile final, ATC will
not authorize:
[a] A preceding arriving aircraft on the same or another runway to pass over or through the localizer
critical area, or;
[b] A preceding departing aircraft or missed approach on the same or another runway to pass through
or over the localizer critical area.
3. In order to ensure that pilot and controller expectations match with respect to critical area protection for
a given approach and landing operation, a flight crew should advise the tower any time it intends to conduct any
autoland operation or use an SA CAT I, any CAT II, or any CAT III line of minima anytime the official weather
observation is at or above a ceiling of 800 feet and 2 miles visibility. If A TC is unable to protect the critical area,
they will advise the flight crew.
EXAMPLE−
Denver Tower , United 1153, Request Autoland (runway) ATC replies with:
United 1153, Denver Tower , Roger, Critical Areas not protected.
Navigation Aids 1−1−17
AIM 2/20/25
4. Pilots are cautioned that even when the critical areas are considered to be protected, unless the official
weather observation including controller observations indicates a ceiling less than 200 feet or RVR less than 2000
feet, A TC may still authorize a preceding arriving, departing, or missed approach aircraft to pass through or over
the localizer critical area and that this may cause signal disturbances that could result in an undesired aircraft state
during the final stages of the approach, landing, and rollout.
5. Pilots are cautioned that vehicular traffic not subject to ATC may cause momentary deviation to ILS
course or glide slope signals. Also, critical areas are not protected at uncontrolled airports or at airports with an
operating control tower when weather or visibility conditions are above those requiring protective measures.
Aircraft conducting coupled or autoland operations should be especially alert in monitoring automatic flight
control systems and be prepared to intervene as necessary. (See FIG 1−1−8.)
NOTE−
Unless otherwise coordinated through Flight Standards, ILS signals to Category I runways are not flight inspected below
the point that is 100 feet less than the decision altitude (DA). Guidance signal anomalies may be encountered below this
altitude.
1−1−10. Simplified Directional Facility (SDF)
a. The SDF provides a final approach course similar to that of the ILS localizer. It does not provide glide slope
information. A clear understanding of the ILS localizer and the additional factors listed below completely
describe the operational characteristics and use of the SDF.
b. The SDF transmits signals within the range of 108.10 to 111.95 MHz.
c. The approach techniques and procedures used in an SDF instrument approach are essentially the same as
those employed in executing a standard localizer approach except the SDF course may not be aligned with the
runway and the course may be wider, resulting in less precision.
d. Usable off−course indications are limited to 35 degrees either side of the course centerline. Instrument
indications received beyond 35 degrees should be disregarded.
e. The SDF antenna may be offset from the runway centerline. Because of this, the angle of convergence
between the final approach course and the runway bearing should be determined by reference to the instrument
approach procedure chart. This angle is generally not more than 3 degrees. However, it should be noted that
inasmuch as the approach course originates at the antenna site, an approach which is continued beyond the
runway threshold will lead the aircraft to the SDF offset position rather than along the runway centerline.
f. The SDF signal is fixed at either 6 degrees or 12 degrees as necessary to provide maximum flyability and
optimum course quality.
g. Identification consists of a three−letter identifier transmitted in Morse Code on the SDF frequency. The
appropriate instrument approach chart will indicate the identifier used at a particular airport.
1−1−18 Navigation Aids
2/20/25 AIM
FIG 1−1−8
FAA Instrument Landing Systems
Navigation Aids 1−1−19
AIM 2/20/25
1−1−11. NAVAID Identifier Removal During Maintenance
During periods of routine or emergency maintenance, coded identification (or code and voice, where applicable)
is removed from certain FAA NA V AIDs. Removal of identification serves as a warning to pilots that the facility
is officially off the air for tune−up or repair and may be unreliable even though intermittent or constant signals
are received.
NOTE−
During periods of maintenance VHF ranges may radiate a T−E−S−T code (- -).
NOTE−
DO NOT attempt to fly a procedure that is NOTAMed out of service even if the identification is present. In certain cases,
the identification may be transmitted for short periods as part of the testing.
1−1−12. NAVAIDs with Voice
a. V oice equipped en route radio navigational aids are under the operational control of either a Flight Service
Station (FSS) or an approach control facility. Facilities with two −way voice communication available are
indicated in the Chart Supplement and aeronautical charts.
b. Unless otherwise noted on the chart, all radio navigation aids operate continuously except during
shutdowns for maintenance. Hours of operation of facilities not operating continuously are annotated on charts
and in the Chart Supplement.
1−1−13. User Reports Requested on NAVAID Outages
a. Users of the National Airspace System (NAS) can render valuable assistance in the early correction of
NA V AID malfunctions or GNSS problems and are encouraged to report their observations of undesirable
avionics performance. Although NA V AIDs are monitored by electronic detectors, adverse effects of electronic
interference, new obstructions, or changes in terrain near the NA VAID can exist without detection by the ground
monitors. Some of the characteristics of malfunction or deteriorating performance which should be reported are:
erratic course or bearing indications; intermittent, or full, flag alarm; garbled, missing or obviously improper
coded identification; poor quality communications reception; or, in the case of frequency interference, an audible
hum or tone accompanying radio communications or NA V AID identification. GNSS problems are often
characterized by navigation degradation or service loss indications. For instance, pilots conducting operations
in areas where there is GNSS interference may be unable to use GPS for navigation, and ADS −B may be
unavailable for surveillance. Radio frequency interference may affect both navigation for the pilot and
surveillance by the air traffic controller. Depending on the equipment and integration, either an advisory light
or message may alert the pilot. Air traffic controllers monitoring ADS−B reports may stop receiving ADS−B
position messages and associated aircraft tracks.
b. Malfunctioning, faulty, inappropriately installed, operated, or modified GPS re−radiator systems, intended
to be used for aircraft maintenance activities, have resulted in unintentional disruption of aviation GPS receivers.
This type of disruption could result in unflagged, erroneous position −information output to primary flight
displays/indicators and to other aircraft and air traffic control systems. Since Receiver Autonomous Integrity
Monitoring (RAIM) is only partially effective against this type of disruption (effectively a “signal spoofing”),
the pilot may not be aware of any erroneous navigation indications; ATC may be the only means available to
identify these disruptions and detect unexpected aircraft positions while monitoring aircraft for IFR separation.
c. Pilots encountering navigation error events should transition to another source of navigation and request
amended clearances from ATC as necessary.
d. Pilots are encouraged to submit detailed reports of NA V AID or GPS anomaly as soon as practical. Pilot
reports of navigation error events should contain the following information:
1. Date and time the anomaly was observed, and NA V AID ID (or GPS).
2. Location of the aircraft at the time the anomaly started and ended (e.g., latitude/longitude or
bearing/distance from a reference point),
1−1−20 Navigation Aids
2/20/25 AIM
3. Heading, altitude, type of aircraft (make/model/call sign),
4. Type of avionics/receivers in use (e.g., make/model/software series or version),
5. Number of satellites being tracked, if applicable,
6. Description of the position/navigation/timing anomaly observed, and duration of the event,
7. Consequences/operational impact(s) of the NA V AID or GPS anomaly,
8. Actions taken to mitigate the anomaly and/or remedy provided by the ATC facility,
9. Post flight pilot/maintenance actions taken.
e. Pilots operating an aircraft in controlled airspace under IFR shall comply with CFR § 91.187 and promptly
report as soon as practical to ATC any malfunctions of navigational equipment occurring in flight; pilots should
submit initial reports:
1. Immediately, by radio to the controlling ATC facility or FSS.
2. By telephone to the nearest ATC facility controlling the airspace where the disruption was experienced.
3. Additionally, GPS problems should be reported, post flight, by Internet via the GPS Anomaly Reporting
Form at http://www.faa.gov/air_traffic/nas/gps_reports/.
f. To minimize ATC workload, GPS anomalies associated with known testing NOTAMs should NOT be
reported in−flight to ATC in detail; EXCEPT when:
1. GPS degradation is experienced outside the NOTAMed area,
2. Pilot observes any unexpected consequences (e.g., equipment failure, suspected spoofing, failure of
unexpected aircraft systems, such as TAWS).
1−1−14. LORAN
NOTE−
In accordance with the 2010 DHS Appropriations Act, the U.S. Coast Guard (USCG) terminated the transmission of all U.S.
LORAN−C signals on 08 Feb 2010. The USCG also terminated the transmission of the Russian American signals on 01 Aug
2010, and the Canadian LORAN −C signals on 03 Aug 2010. For more information, visit http://www.navcen.uscg.gov.
Operators should also note that TSO−C60b, AIRBORNE AREA NAVIGATION EQUIPMENT USING LORAN−C INPUTS,
has been canceled by the F AA.
1−1−15. Inertial Reference Unit (IRU), Inertial Navigation System (INS), and Attitude Heading
Reference System (AHRS)
a. IRUs are self−contained systems comprised of gyros and accelerometers that provide aircraft attitude
(pitch, roll, and heading), position, and velocity information in response to signals resulting from inertial effects
on system components. Once aligned with a known position, IRUs continuously calculate position and velocity.
IRU position accuracy decays with time. This degradation is known as “drift.”
b. INSs combine the components of an IRU with an internal navigation computer. By programming a series
of waypoints, these systems will navigate along a predetermined track.
c. AHRSs are electronic devices that provide attitude information to aircraft systems such as weather radar
and autopilot, but do not directly compute position information.
d. Aircraft equipped with slaved compass systems may be susceptible to heading errors caused by exposure
to magnetic field disturbances (flux fields) found in materials that are commonly located on the surface or buried
under taxiways and ramps. These materials generate a magnetic flux field that can be sensed by the aircraft’s
compass system flux detector or “gate,” which can cause the aircraft’s system to align with the material’s
magnetic field rather than the earth’s natural magnetic field. The system’s erroneous heading may not
Navigation Aids 1−1−21
AIM 2/20/253/15/077110.65R CHG 2AIM 8/7/25
self-correct. Prior to take off pilots should be aware that a heading misalignment may have occurred during taxi.
Pilots are encouraged to follow the manufacturer’s or other appropriate procedures to correct possible heading
misalignment before take off is commenced.
1−1−16. Doppler Radar
Doppler Radar is a semiautomatic self− contained dead reckoning navigation system (radar sensor plus
computer) which is not continuously dependent on information derived from ground based or external aids. The
system employs radar signals to detect and measure ground speed and drift angle, using the aircraft compass
system as its directional reference. Doppler is less accurate than INS, however, and the use of an external
reference is required for periodic updates if acceptable position accuracy is to be achieved on long range flights.
1−1−17. Global Positioning System (GPS)
a. System Overview
1. System Description. The Global Positioning System is a space-based radio navigation system used to
determine precise position anywhere in the world. The 24 satellite constellation is designed to ensure at least five
satellites are always visible to a user worldwide. A minimum of four satellites is necessary for receivers to
establish an accurate three−dimensional position. The receiver uses data from satellites above the mask angle
(the lowest angle above the horizon at which a receiver can use a satellite). The Department of Defense (DoD)
is responsible for operating the GPS satellite constellation and monitors the GPS satellites to ensure proper
operation. Each satellite’s orbital parameters (ephemeris data) are sent to each satellite for broadcast as part of
the data message embedded in the GPS signal. The GPS coordinate system is the Cartesian earth −centered,
earth−fixed coordinates as specified in the World Geodetic System 1984 (WGS−84).
2. System Availability and Reliability.
(a) The status of GPS satellites is broadcast as part of the data message transmitted by the GPS satellites.
GPS status information is also available by means of the U.S. Coast Guard navigation information service: (703)
313−5907, Internet: http://www.navcen.uscg.gov/. Additionally, satellite status is available through the Notice
to Airmen (NOTAM) system.
(b) GNSS operational status depends on the type of equipment being used. For GPS −only equipment
TSO−C129 or TSO-C196(), the operational status of non −precision approach capability for flight planning
purposes is provided through a prediction program that is embedded in the receiver or provided separately.
3. Receiver Autonomous Integrity Monitoring (RAIM). RAIM is the capability of a GPS receiver to
perform integrity monitoring on itself by ensuring available satellite signals meet the integrity requirements for
a given phase of flight. Without RAIM, the pilot has no assurance of the GPS position integrity. RAIM provides
immediate feedback to the pilot. This fault detection is critical for performance-based navigation (PBN)(see
paragraph 1−2−1, Performance−Based Navigation (PBN) and Area Navigation (RNA V), for an introduction to
PBN), because delays of up to two hours can occur before an erroneous satellite transmission is detected and
corrected by the satellite control segment.
(a) In order for RAIM to determine if a satellite is providing corrupted information, at least one satellite,
in addition to those required for navigation, must be in view for the receiver to perform the RAIM function.
RAIM requires a minimum of 5 satellites, or 4 satellites and barometric altimeter input (baro−aiding), to detect
an integrity anomaly. Baro−aiding is a method of augmenting the GPS integrity solution by using a non-satellite
input source in lieu of the fifth satellite. Some GPS receivers also have a RAIM capability, called fault detection
and exclusion (FDE), that excludes a failed satellite from the position solution; GPS receivers capable of FDE
require 6 satellites or 5 satellites with baro−aiding. This allows the GPS receiver to isolate the corrupt satellite
signal, remove it from the position solution, and still provide an integrity-assured position. To ensure that
baro−aiding is available, enter the current altimeter setting into the receiver as described in the operating manual.
Do not use the GPS derived altitude due to the large GPS vertical errors that will make the integrity monitoring
function invalid.
1−1−22 Navigation Aids
2/20/25 AIM
(b) There are generally two types of RAIM fault messages. The first type of message indicates that there
are not enough satellites available to provide RAIM integrity monitoring. The GPS navigation solution may be
acceptable, but the integrity of the solution cannot be determined. The second type indicates that the RAIM
integrity monitor has detected a potential error and that there is an inconsistency in the navigation solution for
the given phase of flight. Without RAIM capability, the pilot has no assurance of the accuracy of the GPS
position.
4. Selective Availability. Selective Availability (SA) is a method by which the accuracy of GPS is
intentionally degraded. This feature was designed to deny hostile use of precise GPS positioning data. SA was
discontinued on May 1, 2000, but many GPS receivers are designed to assume that SA is still active. New
receivers may take advantage of the discontinuance of SA based on the performance values in ICAO Annex 10.
b. Operational Use of GPS. U.S. civil operators may use approved GPS equipment in oceanic airspace,
certain remote areas, the National Airspace System and other States as authorized (please consult the applicable
Aeronautical Information Publication). Equipage other than GPS may be required for the desired operation. GPS
navigation is used for both Visual Flight Rules (VFR) and Instrument Flight Rules (IFR) operations.
1. VFR Operations
(a) GPS navigation has become an asset to VFR pilots by providing increased navigational capabilities
and enhanced situational awareness. Although GPS has provided many benefits to the VFR pilot, care must be
exercised to ensure that system capabilities are not exceeded. VFR pilots should integrate GPS navigation with
electronic navigation (when possible), as well as pilotage and dead reckoning.
(b) GPS receivers used for VFR navigation vary from fully integrated IFR/VFR installation used to
support VFR operations to hand−held devices. Pilots must understand the limitations of the receivers prior to
using in flight to avoid misusing navigation information. (See TBL 1−1−6.) Most receivers are not intuitive. The
pilot must learn the various keystrokes, knob functions, and displays that are used in the operation of the receiver.
Some manufacturers provide computer−based tutorials or simulations of their receivers that pilots can use to
become familiar with operating the equipment.
(c) When using GPS for VFR operations, RAIM capability, database currency, and antenna location are
critical areas of concern.
(1) RAIM Capability. VFR GPS panel mount receivers and hand−held units have no RAIM alerting
capability. This prevents the pilot from being alerted to the loss of the required number of satellites in view, or
the detection of a position error. Pilots should use a systematic cross−check with other navigation techniques to
verify position. Be suspicious of the GPS position if a disagreement exists between the two positions.
(2) Database Currency. Check the currency of the database. Databases must be updated for IFR
operations and should be updated for all other operations. However, there is no requirement for databases to be
updated for VFR navigation. It is not recommended to use a moving map with an outdated database in and around
critical airspace. Pilots using an outdated database should verify waypoints using current aeronautical products;
for example, Chart Supplement, Sectional Chart, or En Route Chart.
(3) Antenna Location. The antenna location for GPS receivers used for IFR and VFR operations may
differ. VFR antennae are typically placed for convenience more than performance, while IFR installations
ensure a clear view is provided with the satellites. Antennae not providing a clear view have a greater opportunity
to lose the satellite navigational signal. This is especially true in the case of hand−held GPS receivers. Typically,
suction cups are used to place the GPS antennas on the inside of cockpit windows. While this method has great
utility, the antenna location is limited to the cockpit or cabin which rarely provides a clear view of all available
satellites. Consequently, signal losses may occur due to aircraft structure blocking satellite signals, causing a
loss of navigation capability. These losses, coupled with a lack of RAIM capability, could present erroneous
position and navigation information with no warning to the pilot. While the use of a hand−held GPS for VFR
operations is not limited by regulation, modification of the aircraft, such as installing a panel− or yoke−mounted
holder, is governed by 14 CFR part 43. Consult with your mechanic to ensure compliance with the regulation
and safe installation.
Navigation Aids 1−1−23
AIM 2/20/253/15/077110.65R CHG 2AIM 1/22/263/15/077110.65R CHG 2AIM 1/22/26
(d) Do not solely rely on GPS for VFR navigation. No design standard of accuracy or integrity is used
for a VFR GPS receiver. VFR GPS receivers should be used in conjunction with other forms of navigation during
VFR operations to ensure a correct route of flight is maintained. Minimize head−down time in the aircraft by
being familiar with your GPS receiver’s operation and by keeping eyes outside scanning for traffic, terrain, and
obstacles.
(e) VFR Waypoints
(1) VFR waypoints provide VFR pilots with a supplementary tool to assist with position awareness
while navigating visually in aircraft equipped with area navigation receivers. VFR waypoints should be used as
a tool to supplement current navigation procedures. The uses of VFR waypoints include providing navigational
aids for pilots unfamiliar with an area, waypoint definition of existing reporting points, enhanced navigation in
and around Class B and Class C airspace, enhanced navigation around Special Use Airspace, and entry points
for commonly flown mountain passes. VFR pilots should rely on appropriate and current aeronautical charts
published specifically for visual navigation. If operating in a terminal area, pilots should take advantage of the
Terminal Area Chart available for that area, if published. The use of VFR waypoints does not relieve the pilot
of any responsibility to comply with the operational requirements of 14 CFR part 91.
(2) VFR waypoint names (for computer entry and flight plans) consist of five letters beginning with
the letters “VP” and are retrievable from navigation databases. The VFR waypoint names are not intended to be
pronounceable, and they are not for use in ATC communications. On VFR charts, stand−alone VFR waypoints
will be portrayed using the same four−point star symbol used for IFR waypoints. VFR waypoints collocated with
visual check−points on the chart will be identified by small magenta flag symbols. VFR waypoints collocated
with visual check−points will be pronounceable based on the name of the visual check−point and may be used
for ATC communications. Each VFR waypoint name will appear in parentheses adjacent to the geographic
location on the chart. Latitude/longitude data for all established VFR waypoints is accessible through FAA Order
JO 7350.9, Location Identifiers.
(3) VFR waypoints may not be used on IFR flight plans. VFR waypoints are not recognized by the IFR
system and will be rejected for IFR routing purposes.
(4) Pilots may use the five−letter identifier as a waypoint in the route of flight section on a VFR flight
plan. Pilots may use the VFR waypoints only when operating under VFR conditions. The point may represent
an intended course change or describe the planned route of flight. This VFR filing would be similar to how a VOR
would be used in a route of flight.
(5) VFR waypoints intended for use during flight should be loaded into the receiver while on the
ground. Once airborne, pilots should avoid programming routes or VFR waypoint chains into their receivers.
(6) Pilots should be vigilant to see and avoid other traffic when near VFR waypoints. With the increased
use of GPS navigation and accuracy, expect increased traffic near VFR waypoints. Regardless of the class of
airspace, monitor the available ATC frequency for traffic information on other aircraft operating in the vicinity.
See paragraph 7−6−3, VFR in Congested Areas, for more information.
(7) Mountain pass entry points are marked for convenience to assist pilots with flight planning and
visual navigation. Do not attempt to fly a mountain pass directly from VFR waypoint to VFR waypoint—they
do not create a path through the mountain pass. Alternative routes are always available. It is the pilot in
command’s responsibility to choose a suitable route for the intended flight and known conditions.
REFERENCE−
AIM, Para 7−6−7, Mountain Flying.
2. IFR Use of GPS
(a) General Requirements. Authorization to conduct any GPS operation under IFR requires:
(1) GPS navigation equipment used for IFR operations must be approved in accordance with the
requirements specified in Technical Standard Order (TSO) TSO −C129(), TSO−C196(), TSO−C145(), or
TSO−C146(), and the installation must be done in accordance with Advisory Circular AC 20−138, Airworthiness
1−1−24 Navigation Aids
2/20/25 AIM
Approval of Positioning and Navigation Systems. Equipment approved in accordance with TSO−C115a does
not meet the requirements of TSO −C129. Visual flight rules (VFR) and hand −held GPS systems are not
authorized for IFR navigation, instrument approaches, or as a principal instrument flight reference.
(2) Aircraft using un-augmented GPS (TSO-C129() or TSO-C196()) for navigation under IFR must
be equipped with an alternate approved and operational means of navigation suitable for navigating the proposed
route of flight. (Examples of alternate navigation equipment include VOR or DME/DME/IRU capability).
Active monitoring of alternative navigation equipment is not required when RAIM is available for integrity
monitoring. Active monitoring of an alternate means of navigation is required when the GPS RAIM capability
is lost.
(3) Procedures must be established for use in the event that the loss of RAIM capability is predicted
to occur. In situations where RAIM is predicted to be unavailable, the flight must rely on other approved
navigation equipment, re-route to where RAIM is available, delay departure, or cancel the flight.
(4) The GPS operation must be conducted in accordance with the FAA −approved aircraft flight
manual (AFM) or flight manual supplement. Flight crew members must be thoroughly familiar with the
particular GPS equipment installed in the aircraft, the receiver operation manual, and the AFM or flight manual
supplement. Operation, receiver presentation and capabilities of GPS equipment vary. Due to these differences,
operation of GPS receivers of different brands, or even models of the same brand, under IFR should not be
attempted without thorough operational knowledge. Most receivers have a built −in simulator mode, which
allows the pilot to become familiar with operation prior to attempting operation in the aircraft.
(5) Aircraft navigating by IFR−approved GPS are considered to be performance−based navigation
(PBN) aircraft and have special equipment suffixes. File the appropriate equipment suffix in accordance with
Appendix 4, TBL 4−2, on the ATC flight plan. If GPS avionics become inoperative, the pilot should advise ATC
and amend the equipment suffix.
(6) Prior to any GPS IFR operation, the pilot must review appropriate NOTAMs and aeronautical
information. (See GPS NOTAMs/Aeronautical Information).
(b) Database Requirements. The onboard navigation data must be current and appropriate for the region
of intended operation and should include the navigation aids, waypoints, and relevant coded terminal airspace
procedures for the departure, arrival, and alternate airfields.
(1) Further database guidance for terminal and en route requirements may be found in AC 90-100, U.S.
Terminal and En Route Area Navigation (RNA V) Operations.
(2) Further database guidance on Required Navigation Performance (RNP) instrument approach
operations, RNP terminal, and RNP en route requirements may be found in AC 90-105, Approval Guidance for
RNP Operations and Barometric Vertical Navigation in the U.S. National Airspace System.
(3) All approach procedures to be flown must be retrievable from the current airborne navigation
database supplied by the equipment manufacturer or other FAA−approved source. The system must be able to
retrieve the procedure by name from the aircraft navigation database, not just as a manually entered series of
waypoints. Manual entry of waypoints using latitude/longitude or place/bearing is not permitted for approach
procedures.
(4) Prior to using a procedure or waypoint retrieved from the airborne navigation database, the pilot
should verify the validity of the database. This verification should include the following preflight and inflight
steps:
[a] Preflight:
[1] Determine the date of database issuance, and verify that the date/time of proposed use is
before the expiration date/time.
[2] V erify that the database provider has not published a notice limiting the use of the specific
waypoint or procedure.
Navigation Aids 1−1−25
AIM 2/20/253/15/077110.65R CHG 2AIM 1/22/26
[b] Inflight:
[1] Determine that the waypoints and transition names coincide with names found on the
procedure chart. Do not use waypoints which do not exactly match the spelling shown on published procedure
charts.
[2] Determine that the waypoints are logical in location, in the correct order, and their
orientation to each other is as found on the procedure chart, both laterally and vertically.
NOTE−
There is no specific requirement to check each waypoint latitude and longitude, type of waypoint and/or altitude constraint,
only the general relationship of waypoints in the procedure, or the logic of an individual waypoint’ s location.
[3] If the cursory check of procedure logic or individual waypoint location, specified in [b]
above, indicates a potential error, do not use the retrieved procedure or waypoint until a verification of latitude
and longitude, waypoint type, and altitude constraints indicate full conformity with the published data.
(5) Air carrier and commercial operators must meet the appropriate provisions of their approved
operations specifications.
[a] During domestic operations for commerce or for hire, operators must have a second navigation
system capable of reversion or contingency operations.
[b] Operators must have two independent navigation systems appropriate to the route to be flown
or one system that is suitable and a second, independent backup system that allows the operator to proceed safely
to a suitable airport, complete an instrument approach; and the aircraft must have sufficient fuel (reference 14
CFR 121.349, 125.203, 129.17, and 135.165). These rules ensure the safety of the operation by preventing a
single point of failure.
NOTE−
An aircraft approved for multi-sensor navigation and equipped with a single navigation system must maintain an ability to
navigate or proceed safely in the event that any one component of the navigation system fails, including the flight
management system (FMS). Retaining an FMS-independent VOR capability would satisfy this requirement.
[c] The requirements for a second system apply to the entire set of equipment needed to achieve the
navigation capability, not just the individual components of the system such as the radio navigation receiver. For
example, to use two RNA V systems (e.g., GPS and DME/DME/IRU) to comply with the requirements, the
aircraft must be equipped with two independent radio navigation receivers and two independent navigation
computers (e.g., flight management systems (FMS)). Alternatively, to comply with the requirements using a
single RNA V system with an installed and operable VOR capability, the VOR capability must be independent
of the FMS.
[d] Due to low risk of disruption or manipulation of GPS signals beyond 50 NM offshore, FAA
differentiates between extended and non−extended over−water operations. To satisfy the requirement of two
independent navigation systems:
[1] For all extended over−water operations (defined in 14 CFR Part 1 as greater than 50 NM from
the nearest shoreline), operators may consider dual GPS −based systems to meet the “independent” criteria
stipulated by regulation, e.g. §121.349, §135.165.
[2] For all “non −extended overwater” operations, if the primary navigation system is
GPS−based, the second system must be independent of GPS (for example, VOR or DME/DME/IRU). This
allows continued navigation in case of failure of the GPS or WAAS services. Recognizing that GPS interference
and test events resulting in the loss of GPS services have become more common, the FAA requires operators
conducting IFR operations under 14 CFR 121.349, 125.203, 129.17 and 135.65 to retain a non−GPS navigation
capability, for example either DME/DME, IRU, or VOR for en route and terminal operations, and VOR and ILS
for final approach. Since this system is to be used as a reversionary capability, single equipage is sufficient.
3. Oceanic, Domestic, En Route, and Terminal Area Operations
1−1−26 Navigation Aids
AIM2/20/251/22/26 AIM
(a) Conduct GPS IFR operations in oceanic areas only when approved avionics systems are installed.
TSO−C196() users and TSO−C129() GPS users authorized for Class A1, A2, B1, B2, C1, or C2 operations may
use GPS in place of another approved means of long−range navigation, such as dual INS. (See TBL 1 −1−5
and TBL 1−1−6.) Aircraft with a single installation GPS, meeting the above specifications, are authorized to
operate on short oceanic routes requiring one means of long−range navigation (reference AC 20-138, Appendix
1).
(b) Conduct GPS domestic, en route, and terminal IFR operations only when approved avionics
systems are installed. Pilots may use GPS via TSO−C129() authorized for Class A1, B1, B3, C1, or C3 operations
GPS via TSO-C196(); or GPS/WAAS with either TSO-C145() or TSO-C146(). When using TSO-C129() or
TSO-C196() receivers, the avionics n ecessary to receive all of the ground−based facilities appropriate for the
route to the destination airport and any required alternate airport must be installed and operational.
Ground−based facilities necessary for these routes must be operational.
(1) GPS en route IFR operations may be conducted in Alaska outside the operational service volume
of ground−based navigation aids when a TSO−C145() or TSO−C146() GPS/wide area augmentation system
(WAAS) system is installed and operating. WAAS is the U.S. version of a satellite-based augmentation system
(SBAS).
[a] In Alaska, aircraft may operate on GNSS Q-routes with GPS (TSO-C129 () or TSO-C196 ())
equipment while the aircraft remains in Air Traffic Control (ATC) radar surveillance or with GPS/WAAS
(TSO-C145 () or TSO-C146 ()) which does not require ATC radar surveillance.
[b] In Alaska, aircraft may only operate on GNSS T-routes with GPS/WAAS (TSO-C145 () or
TSO-C146 ()) equipment.
(2) Ground−based navigation equipment is not required to be installed and operating for en route IFR
operations when using GPS/W AAS navigation systems. All operators should ensure that an alternate means of
navigation is available in the unlikely event the GPS/W AAS navigation system becomes inoperative.
(3) Q-routes and T-routes outside Alaska. Q-routes require system performance currently met by GPS,
GPS/WAAS, or DME/DME/IRU RNA V systems that satisfy the criteria discussed in AC 90−100, U.S. Terminal
and En Route Area Navigation (RNA V) Operations. T-routes require GPS or GPS/WAAS equipment.
REFERENCE−
AIM, Para 5−3−4, Airways and Route Systems.
(c) GPS IFR approach/departure operations can be c onducted when approved avionics systems are
installed and the following requirements are met:
(1) The aircraft is TSO−C145() or TSO−C146() or TSO−C196() or TSO−C129() in Class A1, B1, B3,
C1, or C3; and
(2) The approach/departure must be retrievable from the current airborne navigation database in the
navigation computer. The system must be able to retrieve the procedure by name from the aircraft navigation
database. Manual entry of waypoints using latitude/longitude or place/bearing is not permitted for approach
procedures.
(3) The authorization to fly instrument approaches/departures with GPS is limited to U.S. airspace.
(4) The use of GPS in any other airspace must be expressly authorized by the FAA Administrator.
(5) GPS instrument approach/departure operations outside the U.S. must be authorized by the
appropriate sovereign authority.
4. Departures and Instrument Departure Procedures (DPs)
The GPS receiver must be set to terminal (±1 NM) CDI sensitivity and the navigation routes contained in the
database in order to fly published IFR charted departures and DPs. Terminal RAIM s hould be automatically
provided by the receiver. (Terminal RAIM for departure may not be available unless the waypoints are part of
Navigation Aids 1−1−27
AIM 2/20/253/15/077110.65R CHG 2AIM 1/22/26
the active flight plan rather than proceeding direct to the first destination.) Certain segments of a DP may require
some manual intervention by the pilot, especially when radar vectored to a course or required to intercept a
specific course to a waypoint. The database may not contain all of the transitions or departures from all runways
and some GPS receivers do not contain DPs in the database. It is necessary that helicopter procedures be flown
at 70 knots or less since helicopter departure procedures and missed approaches use a 20:1 obstacle clearance
surface (OCS), which is double the fixed −wing OCS, and turning areas are based on this speed as well.
5. GPS Instrument Approach Procedures
(a) GPS overlay approaches are designated non−precision instrument approach procedures that pilots are
authorized to fly using GPS avionics. Localizer (LOC), localizer type directional aid (LDA), and simplified
directional facility (SDF) procedures are not authorized. Overlay procedures are identified by the “name of
the procedure” and “or GPS” (e.g., VOR/DME or GPS RWY 15) in the title. Authorized procedures must be
retrievable from a current onboard navigation databa se. The navigation database may also enhance position
orientation by displaying a map containing information on conventional NA V AID approaches. This approach
information should not be confused with a GPS overlay approach (see the receiver operating manual, AFM,
or AFM Supplement for details on how to identify these approaches in the navigation database).
NOTE−
Overlay approaches do not adhere to the design criteria described in paragraph 5 −4−5m, Area Navigation (RNAV)
Instrument Approach Charts, for stand−alone GPS approaches. Overlay approach criteria is based on the design criteria
used for ground−based NAVAID approaches.
(b) Stand−alone approach procedures specifically designed for GPS systems have replaced many of the
original overlay approaches. All approaches that contain “GPS” in the title (e.g., “VOR or GPS RWY 24,” “GPS
RWY 24,” or “RNAV (GPS) RWY 24”) can be flown using GPS. GPS−equipped aircraft do not need underlying
ground−based NA V AIDs or associated aircraft avionics to fly the approach. Monitoring the underlying approach
with ground−based NA V AIDs is suggested when able. Existing overlay approaches may be requested using the
GPS title; for example, the VOR or GPS RWY 24 may be requested as “GPS RWY 24.” Some GPS procedures
have a Terminal Arrival Area (TAA) with an underlining RNA V approach.
(c) For flight planning purposes, TSO-C129() and TSO-C196() −equipped users (GPS users) whose
navigation systems have fault detection and exclusion (FDE) capability, who perform a preflight RAIM
prediction for the approach integrity at the airport where the RNA V (GPS) approach will be flown, and have
proper knowledge and any required training and/or approval to conduct a GPS-based IAP, may file based on
a GPS−based IAP at either the destination or the alternate airport, but not at both locations. At the alternate
airport, pilots may plan for:
(1) Lateral navigation (LNA V) or circling minimum descent altitude (MDA);
(2) LNA V/vertical navigation (LNA V/VNA V) DA, if equipped with and using approved barometric
vertical navigation (baro-VNA V) equipment;
(3) RNP 0.3 DA on an RNA V (RNP) IAP, if they are specifically authorized users using approved
baro-VNA V equipment and the pilot has verified required navigation performance (RNP) availability through
an approved prediction program.
(d) If the above conditions cannot be met, any required alternate airport must have an approved
instrument approach procedure other than GPS−based that is anticipated to be operational and available at the
estimated time of arrival, and which the aircraft is equipped to fly.
(e) Procedures for Accomplishing GPS Approaches
(1) An RNA V (GPS) procedure may be associated with a Terminal Arrival Area (TAA). The basic
design of the RNA V procedure is the “T” design or a modification of the “T” (See Paragraph 5-4-5d, Terminal
Arrival Area (TAA), for complete information).
(2) Pilots cleared by ATC for an RNA V (GPS) approach should fly the full approach from an Initial
Approach W aypoint (IAWP) or feeder fix. Randomly joining an approach at an intermediate fix does not assure
terrain clearance.
1−1−28 Navigation Aids
AIM2/20/251/22/26 AIM
(3) When an approach has been loaded in the navigation system, GPS receivers will give an “arm”
annunciation 30 NM straight line distance from the airport/heliport reference point. Pilots should arm the
approach mode at this time if not already armed (some receivers arm automatically). Without arming, the
receiver will not change from en route CDI and RAIM sensitivity of ±5 NM either side of centerline to ±1 NM
terminal sensitivity. Where the IAWP is inside this 30 mile point, a CDI sensitivity change will occur once the
approach mode is armed and the aircraft is inside 30 NM. Where the IAWP is beyond 30 NM from the
airport/heliport reference point and the approach is armed, the CDI sensitivity will not change until the aircraft
is within 30 miles of the airport/heliport reference point. Feeder route obstacle clearance is predicated on the
receiver being in terminal (±1 NM) CDI sensitivity and RAIM within 30 NM of the airport/heliport reference
point; therefore, the receiver should always be armed (if required) not later than the 30 NM annunciation.
(4) The pilot must be aware of what bank angle/turn rate the particular receiver uses to compute turn
anticipation, and whether wind and airspeed are included in the receiver’s calculations. This information should
be in the receiver operating manual. Over or under banking the turn onto the final approach course may
significantly delay getting on course and may result in high descent rates to achieve the next segment altitude.
(5) When within 2 NM of the Final Approach Waypoint (FAWP) with the approach mode armed, the
approach mode will switch to active, which results in RAIM and CDI changing to approach sensitivity.
Beginning 2 NM prior to the FAWP, the full scale CDI sensitivity will smoothly change from ±1 NM to ±0.3
NM at the FAWP. As sensitivity changes from ±1 NM to ±0.3 NM approaching the FAWP, with the CDI not
centered, the corresponding increase in CDI displacement may give the impression that the aircraft is moving
further away from the intended course even though it is on an acceptable intercept heading. Referencing the
digital track displacement information (cross track error), if it is available in the approach mode, may help the
pilot remain position oriented in this situation. Being established on the final approach course prior to the
beginning of the sensitivity change at 2 NM will help prevent problems in interpreting the CDI display during
ramp down. Therefore, requesting or accepting vectors which will cause the aircraft to intercept the final
approach course within 2 NM of the FAWP is not recommended.
(6) When receiving vectors to final, most receiver operating manuals suggest placing the receiver in
the non−sequencing mode on the FAWP and manually setting the course. This provides an extended final
approach course in cases where the aircraft is vectored onto the final approach course outside of any existing
segment which is aligned with the runway. Assigned altitudes must be maintained until established on a
published segment of the approach. Required altitudes at waypoints outside the FAWP or stepdown fixes must
be considered. Calculating the distance to the FAWP may be required in order to descend at the proper location.
(7) Overriding an automatically selected sensitivity during an approach will cancel the approach mode
annunciation. If the approach mode is not armed by 2 NM prior to the F AWP, the approach mode will not become
active at 2 NM prior to the FAWP, and the equipment will flag. In these conditions, the RAIM and CDI sensitivity
will not ramp down, and the pilot should not descend to MDA, but fly to the MAWP and execute a missed
approach. The approach active annunciator and/or the receiver should be checked to ensure the approach mode
is active prior to the FAWP.
(8) Do not attempt to fly an approach unless the procedure in the onboard database is current and
identified as “GPS” on the approach chart. The navigation database may contain information a bout
non−overlay approach procedures that enhances position orientation generally by providing a map, while flying
these approaches using conventional NA V AIDs. This approach information should not be confused with a GPS
overlay approach (see the receiver operating manual, AFM, or AFM Supplement for details on how to identify
these procedures in the navigation database). Flying point to point on the approach does not assure compliance
with the published approach procedure. The proper RAIM sensitivity will not be available and the CDI sensitivity
will not automatically change to ±0.3 NM. Manually setting CDI sensitivity does not automatically change the
RAIM sensitivity on some receivers. Some existing non −precision approach procedures cannot be coded for
use with GPS and will not be available as overlays.
(9) Pilots should pay particular attention to the exact operation of their GPS receivers for performing
holding patterns and in the case of overlay approaches, operations such as procedure turns. These procedures
Navigation Aids 1−1−29
AIM 2/20/253/15/077110.65R CHG 2AIM 1/22/26
may require manual intervention by the pilot to stop the sequencing of waypoints by the receiver and to resume
automatic GPS navigation sequencing once the maneuver is complete. The same waypoint may appear in the
route of flight more than once consecutively (for example, IAWP, FAWP, MAHWP on a procedure turn). Care
must be exercised to ensure that the receiver is sequenced to the appropriate waypoint for the segment of the
procedure being flown, especially if one or more fly−overs are skipped (for example, FAWP rather than IAWP
if the procedure turn is not flown). The pilot may have to sequence past one or more fly −overs of the same
waypoint in order to start GPS automatic sequencing at the proper place in the sequence of waypoints.
(10) Incorrect inputs into the GPS receiver are especially critical during approaches. In some cases,
an incorrect entry can cause the receiver to leave the approach mode.
(11) A fix on an overlay approach identified by a DME fix will not be in the waypoint sequence on the
GPS receiver unless there is a published name assigned to it. When a name is assigned, the along track distance
(ATD) to the waypoint may be zero rather than the DME stated on the approach chart. The pilot should be alert
for this on any overlay procedure where the original approach used DME.
(12) If a visual descent point (VDP) is published, it will not be included in the sequence of waypoints.
Pilots are expected to use normal piloting techniques for beginning the visual descent, such as ATD.
(13) Unnamed stepdown fixes in the final approach segment may or may not be coded in the waypoint
sequence of the aircraft’s navigation database and must be identified using ATD. Stepdown fixes in the final
approach segment of RNA V (GPS) approaches are being named, in addition to being identified by ATD.
However, GPS avionics may or may not accommodate waypoints between the FAF and MAP. Pilots must know
the capabilities of their GPS equipment and continue to identify stepdown fixes using ATD when necessary.
(f) Missed Approach
(1) A GPS missed approach requires pilot action to sequence the receiver past the MAWP to the
missed approach portion of the procedure. The pilot must be thoroughly familiar with the activation procedure
for the particular GPS receiver installed in the aircraft and must initiate appropriate action after the MAWP.
Activating the missed approach prior to the MAWP will cause CDI sensitivity to immediately change to terminal
(±1NM) sensitivity and the receiver will continue to navigate to the MAWP . The receiver will not sequence
past the MAWP. Turns should not begin prior to the MAWP. If the missed approach is not activated, the GPS
receiver will display an extension of the inbound final approach course and the ATD will increase from the
MAWP until it is manually sequenced after crossing the MAWP.
(2) Missed approach routings in which the first track is via a course rather than direct to the next
waypoint require additional action by the pilot to set the course. Being familiar with all of the inputs required
is especially critical during this phase of flight.
(g) Receiver Autonomous Integrity Monitoring (RAIM)
(1) RAIM outages may occur due to an insufficient number of satellites or due to unsuitable satellite
geometry which causes the error in the position solution to become too large. Loss of satellite reception and
RAIM warnings may occur due to aircraft dynamics (changes in pitch or bank angle). Antenna location on the
aircraft, satellite position relative to the horizon, and aircraft attitude may affect reception of one or more
satellites. Since the relative positions of the satellites are constantly changing, prior experience with the airport
does not guarantee reception at all times, and RAIM availability should always be checked.
(2) Civilian pilots may obtain GPS RAIM availability information for nonprecision approach
procedures by using a manufacturer−supplied RAIM prediction tool, or using the Service Availability Prediction
Tool (SAPT) on the FAA en route and terminal RAIM prediction website. Pilots can also request GPS RAIM
aeronautical information from a flight service station during preflight briefings. GPS RAIM aeronautical
information can be obtained for a period of 3 hours (for example, if you are scheduled to arrive at 1215 hours,
then the GPS RAIM information is available from 1100 to 1400 hours) or a 24−hour timeframe at a particular
airport. FAA briefers will provide RAIM information for a period of 1 hour before to 1 hour after the ETA hour,
unless a specific timeframe is requested by the pilot. If flying a published GPS departure, a RAIM prediction
should also be requested for the departure airport.
1−1−30 Navigation Aids
AIM2/20/251/22/26 AIM
(3) The military provides airfield specific GPS RAIM NOTAMs for nonprecision approach procedures
at military airfields. The RAIM outages are issued as M−series NOTAMs and may be obtained for up to 24 hours
from the time of request.
(4) Receiver manufacturers and/or database suppliers may supply “NOTAM” type information
concerning database errors. Pilots should check these sources when available, to ensure that they have the most
current information concerning their electronic database.
(5) If RAIM is not available, use another type of navigation and approach system; select another route
or destination; or delay the trip until RAIM is predicted to be available on arrival. On longer flights, pilots should
consider rechecking the RAIM prediction for the destination during the flight. This may provide an early
indication that an unscheduled satellite outage has occurred since takeoff.
(6) If a RAIM failure/status annunciation occurs prior to the final approach waypoint (FAWP), the
approach should not be completed since GPS no longer provides the required integrity. The receiver performs
a RAIM prediction by 2 NM prior to the FAWP to ensure that RAIM is available as a condition for entering the
approach mode. The pilot should ensure the receiver has sequenced from “Armed” to “Approach” prior to the
FAWP (normally occurs 2 NM prior). Failure to sequence may be an indication of the detection of a satellite
anomaly, failure to arm the receiver (if required), or other problems which preclude flying the approach.
(7) If the receiver does not sequence into the approach mode or a RAIM failure/status annunciation
occurs prior to the FAWP, the pilot must not initiate the approach nor descend, but instead, proceed to the missed
approach waypoint (MAWP) via the FAWP, perform a missed approach, and contact ATC as soon as practical.
The GPS receiver may continue to operate after a RAIM flag/status annunciation appears, but the navigation
information should be considered advisory only. Refer to the receiver operating manual for specific indications
and instructions associated with loss of RAIM prior to the FAF.
(8) If the RAIM flag/status annunciation appears after the FAWP, the pilot should initiate a climb and
execute the missed approach. The GPS receiver may continue to operate after a RAIM flag/status annunciation
appears, but the navigation information should be considered advisory only. Refer to the receiver operating
manual for operating mode information during a RAIM annunciation.
(h) Waypoints
(1) GPS receivers navigate from one defined point to another retrieved from the aircraft’s onboard
navigational database. These points are waypoints (5-letter pronounceable name), existing VHF intersections,
DME fixes with 5−letter pronounceable names and 3-letter NA V AID IDs. Each waypoint is a geographical
location defined by a latitude/longitude geographic coordinate. These 5−letter waypoints, VHF intersections,
5−letter pronounceable DME fixes and 3 −letter NA V AID IDs are published on various FAA aeronautical
navigation products (IFR Enroute Charts, VFR Charts, Terminal Procedures Publications, etc.).
(2) A Computer Navigation Fix (CNF) is also a point defined by a latitude/longitude coordinate and
is required to support Performance−Based Navigation (PBN) operations. The GPS receiver uses CNFs in
conjunction with waypoints to navigate from point to point. However, CNFs are not recognized by ATC. ATC
does not maintain CNFs in their database and they do not use CNFs for any air traffic control purpose. CNFs may
or may not be charted on FAA aeronautical navigation products, are listed in the chart legends, and are for
advisory purposes only. Pilots are not to use CNFs for point to point navigation (proceed direct), filing a flight
plan, or in aircraft/ATC communications. CNFs that do appear on aeronautical charts allow pilots increased
situational awareness by identifying points in the aircraft database route of flight with points on the aeronautical
chart. CNFs are random five-letter identifiers, not pronounceable like waypoints and placed in parenthesis.
Eventually, all CNFs will begin with the letters “CF” followed by three consonants (for example, CFWBG). This
five-letter identifier will be found next to an “x” on enroute charts and possibly on an approach chart. On
instrument approach procedures (charts) in the terminal procedures publication, CNFs may represent unnamed
DME fixes, beginning and ending points of DME arcs, and sensor (ground-based signal i.e., VOR, NDB, ILS)
final approach fixes on GPS overlay approaches. These CNFs provide the GPS with points on the procedure that
allow the overlay approach to mirror the ground-based sensor approach. These points should only be used by
Navigation Aids 1−1−31
AIM 2/20/253/15/077110.65R CHG 2AIM 1/22/26
the GPS system for navigation and should not be used by pilots for any other purpose on the approach. The CNF
concept has not been adopted or recognized by the International Civil Aviation Organization (ICAO).
(3) GPS approaches use fly−over and fly−by waypoints to join route segments on an approach. Fly−by
waypoints connect the two segments by allowing the aircraft to turn prior to the current waypoint in order to roll
out on course to the next waypoint. This is known as turn anticipation and is compensated for in the airspace and
terrain clearances. The missed approach waypoint (MAWP) will always be a fly −over waypoint. A holding
waypoint will always be designed as a fly−over waypoint in the navigational database but may be charted as a
fly−by event unless the holding waypoint is used for another purpose in the procedure and both events require
the waypoint to be a fly−over event. Some waypoints may have dual use; for example, as a fly−by waypoint when
used as an IF for a NoPT route and as a fly−over waypoint when the same waypoint is also used as an IAF/IF
hold−in−lieu of PT. Since the waypoint can only be charted one way, when this situation occurs, the fly −by
waypoint symbol will be charted in all uses of the waypoint.
(4) Unnamed waypoints for each airport will be uniquely identified in the database. Although the
identifier may be used at different airports (for example, RW36 will be the identifier at each airport with a runway
36), the actual point, at each airport, is defined by a specific latitude/longitude coordinate.
(5) The runway threshold waypoint, normally the MAWP, may have a five −letter identifier (for
example, SNEEZ) or be coded as RW## (for example, RW36, RW36L). MAWPs located at the runway threshold
are being changed to the RW## identifier, while MAWPs not located at the threshold will have a five −letter
identifier. This may cause the approach chart to differ from the aircraft database until all changes are complete.
The runway threshold waypoint is also used as the center of the Minimum Safe Altitude (MSA) on most GPS
approaches.
(i) Position Orientation. Pilots should pay particular attention to position orientation while using
GPS. Distance and track information are provided to the next active waypoint, not to a fixed navigation aid.
Receivers may sequence when the pilot is not flying along an active route, such as when being vectored or
deviating for weather, due to the proximity to another waypoint in the route. This can be prevented by placing
the receiver in the non-sequencing mode. When the receiver is in the non-sequencing mode, bearing and
distance are provided to the selected waypoint and the receiver will not sequence to the next waypoint in the
route until placed back in the auto sequence mode or the pilot selects a different waypoint. The pilot may have
to compute the ATD to stepdown fixes and other points on overlay approaches, due to the r eceiver showing
ATD to the next waypoint rather than DME to the VOR or ILS ground station.
(j) Impact of Magnetic Variation on PBN Systems
(1) Differences may exist between PBN systems and the charted magnetic courses on ground−based
NA V AID instrument flight procedures (IFP), enrout e charts, approach charts, and Standard Instrument
Departure/Standard Terminal Arrival (SID/STAR) charts. These differences are due to the magnetic variance
used to calculate the magnetic course. Every leg of an instrument procedure is first computed along a desired
ground track with reference to true north. A magnetic variation correction is then applied to the true course in
order to calculate a magnetic course for publication. The type of procedure will determine what magnetic
variation value is added to the true course. A ground−based NA V AID IFP applies the facility magnetic variation
of record to the true course to get the charted magnetic course. Magnetic courses on PBN procedures are
calculated two different ways. SID/STAR procedures use the airport magnetic variation of record, while IFR
enroute charts use magnetic reference bearing. PBN systems make a correction to true north by adding a magnetic
variation calculated with an algorithm based on aircraft position, or by adding the magnetic variation coded in
their navigational database. This may result in the PBN system and the procedure designer using a different
magnetic variation, which causes the magnetic course displayed by the PBN system and the magnetic course
charted on the IFP plate to be different. It is important to understand, however, that PBN systems, (with the
exception of VOR/DME RNA V equipment) navigate by reference to true north and display magnetic course only
for pilot reference. As such, a properly functioning PBN system, containing a current and accurate
navigational database , should fly the correct ground track for any loaded instrument procedure, despite
differences in displayed magnetic course that may be attributed to magnetic variation application. Should
1−1−32 Navigation Aids
AIM2/20/251/22/26 AIM
significant differences between the approach chart and the PBN system avionics’ application of the navigation
database arise, the published approach chart, supplemented by NOTAMs, holds precedence.
(2) The course into a waypoint may not always be 180 degrees different from the course leaving the
previous waypoint, due to the PBN system avionics’ computation of geodesic paths, distance between
waypoints, and differences in magnetic variation application. Variations in distances may also occur since PBN
system distance−to−waypoint values are ATDs computed to the next waypoint and the DME values published
on underlying procedures are slant −range distances measured to the station. This difference increases with
aircraft altitude and proximity to the NA V AID.
(k) GPS Familiarization
Pilots should practice GPS approaches in visual meteorological conditions (VMC) until thoroughly proficient
with all aspects of their equipment (receiver and installation) prior to attempting flight in instrument
meteorological conditions (IMC). Pilots should be proficient in the following areas:
(1) Using the receiver autonomous integrity monitoring (RAIM) prediction function;
(2) Inserting a DP into the flight plan, including setting terminal CDI sensitivity, if required, and the
conditions under which terminal RAIM is available for departure;
(3) Programming the destination airport;
(4) Programming and flying the approaches (especially procedure turns and arcs);
(5) Changing to another approach after selecting an approach;
(6) Programming and flying “direct” missed approaches;
(7) Programming and flying “routed” missed approaches;
(8) Entering, flying, and exiting holding patterns, particularly on approaches with a second waypoint
in the holding pattern;
(9) Programming and flying a “route” from a holding pattern;
(10) Programming and flying an approach with radar vectors to the intermediate segment;
(11) Indication of the actions required for RAIM failure both before and after the FAWP; and
(12) Programming a radial and distance from a VOR (often used in departure instructions).
Navigation Aids 1−1−33
AIM 2/20/25
TBL 1−1−5
GPS IFR Equipment Classes/Categories
TSO−C129
Equipment
Class RAIM
Int. Nav. Sys. to
Prov. RAIM
Equiv.
Oceanic En Route Terminal
Non−precision
Approach
Capable
Class A − GPS sensor and navigation capability.
A1 yes yes yes yes yes
A2 yes yes yes yes no
Class B − GPS sensor data to an integrated navigation system (i.e., FMS, multi−sensor navigation system, etc.).
B1 yes yes yes yes yes
B2 yes yes yes yes no
B3 yes yes yes yes yes
B4 yes yes yes yes no
Class C − GPS sensor data to an integrated navigation system (as in Class B) which provides enhanced guidance to an autopilot, or
flight director, to reduce flight tech. errors. Limited to 14 CFR part 121 or equivalent criteria.
C1 yes yes yes yes yes
C2 yes yes yes yes no
C3 yes yes yes yes yes
C4 yes yes yes yes no
TBL 1−1−6
GPS Approval Required/Authorized Use
Equipment
Type1
Installation
Approval
Required
Operational
Approval
Required
IFR
En Route2
IFR
Terminal2
IFR
Approach3
Oceanic
Remote
In Lieu of
ADF and/or
DME3
Hand held4 X5
VFR Panel Mount4 X
IFR En Route
and Terminal
X X X X X
IFR Oceanic/
Remote
X X X X X X
IFR En Route,
Terminal, and
Approach
X X X X X X
NOTE−
1To determine equipment approvals and limitations, refer to the AFM, AFM supplements, or pilot guides.
2Requires verification of data for correctness if database is expired.
3Requires current database or verification that the procedure has not been amended since the expiration of the database.
4VFR and hand−held GPS systems are not authorized for IFR navigation, instrument approaches, or as a primary instrument
flight reference. During IFR operations they may be considered only an aid to situational awareness.
5Hand−held receivers require no approval. However, any aircraft modification to support the hand −held receiver;
i.e., installation of an external antenna or a permanent mounting bracket, does require approval.
1−1−18. Wide Area Augmentation System (WAAS)
a. General
1. The FAA developed the WAAS to improve the accuracy, integrity and availability of GPS signals. WAAS
will allow GPS to be used, as the aviation navigation system, from takeoff through approach when it is complete.
WAAS is a critical component of the FAA’s strategic objective for a seamless satellite navigation system for civil
aviation, improving capacity and safety.
1−1−34 Navigation Aids
2/20/25 AIM
2. The International Civil Aviation Organization (ICAO) has defined Standards and Recommended
Practices (SARPs) for satellite−based augmentation systems (SBAS) such as WAAS. India and Europe are
building similar systems: EGNOS, the European Geostationary Navigation Overlay System; and India’s GPS
and Geo−Augmented Navigation (GAGAN) system. The merging of these systems will create an expansive
navigation capability similar to GPS, but with greater accuracy, availability, and integrity.
3. Unlike traditional ground−based navigation aids, WAAS will cover a more extensive service area.
Precisely surveyed wide−area reference stations (WRS) are linked to form the U.S. WAAS network. Signals from
the GPS satellites are monitored by these WRSs to determine satellite clock and ephemeris corrections and to
model the propagation effects of the ionosphere. Each station in the network relays the data to a wide−area master
station (WMS) where the correction information is computed. A correction message is prepared and uplinked
to a geostationary earth orbit satellite (GEO) via a GEO uplink subsystem (GUS) which is located at the ground
earth station (GES). The message is then broadcast on the same frequency as GPS (L1, 1575.42 MHz) to WAAS
receivers within the broadcast coverage area of the WAAS GEO.
4. In addition to providing the correction signal, the WAAS GEO provides an additional pseudorange
measurement to the aircraft receiver, improving the availability of GPS by providing, in effect, an additional GPS
satellite in view. The integrity of GPS is improved through real−time monitoring, and the accuracy is improved
by providing differential corrections to reduce errors. The performance improvement is sufficient to enable
approach procedures with GPS/WAAS glide paths (vertical guidance).
5. The FAA has completed installation of 3 GEO satellite links, 38 WRSs, 3 WMSs, 6 GES, and the required
terrestrial communications to support the WAAS network including 2 operational control centers. Prior to the
commissioning of the WAAS for public use, the FAA conducted a series of test and validation activities. Future
dual frequency operations are planned.
6. GNSS navigation, including GPS and WAAS, is referenced to the WGS−84 coordinate system. It should
only be used where the Aeronautical Information Publications (including electronic data and aeronautical charts)
conform to WGS−84 or equivalent. Other countries’ civil aviation authorities may impose additional limitations
on the use of their SBAS systems.
b. Instrument Approach Capabilities
1. A class of approach procedures which provide vertical guidance, but which do not meet the ICAO Annex
10 requirements for precision approaches has been developed to support satellite navigation use for aviation
applications worldwide. These procedures are not precision and are referred to as Approach with Vertical
Guidance (APV), are defined in ICAO Annex 6, and include approaches such as the LNAV/VNA V and localizer
performance with vertical guidance (LPV). These approaches provide vertical guidance, but do not meet the
more stringent standards of a precision approach. Properly certified WAAS receivers will be able to fly to LPV
minima and LNA V/VNA V minima, using a WAAS electronic glide path, which eliminates the errors that can
be introduced by using Barometric altimetry.
2. LPV minima takes advantage of the high accuracy guidance and increased integrity provided by WAAS.
This WAAS generated angular guidance allows the use of the same TERPS approach criteria used for ILS
approaches. LPV minima may have a decision altitude as low as 200 feet height above touchdown with visibility
minimums as low as 1/2 mile, when the terrain and airport infrastructure support the lowest minima. LPV minima
is published on the RNA V (GPS) approach charts (see paragraph 5 −4−5, Instrument Approach Procedure
Charts).
3. A different WAAS-based line of minima, called Localizer Performance (LP) is being added in locations
where the terrain or obstructions do not allow publication of vertically guided LPV minima. LP takes advantage
of the angular lateral guidance and smaller position errors provided by WAAS to provide a lateral only procedure
similar to an ILS Localizer. LP procedures may provide lower minima than a LNA V procedure due to the
narrower obstacle clearance surface.
NOTE−
WAAS receivers certified prior to TSO −C145b and TSO−C146b, even if they have LPV capability, do not contain LP
Navigation Aids 1−1−35
AIM 2/20/253/15/077110.65R CHG 2AIM 8/7/25
capability unless the receiver has been upgraded. Receivers capable of flying LP procedures must contain a statement in
the Aircraft Flight Manual (AFM), AFM Supplement, or Approved Supplemental Flight Manual stating that the receiver has
LP capability, as well as the capability for the other WAAS and GPS approach procedure types.
4. WAAS provides a level of service that supports all phases of flight, including RNA V (GPS) approaches
to LNA V , LP , LNA V/VNA V , and LPV lines of minima, within system coverage. Some locations close to the edge
of the coverage may have a lower availability of vertical guidance.
c. General Requirements
1. WAAS avionics must be certified in accordance with Technical Standard Order (TSO) TSO −C145(),
Airborne Navigation Sensors Using the (GPS) Augmented by the Wide Area Augmentation System (WAAS);
or TSO−C146(), Stand−Alone Airborne Navigation Equipment Using the Global Positioning System (GPS)
Augmented by the Wide Area Augmentation System (WAAS), and installed in accordance with AC 20−138,
Airworthiness Approval of Positioning and Navigation Systems.
2. GPS/WAAS operation must be conducted in accordance with the FAA−approved aircraft flight manual
(AFM) and flight manual supplements. Flight manual supplements will state the level of approach procedure that
the receiver supports. IFR approved WAAS receivers support all GPS only operations as long as lateral capability
at the appropriate level is functional. WAAS monitors both GPS and WAAS satellites and provides integrity.
3. GPS/WAAS equipment is inherently capable of supporting oceanic and remote operations if the operator
obtains a fault detection and exclusion (FDE) prediction program.
4. Air carrier and commercial operators must meet the appropriate provisions of their approved operations
specifications.
5. Prior to GPS/WAAS IFR operation, the pilot must review appropriate Notices to Airmen (NOTAMs) and
aeronautical information. This information is available on request from a Flight Service Station. The FAA will
provide NOTAMs to advise pilots of the status of the WAAS and level of service available.
(a) The term MAY NOT BE A VBL is used in conjunction with WAAS NOTAMs and indicates that due
to ionospheric conditions, lateral guidance may still be available when vertical guidance is unavailable. Under
certain conditions, both lateral and vertical guidance may be unavailable. This NOTAM language is an advisory
to pilots indicating the expected level of WAAS service (LNA V/VNA V , LPV , LP) may not be available.
EXAMPLE−
!FDC FDC NAV WAAS VNAV/LPV/LP MINIMA MAY NOT BE AVBL 1306111330-1306141930EST
or
!FDC FDC NAV WAAS VNAV/LPV MINIMA NOT AVBL, WAAS LP MINIMA MAY NOT BE AVBL
1306021200-1306031200EST
WAAS MAY NOT BE A VBL NOTAMs are predictive in nature and published for flight planning purposes.
Upon commencing an approach at locations NOTAMed WAAS MAY NOT BE A VBL, if the WAAS avionics
indicate LNA V/VNA V or LPV service is available, then vertical guidance may be used to complete the approach
using the displayed level of service. Should an outage occur during the approach, reversion to LNA V minima
or an alternate instrument approach procedure may be required. When GPS testing NOTAMS are published and
testing is actually occurring, Air Traffic Control will advise pilots requesting or cleared for a GPS or RNA V
(GPS) approach that GPS may not be available and request intentions. If pilots have reported GPS anomalies,
Air Traffic Control will request the pilot’s intentions and/or clear the pilot for an alternate approach, if available
and operational.
(b) WAAS area-wide NOTAMs are originated when WAAS assets are out of service and impact the
service area. Area−wide WAAS NOT A V AILABLE (A VBL) NOTAMs indicate loss or malfunction of the
WAAS system. In flight, Air Traffic Control will advise pilots requesting a GPS or RNA V (GPS) approach of
WAAS NOT A VBL NOTAMs if not contained in the ATIS broadcast.
EXAMPLE−
For unscheduled loss of signal or service, an example NOTAM is: !FDC FDC NAV WAAS NOT AVBL 1311 160600−
1−1−36 Navigation Aids
2/20/25 AIM
1311191200EST.
For scheduled loss of signal or service, an example NOTAM is : !FDC FDC NAV WAAS NOT AVBL 1312041015-
1312082000EST.
(c) Site−specific WAAS MAY NOT BE A VBL NOTAMs indicate an expected level of service; for
example, LNA V/VNA V , LP, or LPV may not be available. Pilots must request site−specific WAAS NOTAMs
during flight planning. In flight, Air Traffic Control will not advise pilots of WAAS MAY NOT BE A VBL
NOTAMs.
NOTE−
Though currently unavailable, the F AA is updating its prediction tool software to provide this site-service in the future.
(d) Most of North America has redundant coverage by two or more geostationary satellites. One
exception is the northern slope of Alaska. If there is a problem with the satellite providing coverage to this area,
a NOTAM similar to the following example will be issued:
EXAMPLE−
!FDC 4/3406 (P AZA A0173/14) ZAN NAV WAAS SIGNAL MAY NOT BE AVBL NORTH OF LINE FROM 7000N150000W
TO 6400N16400W. RMK WAAS USERS SHOULD CONFIRM RAIM AVAILABILITY FOR IFR OPERATIONS IN THIS
AREA. T-ROUTES IN THIS SECTOR NOT AVBL. ANY REQUIRED ALTERNATE AIRPORT IN THIS AREA MUST HAVE
AN APPROVED INSTRUMENT APPROACH PROCEDURE OT HER THAN GPS THAT IS ANTICIP ATED TO BE
OPERATIONAL AND AVAILABLE AT THE ESTIMATED TIME OF ARRIVAL AND WHICH THE AIRCRAFT IS
EQUIPPED TO FLY. 1406030812-1406050812EST .
6. When GPS−testing NOTAMS are published and testing is actually occurring, Air Traffic Control will
advise pilots requesting or cleared for a GPS or RNA V (GPS) approach that GPS may not be available and request
intentions. If pilots have reported GPS anomalies, Air Traffic Control will request the pilot’s intentions and/or
clear the pilot for an alternate approach, if available and operational.
EXAMPLE−
Here is an example of a GPS testing NOTAM:
!GPS 06/001 ZAB NAV GPS (INCLUDING WAAS, GBAS, AND ADS-B) MAY NOT BE AVAILABLE WITHIN A 468NM
RADIUS CENTERED AT 330702N1062540W (TCS 093044) FL400-UNL DECREASING IN AREA WITH A DECREASE
IN ALTITUDE DEFINED AS: 425NM RADIUS AT FL250, 360NM RADIUS AT 10000FT, 354NM RADIUS AT 4000FT
AGL, 327NM RADIUS AT 50FT AGL. 1406070300-1406071200.
7. When the approach chart is annotated with the
symbol, site−specific WAAS MAY NOT BE A VBL
NOTAMs or Air Traffic advisories are not provided for outages in WAAS LNA V/VNA V and LPV vertical
service. Vertical outages may occur daily at these locations due to being close to the edge of WAAS system
coverage. Use LNA V or circling minima for flight planning at these locations, whether as a destination or
alternate. For flight operations at these locations, when the WAAS avionics indicate that LNAV/VNA V or LPV
service is available, then the vertical guidance may be used to complete the approach using the displayed level
of service. Should an outage occur during the procedure, reversion to LNA V minima may be required.
NOTE−
Area−wide WAAS NOT AVBL NOTAMs apply to all airports in the WAAS NOT AVBL area designated in the NOTAM,
including approaches at airports where an approach chart is annotated with the
symbol.
8. GPS/WAAS was developed to be used within GEO coverage over North America without the need for
other radio navigation equipment appropriate to the route of flight to be flown. Outside the WAAS coverage or
in the event of a WAAS failure, GPS/WAAS equipment reverts to GPS −only operation and satisfies the
requirements for basic GPS equipment. (See paragraph 1−1−17 for these requirements).
9. Unlike TSO−C129 avionics, which were certified as a supplement to other means of navigation, WAAS
avionics are evaluated without reliance on other navigation systems. As such, installation of WAAS avionics
does not require the aircraft to have other equipment appropriate to the route to be flown. (See paragraph
1−1−17 d for more information on equipment requirements.)
(a) Pilots with W AAS receivers may flight plan to use any instrument approach procedure authorized for
use with their WAAS avionics as the planned approach at a required alternate, with the following restrictions.
Navigation Aids 1−1−37
AIM 2/20/25
When using WAAS at an alternate airport, flight planning must be based on flying the RNA V (GPS) LNA V or
circling minima line, or minima on a GPS approach procedure, or conventional approach procedure with “or
GPS” in the title. Code of Federal Regulation (CFR) part 91 non−precision weather requirements must be used
for planning. Upon arrival at an alternate, when the WAAS navigation system indicates that LNA V/VNA V or
LPV service is available, then vertical guidance may be used to complete the approach using the displayed level
of service. The FAA has begun removing the
NA (Alternate Minimums Not Authorized) symbol from select
RNA V (GPS) and GPS approach procedures so they may be used by approach approved WAAS receivers at
alternate airports. Some approach procedures will still require the
NA for other reasons, such as no weather
reporting, so it cannot be removed from all procedures. Since every procedure must be individually evaluated,
removal of the
NA from RNA V (GPS) and GPS procedures will take some time.
NOTE−
Properly trained and approved, as required, TSO-C145() and TSO-C146() equipped users (WAAS users) with and using
approved baro-VNAV equipment may plan for LNAV/VNAV DA at an alternate airport. Specifically authorized WAAS users
with and using approved baro-VNAV equipment may also plan for RNP 0.3 DA at the alternate airport as long as the pilot
has verified RNP availability through an approved prediction program.
d. Flying Procedures with WAAS
1. WAAS receivers support all basic GPS approach functions and provide additional capabilities. One of
the major improvements is the ability to generate glide path guidance, independent of ground equipment or
barometric aiding. This eliminates several problems such as hot and cold temperature effects, incorrect altimeter
setting, or lack of a local altimeter source. It also allows approach procedures to be built without the cost of
installing ground stations at each airport or runway. Some approach certified receivers may only generate a glide
path with performance similar to Baro−VNA V and are only approved to fly the LNA V/VNA V line of minima
on the RNA V (GPS) approach charts. Receivers with additional capability (including faster update rates and
smaller integrity limits) are approved to fly the LPV line of minima. The lateral integrity changes dramatically
from the 0.3 NM (556 meter) limit for GPS, LNA V , and LNA V/VNA V approach mode, to 40 meters for LPV .
It also provides vertical integrity monitoring, which bounds the vertical error to 50 meters for LNAV/VNA V and
LPVs with minima of 250’ or above, and bounds the vertical error to 35 meters for LPVs with minima below
250’.
2. When an approach procedure is selected and active, the receiver will notify the pilot of the most accurate
level of service supported by the combination of the W AAS signal, the receiver, and the selected approach, using
the naming conventions on the minima lines of the selected approach procedure. For example, if an approach
is published with LPV minima and the receiver is only certified for LNAV/VNA V , the equipment would indicate
“LNA V/VNA V available,” even though the WAAS signal would support LPV . If flying an existing
LNA V/VNA V procedure with no LPV minima, the receiver will notify the pilot “LNAV/VNA V available,” even
if the receiver is certified for LPV and the signal supports LPV . If the signal does not support vertical guidance
on procedures with LPV and/or LNA V/VNA V minima, the receiver annunciation will read “LNA V available.”
On lateral only procedures with LP and LNA V minima the receiver will indicate “LP available” or “LNA V
available” based on the level of lateral service available. Once the level of service notification has been given,
the receiver will operate in this mode for the duration of the approach procedure, unless that level of service
becomes unavailable. The receiver cannot change back to a more accurate level of service until the next time an
approach is activated.
NOTE−
Receivers do not “fail down” to lower levels of service once the approach has been activated. If only the vertical off flag
appears, the pilot may elect to use the LNAV minima if the rules under which the flight is operating allow changing the type
of approach being flown after commencing the procedure. If the lateral integrity limit is exceeded on an LP approach, a
missed approach will be necessary since there is no way to reset the lateral alarm limit while the approach is active.
3. Another additional feature of WAAS receivers is the ability to exclude a bad GPS signal and continue
operating normally. This is normally accomplished by the WAAS correction information. Outside WAAS
coverage or when WAAS is not available, it is accomplished through a receiver algorithm called FDE. In most
1−1−38 Navigation Aids
2/20/25 AIM
cases this operation will be invisible to the pilot since the receiver will continue to operate with other available
satellites after excluding the “bad” signal. This capability increases the reliability of navigation.
4. Both lateral and vertical scaling for the LNA V/VNA V and LPV approach procedures are different than
the linear scaling of basic GPS. When the complete published procedure is flown, ±1 NM linear scaling is
provided until two (2) NM prior to the FAF, where the sensitivity increases to be similar to the angular scaling
of an ILS. There are two differences in the WAAS scaling and ILS: 1) on long final approach segments, the initial
scaling will be ±0.3 NM to achieve equivalent performance to GPS (and better than ILS, which is less sensitive
far from the runway); 2) close to the runway threshold, the scaling changes to linear instead of continuing to
become more sensitive. The width of the final approach course is tailored so that the total width is usually 700
feet at the runway threshold. Since the origin point of the lateral splay for the angular portion of the final is not
fixed due to antenna placement like localizer, the splay angle can remain fixed, making a consistent width of final
for aircraft being vectored onto the final approach course on different length runways. When the complete
published procedure is not flown, and instead the aircraft needs to capture the extended final approach course
similar to ILS, the vector to final (VTF) mode is used. Under VTF, the scaling is linear at ±1 NM until the point
where the ILS angular splay reaches a width of ±1 NM regardless of the distance from the FAWP.
5. The W AAS scaling is also different than GPS TSO−C129() in the initial portion of the missed approach.
Two differences occur here. First, the scaling abruptly changes from the approach scaling to the missed approach
scaling, at approximately the departure end of the runway or when the pilot selects missed approach guidance
rather than ramping as GPS does. Second, when the first leg of the missed approach is a Track to Fix (TF) leg
aligned within 3 degrees of the inbound course, the receiver will change to 0.3 NM linear sensitivity until the
turn initiation point for the first waypoint in the missed approach procedure, at which time it will abruptly change
to terminal (±1 NM) sensitivity. This allows the elimination of close in obstacles in the early part of the missed
approach that may otherwise cause the DA to be raised.
6. There are two ways to select the final approach segment of an instrument approach. Most receivers use
menus where the pilot selects the airport, the runway, the specific approach procedure and finally the IAF, there
is also a channel number selection method. The pilot enters a unique 5−digit number provided on the approach
chart, and the receiver reca lls the matching final approach segment from the aircraft database. A list of
information including the available IAFs is displayed and the pilot selects the appropriate IAF. The pilot should
confirm that the correct final approach segment was loaded by cross checking the Approach ID, which is also
provided on the approach chart.
7. The Along−Track Distance (ATD) during the final approach segment of an LNA V procedure (with a
minimum descent altitude) will be to the MAWP. On LNA V/VNA V and LPV approaches to a decision altitude,
there is no missed approach waypoint so the along−track distance is displayed to a point normally located at the
runway threshold. In most cases, the MAWP for the LNA V approach is located on the runway threshold at the
centerline, so these distances will be the same. This distance will always vary slightly from any ILS DME that
may be present, since the ILS DME is located further down the runway. Initiation of the missed approach on the
LNA V/VNA V and LPV approaches is still based on reaching the decision altitude without any of the items listed
in 14 CFR section 91.175 being visible, and must not be delayed while waiting for the ATD to reach zero. The
WAAS receiver, unlike a GPS receiver, will automatically sequence past the MAWP if the missed approach
procedure has been designed for RNA V. The pilot may also select missed approach prior to the MAWP; however,
navigation will continue to the MAWP prior to waypoint sequencing taking place.
1−1−19. Ground Based Augmentation System (GBAS) Landing System (GLS)
a. A GBAS ground installation at an airport can provide localized, differential augmentation to the Global
Positioning System (GPS) signal−in−space enabling an aircraft’s GLS precision approach capability. Through
the GBAS service and the aircraft’s GLS installation a pilot may complete an instrument approach offering
three−dimensional angular, lateral, and vertical guidance for exact alignment and descent to a runway. The
operational benefits of a GLS approach are similar to the benefits of an ILS or LPV approach operation.
Navigation Aids 1−1−39
AIM 2/20/25
NOTE−
To remain consistent with international terminology, the F AA will use the term GBAS in place of the former term Local Area
Augmentation System (LAAS).
b. An aircraft’s GLS approach capability relies on the broadcast from a GBAS Ground Facility (GGF)
installation. The GGF installation includes at least four ground reference stations near the airport’s runway(s),
a corrections processor, and a VHF Data Broadcast (VDB) uplink antenna. To use the GBAS GGF output and
be eligible to conduct a GLS approach, the aircraft requires eligibility to conduct RNP approach (RNP APCH)
operations and must meet the additional, specific airworthiness requirements for installation of a GBAS receiver
intended to support GLS approach operations. When the aircraft achieves GLS approach eligibility, the aircraft’s
onboard navigation database may then contain published GLS instrument approach procedures.
c. During a GLS instrument approach procedure, the installation of an aircraft’s GLS capability provides the
pilot three−dimensional (3D) lateral and vertical navigation guidance much like an ILS instrument approach.
GBAS corrections augment the GPS signal−in−space by offering position corrections, ensures the availability
of enhanced integrity parameters, and then transmits the actual approach path definition over the VDB uplink
antenna. A single GBAS ground station can support multiple GLS approaches to one or more runways.
d. Through the GBAS ground station, a GLS approach offers a unique operational service volume distinct
from the traditional ILS approach service volume (see FIG 1−1−9). However, despite the unique service volume,
in the final approach segment, a GLS approach provides precise 3D angular lateral and vertical guidance
mimicking the precision guidance of an ILS approach.
e. Transitions to and segments of the published GLS instrument approach procedures may rely on use of
RNA V 1 or RNP 1 prior to an IAF. Then, during the approach procedure, prior to the aircraft entering the GLS
approach mode, a GLS approach procedure design uses the RNP APCH procedure design criteria to construct
the procedural path (the criteria used to publish procedures titled “RNA V (GPS)” in the US). Thus, a GLS
approach procedure may include paths requiring turns after the aircraft crosses the IAF, prior to the aircraft’s
flight guidance entering the GLS approach flight guidance mode. Likewise, the missed approach procedure for
a GLS approach procedure relies exclusively on the same missed approach criteria supporting an RNP APCH.
f. When maneuvering the aircraft in compliance with an ATC clearance to intercept a GLS approach prior to
the final approach segment (e.g. “being vectored”), the pilot should adhere to the clearance and ensure the aircraft
intercepts the extended GLS final approach course within the specified service volume. Once on the GLS final
approach course, the pilot should ensure the aircraft is in the GLS approach mode pr ior to reaching the
procedure’s glidepath intercept point. Once the aircraft is in the GLS flight guidance mode and captures the GLS
glidepath, the pilot should fly the GLS final approach segment using the same pilot techniques they use to fly
an ILS final approach or the final approach of an RNA V (GPS) approach flown to LPV minimums. See also the
Instrument Procedures Handbook for more information on how to conduct a GLS instrument approach
procedure.
1−1−40 Navigation Aids
AIM2/20/251/22/26 AIM
FIG 1−1−9
GLS Standard Approach Service Volume
1−1−20. Precision Approach Systems other than ILS and GLS
a. General
Approval and use of precision approach systems other than ILS and GLS require the issuance of special
instrument approach procedures.
b. Special Instrument Approach Procedure
1. Special instrument approach procedures must be issued to the aircraft operator if pilot training, aircraft
equipment, and/or aircraft performance is different than published procedures. Special instrument approach
procedures are not distributed for general public use. These procedures are issued to an aircraft operator when
the conditions for operations approval are satisfied.
2. General aviation operators requesting approval for special procedures should contact the local Flight
Standards District Office to obtain a letter of authorization. Air carrier operators requesting approval for use of
special procedures should contact their Certificate Holding District Office for authorization through their
Operations Specification.
REFERENCE−
AIM, Para 5−4−7, Instrument Approach Procedures, Subpara i.
Navigation Aids 1−1−41
2/20/25 AIM
Section 2. Performance −Based Navigation (PBN) and
Area Navigation (RNAV)
1−2−1. General
a. Introduction to PBN. As air travel has evolved, methods of navigation have improved to give operators
more flexibility. PBN exists under the umbrella of area navigation (RNA V). The term RNA V in this context, as
in procedure titles, just means “area navigation,” regardless of the equipment capability of the aircraft. (See
FIG 1−2−1.) Many operators have upgraded their systems to obtain the benefits of PBN. Within PBN there are
two main categories of navigation methods or specifications: area navigation (RNA V) and required navigation
performance (RNP). In this context, the term RNA V x means a specific navigation specification with a specified
lateral accuracy value. For an aircraft to meet the requirements of PBN, a specified RNA V or RNP accuracy must
be met 95 percent of the flight time. RNP is a PBN system that includes onboard performance monitoring and
alerting capability (for example, Receiver Autonomous Integrity Monitoring (RAIM)). PBN also introduces the
concept of navigation specifications (NavSpecs) which are a set of aircraft and aircrew requirements needed to
support a navigation application within a defined airspace concept. For both RNP and RNA V NavSpecs, the
numerical designation refers to the lateral navigation accuracy in nautical miles which is expected to be achieved
at least 95 percent of the flight time by the population of aircraft operating within the airspace, route, or
procedure. This information is detailed in International Civil Aviation Organization’s (ICAO) Doc 9613,
Performance−based Navigation (PBN) Manual and the latest FAA AC 90−105, Approval Guidance for RNP
Operations and Barometric Vertical Navigation in the U.S. National Airspace System and in Remote and Oceanic
Airspace.
FIG 1−2−1
Navigation Specifications
b. Area Navigation (RNA V)
1. General. RNAV is a method of navigation that permits aircraft operation on any desired flight path within
the coverage of ground− or space−based navigation aids or within the limits of the capability of self−contained
aids, or a combination of these. In the future, there will be an increased dependence on the use of RNA V in lieu
of routes defined by ground−based navigation aids. RNA V routes and terminal procedures, including departure
procedures (DPs) and standard terminal arrivals (STARs), are designed with RNA V systems in mind. There are
several potential advantages of RNA V routes and procedures:
Performance−Based Navigation (PBN) and Area Navigation (RNA V) 1−2−1
AIM 2/20/25
(a) Time and fuel savings;
(b) Reduced dependence on radar vectoring, altitude, and speed assignments allowing a reduction in
required ATC radio transmissions; and
(c) More efficient use of airspace.
In addition to information found in this manual, guidance for domestic RNA V DPs, STARs, and routes may also
be found in AC 90−100, U.S. Terminal and En Route Area Navigation (RNA V) Operations.
2. RNA V Operations. RNA V procedures, such as DPs and STARs, demand strict pilot awareness and
maintenance of the procedure centerline. Pilots should possess a working knowledge of their aircraft navigation
system to ensure RNA V procedures are flown in an appropriate manner. In addition, pilots should have an
understanding of the various waypoint and leg types used in RNAV procedures; these are discussed in more detail
below.
(a) Waypoints. A waypoint is a predetermined geographical position that is defined in terms of
latitude/longitude coordinates. Waypoints may be a simple named point in space or associated with existing
navaids, intersections, or fixes. A waypoint is most often used to indicate a change in direction, speed, or altitude
along the desired path. RNA V procedures make use of both fly−over and fly−by waypoints.
(1) Fly−by waypoints. Fly−by waypoints are used when an aircraft should begin a turn to the next
course prior to reaching the waypoint separating the two route segments. This is known as turn anticipation.
(2) Fly−over waypoints. Fly−over waypoints are used when the aircraft must fly over the point prior
to starting a turn.
NOTE−
FIG 1−2−2 illustrates several differences between a fly−by and a fly−over waypoint.
FIG 1−2−2
Fly−by and Fly−over Waypoints
(b) RNA V Leg Types. A leg type describes the desired path proceeding, following, or between
waypoints on an RNA V procedure. Leg types are identified by a two−letter code that describes the path (e.g.,
heading, course, track, etc.) and the termination point (e.g., the path terminates at an altitude, distance, fix, etc.).
Leg types used for procedure design are included in the aircraft navigation database, but not normally provided
on the procedure chart. The narrative depiction of the RNAV chart describes how a procedure is flown. The “path
and terminator concept” defines that every leg of a procedure has a termination point and some kind of path into
that termination point. Some of the available leg types are described below.
1−2−2 Performance−Based Navigation (PBN) and Area Navigation (RNA V)
2/20/25 AIM
(1) Track to Fix. A Track to Fix (TF) leg is intercepted and acquired as the flight track to the following
waypoint. Track to a Fix legs are sometimes called point −to−point legs for this reason. Narrative: “direct
ALPHA, then on course to BRAVO WP .” See FIG 1−2−3.
(2) Direct to Fix. A Direct to Fix (DF) leg is a path described by an aircraft’s track from an initial area
direct to the next waypoint. Narrative: “turn right direct BRAVO WP .” See FIG 1−2−4.
FIG 1−2−3
Track to Fix Leg Type
FIG 1−2−4
Direct to Fix Leg Type
(3) Course to Fix. A Course to Fix (CF) leg is a path that terminates at a fix with a specified course
at that fix. Narrative: “on course 150 to ALPHA WP .” See FIG 1−2−5.
Performance−Based Navigation (PBN) and Area Navigation (RNA V) 1−2−3
AIM 2/20/25
FIG 1−2−5
Course to Fix Leg Type
(4) Radius to Fix. A Radius to Fix (RF) leg is defined as a constant radius circular path around a
defined turn center that terminates at a fix. See FIG 1−2−6.
FIG 1−2−6
Radius to Fix Leg Type
(5) Heading. A Heading leg may be defined as, but not limited to, a Heading to Altitude (V A),
Heading to DME range (VD), and Heading to Manual Termination, i.e., Vector (VM). Narrative: “climb
heading 350 to 1500”, “heading 265, at 9 DME west of PXR VORTAC, right turn heading 360”, “fly heading
090, expect radar vectors to DRYHT INT.”
(c) Navigation Issues. Pilots should be aware of their navigation system inputs, alerts, and
annunciations in order to make better −informed decisions. In addition, the availability and suitability of
particular sensors/systems should be considered.
(1) GPS/WAAS. Operators using TSO−C129(), TSO−C196(), TSO−C145() or TSO−C146() systems
should ensure departure and arrival airports are entered to ensure proper RAIM availability and CDI sensitivity.
(2) DME/DME. Operators should be aware that DME/DME position updating is dependent on
navigation system logic and DME facility proximity, availability, geometry, and signal masking.
1−2−4 Performance−Based Navigation (PBN) and Area Navigation (RNA V)
2/20/25 AIM
(3) VOR/DME. Unique VOR characteristics may result in less accurate values from VOR/DME
position updating than from GPS or DME/DME position updating.
(4) Inertial Navigation. Inertial reference units and inertial navigation systems are often coupled
with other types of navigation inputs, e.g., DME/D ME or GPS, to improve overall navigation system
performance.
NOTE−
Specific inertial position updating requirements may apply.
(d) Flight Management System (FMS). An FMS is an integrated suite of sensors, receivers, and
computers, coupled with a navigation database. These systems generally provide performance and RNA V
guidance to displays and automatic flight control systems.
Inputs can be accepted from multiple sources such as GPS, DME, VOR, LOC and IRU. These inputs may be
applied to a navigation solution one at a time or in combination. Some FMSs provide for the detection and
isolation of faulty navigation information.
When appropriate navigation signals are available, FMSs will normally rely on GPS and/or DME/DME (that
is, the use of distance information from two or more DME stations) for position updates. Other inputs may also
be incorporated based on FMS system architecture and navigation source geometry.
NOTE−
DME/DME inputs coupled with one or more IRU(s) are often abbreviated as DME/DME/IRU or D/D/I.
(e) RNA V Navigation Specifications (Nav Specs)
Nav Specs are a set of aircraft and aircrew requirements needed to support a navigation application within a
defined airspace concept. For both RNP and RNA V designations, the numerical designation refers to the lateral
navigation accuracy in nautical miles which is expected to be achieved at least 95 percent of the flight time by
the population of aircraft operating within the airspace, route, or procedure. (See FIG 1−2−1.)
(1) RNA V 1. Typically RNA V 1 is used for DPs and STARs and appears on the charts. Aircraft must
maintain a total system error of not more than 1 NM for 95 percent of the total flight time.
(2) RNA V 2. Typically RNA V 2 is used for en route operations unless otherwise specified. T-routes
and Q-routes are examples of this Nav Spec. Aircraft must maintain a total system error of not more than 2 NM
for 95 percent of the total flight time.
(3) RNA V 10. Typically RNA V 10 is used in oceanic operations. See paragraph 4−7−1 for specifics
and explanation of the relationship between RNP 10 and RNA V 10 terminology.
1−2−2. Required Navigation Performance (RNP)
a. General. While both RNA V navigation specifications (NavSpecs) and RNP NavSpecs contain specific
performance requirements, RNP is RNA V with the added requirement for onboard performance monitoring and
alerting (OBPMA). RNP is also a statement of navigation performance necessary for operation within a defined
airspace. A critical component of RNP is the ability of the aircraft navigation system to monitor its achieved
navigation performance, and to identify for the pilot whether the operational requirement is, or is not, being met
during an operation. OBPMA capability therefore allows a lessened reliance on air traffic control intervention
and/or procedural separation to achieve the overall safety of the operation. RNP capability of the aircraft is a
major component in determining the separation criteria to ensure that the overall containment of the operation
is met. The RNP capability of an aircraft will vary depending upon the aircraft equipment and the navigation
infrastructure. For example, an aircraft may be eligible for RNP 1, but may not be capable of RNP 1 operations
due to limited NA V AID coverage or avionics failure. The Aircraft Flight Manual (AFM) or avionics documents
for your aircraft should specifically state the aircraft’s RNP eligibilities. Contact the manufacturer of the avionics
or the aircraft if this information is missing or incomplete. NavSpecs should be considered different from one
another, not “better” or “worse” based on the described lateral navigation accuracy. It is this concept that requires
Performance−Based Navigation (PBN) and Area Navigation (RNA V) 1−2−5
AIM 2/20/25
each NavSpec eligbility to be listed separately in the avionics documents or AFM. For example, RNP 1 is
different from RNA V 1, and an RNP 1 eligibility does NOT mean automatic RNP 2 or RNA V 1 eligibility. As
a safeguard, the FAA requires that aircraft navigation databases hold only those procedures that the aircraft
maintains eligibility for. If you look for a specific instrument procedure in your aircraft’s navigation database
and cannot find it, it’s likely that procedure contains PBN elements your aircraft is ineligible for or cannot
compute and fly. Further, optional capabilities such as Radius −to−fix (RF) turns or scalability should be
described in the AFM or avionics documents. Use the capabilities of your avionics suite to verify the appropriate
waypoint and track data after loading the procedure from your database.
b. PBN Operations.
1. Lateral Accuracy Values. Lateral Accuracy values are applicable to a selected airspace, route, or
procedure. The lateral accuracy value is a value typically expressed as a distance in nautical miles from the
intended centerline of a procedure, route, or path. RNP applications also account for potential errors at some
multiple of lateral accuracy value (for example, twice the RNP lateral accuracy values).
(a) RNP NavSpecs. U.S. standard NavSpecs supporting typical RNP airspace uses are as specified
below. Other NavSpecs may include different lateral accuracy values as identified by ICAO or other states. (See
FIG 1−2−1.)
(1) RNP Approach (RNP APCH). In the U.S., RNP APCH procedures are titled RNA V (GPS) and
offer several lines of minima to accommodate varying levels of aircraft equipage: either lateral navigation
(LNA V), LNA V/vertical navigation (LNAV/VNAV), Localizer Performance with Vertical Guidance (LPV), and
Localizer Performance (LP). GPS with or without Space−Based Augmentation System (SBAS) (for example,
WAAS) can provide the lateral information to support LNA V minima. LNAV/VNA V incorporates LNA V lateral
with vertical path guidance for systems and operators capable of either barometric or SBAS vertical. Pilots are
required to use SBAS to fly to the LPV or LP minima. RF turn capability is optional in RNP APCH eligibility.
This means that your aircraft may be eligible for RNP APCH operations, but you may not fly an RF turn unless
RF turns are also specifically listed as a feature of your avionics suite. GBAS Landing System (GLS) procedures
are also constructed using RNP APCH NavSpecs and provide precision approach capability. RNP APCH has
a lateral accuracy value of 1 in the terminal and missed approach segments and essentially scales to RNP 0.3 (or
40 meters with SBAS) in the final approach. (See paragraph 5 −4−18, RNP AR (Authorization Required)
Instrument Procedures.)
(2) RNP Authorization Required Approach (RNP AR APCH). In the U.S., RNP AR APCH
procedures are titled RNA V (RNP). These approaches have stringent equipage and pilot training standards and
require special FAA authorization to fly. Scalability and RF turn capabilities are mandatory in RNP AR APCH
eligibility. RNP AR APCH vertical navigation performance is based upon barometric VNA V or SBAS. RNP AR
is intended to provide specific benefits at specific locations. It is not intended for every operator or aircraft. RNP
AR capability requires specific aircraft performance, design, operational processes, training, and specific
procedure design criteria to achieve the required target level of safety. RNP AR APCH has lateral accuracy values
that can range below 1 in the terminal and missed approach segments and essentially scale to RNP 0.3 or lower
in the final approach. Before conducting these procedures, operators should refer to the latest AC 90 −101,
Approval Guidance for RNP Procedures with AR. (See paragraph 5−4−18.)
(3) RNP Authorization Required Departure (RNP AR DP). Similar to RNP AR approaches, RNP
AR departure procedures have stringent equipage and pilot training standards and require special FAA
authorization to fly. Scalability and RF turn capabilities is mandatory in RNP AR DP eligibility. RNP AR DP
is intended to provide specific benefits at specific locations. It is not intended for every operator or aircraft. RNP
AR DP capability requires specific aircraft performance, design, operational processes, training, and specific
procedure design criteria to achieve the required target level of safety. RNP AR DP has lateral accuracy values
that can scale to no lower than RNP 0.3 in the initial departure flight path. Before conducting these procedures,
operators should refer to the latest AC 90−101, Approval Guidance for RNP Procedures with AR. (See paragraph
5−4−18.)
1−2−6 Performance−Based Navigation (PBN) and Area Navigation (RNA V)
2/20/25 AIM
(4) Advanced RNP (A−RNP). Advanced RNP is a NavSpec with a minimum set of mandatory
functions enabled in the aircraft’s avionics suite. In the U.S., these minimum functions include capability to
calculate and perform RF turns, scalable RNP, and parallel offset flight path generation. Higher continuity (such
as dual systems) may be required for certain oceanic and remote continental airspace. Other “advanced” options
for use in the en route environment (such as fixed radius transitions and Time of Arrival Control) are optional
in the U.S. Typically, an aircraft eligible for A−RNP will also be eligible for operations comprising: RNP APCH,
RNP/RNA V 1, RNP/RNA V 2, RNP 4, and RNP/RNA V 10. A−RNP allows for scalable RNP lateral navigation
values (either 1.0 or 0.3) in the terminal environment. Use of these reduced lateral accuracies will normally
require use of the aircraft’s autopilot and/or flight director. See the latest AC 90−105 for more information on
A−RNP , including NavSpec bundling options, eligibility determinations, and operations approvals.
NOTE−
A−RNP eligible aircraft are NOT automatically eligible for RNP AR APCH or RNP AR DP operations, as RNP AR eligibility
requires a separate determination process and special F AA authorization.
(5) RNP 1. RNP 1 requires a lateral accuracy value of 1 for arrival and departure in the terminal area,
and the initial and intermediate approach phase when used on conventional procedures with PBN segments (for
example, an ILS with a PBN feeder, IAF, or missed approach). RF turn capability is optional in RNP 1 eligibility.
This means that your aircraft may be eligible for RNP 1 operations, but you may not fly an RF turn unless RF
turns are also specifically listed as a feature of your avionics suite.
(6) RNP 2. RNP 2 will apply to both domestic and oceanic/remote operations with a lateral accuracy
value of 2.
(7) RNP 4. RNP 4 will apply to oceanic and remote operations only with a lateral accuracy value of
4. RNP 4 eligibility will automatically confer RNP 10 eligibility.
(8) RNP 10. The RNP 10 NavSpec applies to certain oceanic and remote operations with a lateral
accuracy of 10. In such airspace, the RNA V 10 NavSpec will be applied, so any aircraft eligible for RNP 10 will
be deemed eligible for RNA V 10 operations. Further, any aircraft eligible for RNP 4 operations is automatically
qualified for RNP 10/ RNA V 10 operations. (See also the latest AC 91−70, Oceanic and Remote Continental
Airspace Operations, for more information on oceanic RNP/RNA V operations.)
(9) RNP 0.3. The RNP 0.3 NavSpec requires a lateral accuracy value of 0.3 for all authorized phases
of flight. RNP 0.3 is not authorized for oceanic, remote, or the final approach segment. Use of RNP 0.3 by
slow−flying fixed−wing aircraft is under consideration, but the RNP 0.3 NavSpec initially will apply only to
rotorcraft operations. RF turn capability is optional in RNP 0.3 eligibility. This means that your aircraft may be
eligible for RNP 0.3 operations, but you may not fly an RF turn unless RF turns are also specifically listed as
a feature of your avionics suite.
NOTE−
On terminal procedures or en route charts, do not confuse a charted RNP value of 0.30, or any standard final approach
course segment width of 0.30, with the NavSpec title “RNP 0.3.” Charted RNP values of 0.30 or below should contain two
decimal places (for example, RNP 0.15, or 0.10, or 0.30) whereas the NavSpec title will only state “RNP 0.3.”
(b) Application of Standard Lateral Accuracy Values. U.S. standard lateral accuracy values typically
used for various routes and procedures supporting RNA V operations may be based on use of a specific
navigational system or sensor such as GPS, or on multi−sensor RNA V systems having suitable performance.
(c) Depiction of PBN Requirements. In the U.S., PBN requirements like Lateral Accuracy Values or
NavSpecs applicable to a procedure will be depicted on affected charts and procedures. In the U.S., a specific
procedure’s Performance−Based Navigation (PBN) requirements will be prominently displayed in separate,
standardized notes boxes. For procedures with PBN elements, the “PBN box” will contain the procedure’s
NavSpec(s); and, if required: specific sensors or infrastructure needed for the navigation solution, any additional
or advanced functional requirements, the minimum RNP value, and any amplifying remarks. Items listed in this
PBN box are REQUIRED to fly the procedure’s PBN elements. For example, an ILS with an RNA V missed
approach would require a specific capability to fly the missed approach portion of the procedure. That required
Performance−Based Navigation (PBN) and Area Navigation (RNA V) 1−2−7
AIM 2/20/25
capability will be listed in the PBN box. The separate Equipment Requirements box will list ground −based
equipment and/or airport specific requirements. On procedures with both PBN elements and ground −based
equipment requirements, the PBN requirements box will be listed first. (See FIG 5−4−1.)
c. Other RNP Applications Outside the U.S. The FAA and ICAO member states have led initiatives in
implementing the RNP concept to oceanic operations. For example, RNP−10 routes have been established in the
northern Pacific (NOPAC) which has increased capacity and efficiency by reducing the distance between tracks
to 50 NM. (See paragraph 4−7−1.)
d. Aircraft and Airborne Equipment Eligibility for RNP Operations. Aircraft eligible for RNP operations
will have an appropriate entry including special conditions and limitations in its AFM, avionics manual, or a
supplement. Operators of aircraft not having specific RNP eligibility statements in the AFM or avionics
documents may be issued operational approval including special conditions and limitations for specific RNP
eligibilities.
NOTE−
Some airborne systems use Estimated Position Uncertainty (EPU) as a measure of the current estimated navigational
performance. EPU may also be referred to as Actual Navigation Performance (ANP) or Estimated Position Error (EPE).
TBL 1−2−1
U.S. Standard RNP Levels
RNP Level Typical Application Primary Route
Width (NM) −
Centerline to
Boundary
0.1 to 1.0 RNP AR Approach Segments 0.1 to 1.0
0.3 to 1.0 RNP Approach Segments 0.3 to 1.0
1 Terminal and En Route 1.0
2 En Route 2.0
4 Oceanic/remote areas where performance−based horizontal
separation is applied.
4.0
10 Oceanic/remote areas where performance−based horizontal
separation is applied.
10.0
1−2−3. Use of Suitable Area Navigation (RNAV) Systems on Conventional Procedures and
Routes
a. Discussion. This paragraph sets forth policy, while providing operational and airworthiness guidance
regarding the suitability and use of RNA V systems when operating on, or transitioning to, conventional,
non−RNA V routes and procedures within the U.S. National Airspace System (NAS):
1. Use of a suitable RNA V system as a Substitute Means of Navigation when a V ery−High Frequency (VHF)
Omni−directional Range (VOR), Distance Measuring Equipment (DME), Tactical Air Navigation (TACAN),
VOR/TACAN (VORTAC), VOR/DME, Non−directional Beacon (NDB), or compass locator facility including
locator outer marker and locator middle marker is out −of−service (that is, the navigation aid (NA V AID)
information is not available); an aircraft is not equipped with an Automatic Direction Finder (ADF) or DME;
or the installed ADF or DME on an aircraft is not operational. For example, if equipped with a suitable RNA V
system, a pilot may hold over an out−of−service NDB.
2. Use of a suitable RNA V system as an Alternate Means of Navigation when a VOR, DME, VORTAC,
VOR/DME, TACAN, NDB, or compass locator facility including locator outer marker and locator middle
marker is operational and the respective aircraft is equipped with operational navigation equipment that is
compatible with conventional navaids. For example, if equipped with a suitable RNA V system, a pilot may fly
a procedure or route based on operational VOR using that RNA V system without monitoring the VOR.
1−2−8 Performance−Based Navigation (PBN) and Area Navigation (RNA V)
AIM2/20/258/7/25 AIM
NOTE−
1. Additional information and associated requirements are available in Advisory Circular 90-108 titled “Use of Suitable
RNAV Systems on Conventional Routes and Procedures.”
2. Good planning and knowledge of your RNAV system are critical for safe and successful operations.
3. Pilots planning to use their RNAV system as a substitute means of navigation guidance in lieu of an out −of−service
NAVAID may need to advise ATC of this intent and capability.
4. The navigation database should be current for the duration of the flight. If the AIRAC cycle will change during flight,
operators and pilots should establish procedures to ensure the accuracy of navigation data, including suitability of
navigation facilities used to define the routes and procedures for flight. To facilitate validating database currency, the F AA
has developed procedures for publishing the amendment date that instrument approach procedures were last revised. The
amendment date follows the amendment number, e.g., Amdt 4 14Jan10. Currency of graphic departure procedures and
STARs may be ascertained by the numerical designation in the procedure title. If an amended chart is published for the
procedure, or the procedure amendment date shown on the chart is on or after the expiration date of the database, the
operator must not use the database to conduct the operation.
b. Types of RNA V Systems that Qualify as a Suitable RNA V System. When installed in accordance with
appropriate airworthiness installation requirements and operated in accordance with applicable operational
guidance (for example, aircraft flight manual and Advisory Circular material), the following systems qualify as
a suitable RNA V system:
1. An RNA V system with TSO−C129/ −C145/−C146 equipment, installed in accordance with AC 20−138,
Airworthiness Approval of Global Positioning System (GPS) Navigation Equipment for Use as a VFR and IFR
Supplemental Navigation System, and authorized for in strument flight rules (IFR) en route and terminal
operations (including those systems previously qualified for “GPS in lieu of ADF or DME” operations), or
2. An RNA V system with DME/DME/IRU inputs that is compliant with the equipment provisions of AC
90−100A, U.S. Terminal and En Route Area Navigation (RNA V) Operations, for RNA V routes. A table of
compliant equipment is available at the following website:
https://www.faa.gov/about/office_org/headquarters_ offices/avs/offices/afx/a fs/afs400/afs410/media/
AC90−100compliance.pdf
NOTE−
Approved RNAV systems using DME/DME/IRU, without GPS/WAAS position input, may only be used as a substitute means
of navigation when specifically authorized by a Notice to Airmen (NOTAM) or other F AA guidance for a specific procedure.
The NOTAM or other F AA guidance authorizing the use of DME/DME/IRU systems will also identify any required DME
facilities based on an F AA assessment of the DME navigation infrastructure.
c. Uses of Suitable RNA V Systems. Subject to the operating requirements, operators may use a suitable
RNA V system in the following ways.
1. Determine aircraft position relative to, or distance from a VOR (see NOTE 6 below), TACAN, NDB,
compass locator, DME fix; or a named fix defined by a VOR radial, TACAN course, NDB bearing, or compass
locator bearing intersecting a VOR or localizer course.
2. Navigate to or from a VOR, TACAN, NDB, or compass locator.
3. Hold over a VOR, TACAN, NDB, compass locator, or DME fix.
4. Fly an arc based upon DME.
NOTE−
1. The allowances described in this section apply even when a facility is identified as required on a procedure (for example,
“Note ADF required”).
2. These operations do not include lateral navigation on localizer −based courses (including localizer back− course
guidance) without reference to raw localizer data.
3. Unless otherwise specified, a suitable RNAV system cannot be used for navigation on procedures that are identified as
not authorized (“NA”) without exception by a NOTAM. For example, an operator may not use a RNAV system to navigate
Performance−Based Navigation (PBN) and Area Navigation (RNA V) 1−2−9
AIM 2/20/25
on a procedure affected by an expired or unsatisfactory flight inspection, or a procedure that is based upon a recently
decommissioned NAVAID.
4. Pilots may not substitute for the NAVAID (for example, a VOR or NDB) providing lateral guidance for the final approach
segment. This restriction does not refer to instrument approach procedures with “or GPS” in the title when using GPS or
WAAS. These allowances do not apply to procedures that are identified as not authorized (NA) without exception by a
NOTAM, as other conditions may still exist and result in a procedure not being available. For example, these allowances
do not apply to a procedure associated with an expired or unsatisfactory flight inspection, or is based upon a recently
decommissioned NAVAID.
5. Use of a suitable RNAV system as a means to navigate on the final approach segment of an instrument approach procedure
based on a VOR, TACAN or NDB signal, is allowable. The underlying NAVAID must be operational and the NAVAID
monitored for final segment course alignment.
6. For the purpose of paragraph c, “VOR” includes VOR, VOR/DME, and VORTAC facilities and “compass locator”
includes locator outer marker and locator middle marker.
d. Alternate Airport Considerations. For the purposes of flight planning, any required alternate airport must
have an available instrument approach procedure that does not require the use of GPS. This restriction includes
conducting a conventional approach at the alternate airport using a substitute means of navigation that is based
upon the use of GPS. For example, these restrictions would apply when planning to use GPS equipment as a
substitute means of navigation for an out−of−service VOR that supports an ILS missed approach procedure at
an alternate airport. In this case, some other approach not reliant upon the use of GPS must be available. This
restriction does not apply to RNA V systems using TSO−C145/−C146 WAAS equipment. For further WAAS
guidance, see paragraph 1−1−18.
1. For flight planning purposes, TSO-C129() and TSO-C196() equipped users (GPS users) whose
navigation systems have fault detection and exclusion (FDE) capability, who perform a preflight RAIM
prediction at the airport where the RNA V (GPS) approach will be flown, and have proper knowledge and any
required training and/or approval to conduct a GPS-based IAP, may file based on a GPS-based IAP at either the
destination or the alternate airport, but not at both locations. At the alternate airport, pilots may plan for
applicable alternate airport weather minimums using:
(a) Lateral navigation (LNA V) or circling minimum descent altitude (MDA);
(b) LNA V/vertical navigation (LNA V/VNA V) DA, if equipped with and using approved barometric
vertical navigation (baro-VNA V) equipment;
(c) RNP 0.3 DA on an RNA V (RNP) IAP, if they are specifically authorized users using approved
baro-VNA V equipment and the pilot has verified required navigation performance (RNP) availability through
an approved prediction program.
2. If the above conditions cannot be met, any required alternate airport must have an approved instrument
approach procedure other than GPS that is anticipated to be operational and available at the estimated time of
arrival, and which the aircraft is equipped to fly.
3. This restriction does not apply to TSO-C145() and TSO-C146() equipped users (WAAS users). For
further WAAS guidance, see paragraph 1−1−18.
1−2−4. Recognizing, Mitigating, and Adapting to GPS Jamming and/or Spoofing
a. The low−strength data transmission signals from GPS satellites are vulnerable to various anomalies that
can significantly reduce the reliability of the navigation signal. The GPS signal is vulnerable and has many uses
in aviation (e.g., communication, navigation, surveillance, safety systems and automation); therefore, pilots
must place additional emphasis on closely monitoring aircraft equipment performance for any anomalies and
promptly inform Air Traffic Control (ATC) of any apparent GPS degradation. Pilots should also be prepared to
operate without GPS navigation systems.
b. GPS signals are vulnerable to intentional and unintentional interference from a wide variety of sources,
including radars, microwave links, ionosphere effects, solar activity, multi−path error, satellite communications,
1−2−10 Performance−Based Navigation (PBN) and Area Navigation (RNA V)
AIM2/20/258/7/25 AIM
GPS repeaters, and even some systems onboard the aircraft. In general, these types of unintentional interference
are localized and intermittent. Of greater and growing concern is the intentional and unauthorized interference
of GPS signals by persons using “jammers” or “spoofers” to disrupt air navigation by interfering with the
reception of valid satellite signals.
NOTE−
The U.S. government regularly conducts GPS tests, training activities, and exercises that interfere with GPS signals. These
events are geographically limited, coordinated, scheduled, and advertised via GPS and/or WAAS NOTAMS. Operators of
GPS aircraft should always check for GPS and/or WAAS NOTAMS for their route of flight.
c. Manufacturers, operators, and air traffic controllers should be aware of the general impacts of GPS jamming
and/or spoofing, which include, but are not limited to:
1. Inability to use GPS for navigation.
2. Inability to use hybrid GPS inertial systems for navigation.
3. Loss of, or degraded, performance−based navigation (PBN) capability (e.g., inability to fly required
navigation performance (RNP) procedures).
4. Unreliable triggering of Terrain Awareness and Warning Systems (TAWS).
5. Inaccurate aircraft position on navigation display (e.g., moving map and electronic flight bag).
6. Loss of, or erroneous, Automatic Dependent Surveillance-Broadcast (ADS−B) outputs.
7. Unexpected effects when navigating with conventional NA V AIDS (e.g., if the aircraft is spoofed from
the intended flight path, autotuning will not select the nearby NA V AID).
8. Unanticipated position-dependent flight management system effects (e.g., erroneous insufficient fuel
indication).
9. Failure or degradation of Air Traffic Management (A TM) infrastructure and its associated systems reliant
on GPS, resulting in potential airspace infringements and/or route deviations.
10. Failure of, or erroneous aircraft clocks (resulting in inability to log on to Controller-Pilot Data Link
Communications CPDLC).
11. Erroneous wind and ground speed indications.
d. When flying IFR, pilots should have additional navigation equipment for their intended route to crosscheck
their position. Routine checks of position against VOR or DME information, for example, could help detect a
compromised GPS signal. Pilots transitioning to VOR navigation in response to GPS anomalies should refer to
the Chart Supplement U.S. to identify airports with available conventional approaches associated with the VOR
Minimum Operational Network (MON) program. (Reference 1−1−3f.)
e. Prior to departure, the FAA recommends operators to:
1. Be aware of potential risk locations.
2. Check for any relevant Notices to Airmen (NOTAMs).
3. Plan fuel contingencies.
4. Plan to use conventional NA V AIDs and appropriate arrival/approach procedures at the destination.
5. Follow the detailed guidance from the respective Original Equipment Manufacturer (OEM).
f. During flight, the FAA recommends operators do the following:
1. Be vigilant for any indication that the aircraft’s GPS is disrupted by reviewing the manufacturer’s
guidance for that specific aircraft type and avionics equipage. Verify the aircraft position by means of
conventional NA V AIDs, when available. Indications of jamming and/or spoofing may include:
(a) Changes in actual navigation performance.
Performance−Based Navigation (PBN) and Area Navigation (RNA V) 1−2−11
AIM 2/20/25
(b) Aircraft clock changes (e.g., incorrect time).
(c) Incorrect Flight Management System (FMS) position.
(d) Large shift in displayed GPS position.
(e) Primary Flight Display (PFD)/Navigation Display (ND) warnings about position error.
(f) Other aircraft reporting clock issues, position errors, or requesting vectors.
2. Assess operational risks and limitations linked to the loss of GPS capability, including any on −board
systems requiring inputs from a GPS signal.
3. Ensure NA V AIDs critical to the operation for the intended route/approach are available.
4. Remain prepared to revert to conventional instrument flight procedures.
5. Promptly notify ATC if they experience GPS anomalies. Pilots should not inform ATC of GPS jamming
and/or spoofing when flying through known NOTAMed testing areas unless they require ATC assistance. (See
paragraph 1−1−13)
g. Post flight, the FAA recommends operators to:
1. Document any GPS jamming and/or spoofing in the maintenance log to ensure all faults are cleared.
2. File a detailed report at the reporting site: Report a GPS Anomaly Federal Aviation Administration,
www.faa.gov/air_traffic/nas/gps_reports.
1−2−12 Performance−Based Navigation (PBN) and Area Navigation (RNA V)
AIM2/20/258/7/25 AIM
Chapter 2. Aeronautical Lighting and Other Airport
Visual Aids
Section 1. Airport Lighting Aids
2−1−1. Approach Light Systems (ALS)
a. ALS provide the basic means to transition from instrument flight to visual flight for landing. Operational
requirements dictate the sophistication and configuration of the approach light system for a particular runway.
b. ALS are a configuration of signal lights starting at the landing threshold and extending into the approach
area a distance of 2400 −3000 feet for precision instrument runways and 1400 −1500 feet for nonprecision
instrument runways. Some systems include sequenced flashing lights which appear to the pilot as a ball of light
traveling towards the runway at high speed (twice a second). (See FIG 2−1−1.)
2−1−2. Visual Glideslope Indicators
a. Visual Approach Slope Indicator (V ASI)
1. V ASI installations may consist of either 2, 4, 6, 12, or 16 light units arranged in bars referred to as near,
middle, and far bars. Most V ASI installations consist of 2 bars, near and far, and may consist of 2, 4, or 12 light
units. Some V ASIs consist of three bars, near, middle, and far, which provide an additional visual glide path to
accommodate high cockpit aircraft. This installation may consist of either 6 or 16 light units. V ASI installations
consisting of 2, 4, or 6 light units are located on one side of the runway, usually the left. Where the installation
consists of 12 or 16 light units, the units are located on both sides of the runway.
2. Two−bar V ASI installations provide one visual glide path which is normally set at 3 degrees. Three−bar
V ASI installations provide two visual glide paths. The lower glide path is provided by the near and middle bars
and is normally set at 3 degrees while the upper glide path, provided by the middle and far bars, is normally 1/4
degree higher. This higher glide path is intended for use only by high cockpit aircraft to provide a sufficient
threshold crossing height. Although normal glide path angles are three degrees, angles at some locations may
be as high as 4.5 degrees to give proper obstacle clearance. Pilots of high performance aircraft are cautioned that
use of V ASI angles in excess of 3.5 degrees may cause an increase in runway length required for landing and
rollout.
3. The basic principle of the V ASI is that of color differentiation between red and white. Each light unit
projects a beam of light having a white segment in the upper part of the beam and red segment in the lower part
of the beam. The light units are arranged so that the pilot using the V ASIs during an approach will see the
combination of lights shown below.
4. The V ASI is a system of lights so arranged to provide visual descent guidance information during the
approach to a runway. These lights are visible from 3−5 miles during the day and up to 20 miles or more at night.
The visual glide path of the V ASI provides safe obstruction clearance within plus or minus 10 degrees of the
extended runway centerline and to 4 NM from the runway threshold. Descent, using the V ASI, should not be
initiated until the aircraft is visually aligned with the runway. Lateral course guidance is provided by the runway
or runway lights. In certain circumstances, the safe obstruction clearance area may be reduced by narrowing the
beam width or shortening the usable distance due to local limitations, or the V ASI may be offset from the
extended runway centerline. This will be noted in the Chart Supplement and/or applicable Notices to Airmen
(NOTAMs).
Airport Lighting Aids 2−1−1
AIM 2/20/25
FIG 2−1−1
Precision & Nonprecision Configurations
NOTE−
Civil ALSF−2 may be operated as SSALR during favorable weather conditions.
2−1−2 Airport Lighting Aids
2/20/25 AIM
5. For 2−bar V ASI (4 light units) see FIG 2−1−2.
FIG 2−1−2
2−Bar VASI
Far Bar
= Red
Near Bar = White
Below Glide Path On Glide Path Above Glide Path
6. For 3−bar V ASI (6 light units) see FIG 2−1−3.
FIG 2−1−3
3−Bar VASI
Far Bar
Middle Bar
Near Bar
Below Both
Glide Paths
On Lower
Glide Path
On Upper
Glide Path
Above Both
Glide Paths
7. For other V ASI configurations see FIG 2−1−4.
FIG 2−1−4
V ASI Variations
2 Bar 2 Bar 3 Bar
2 Light Units 12 Light Units 16 Light Units
On Glide Path On Glide Path on Lower Glide Path
Airport Lighting Aids 2−1−3
AIM 2/20/25
b. Precision Approach Path Indicator (PAPI). The precision approach path indicator (PAPI) uses light units
similar to the V ASI but are installed in a single row of either two or four light units. These lights are visible from
about 5 miles during the day and up to 20 miles at night. The visual glide path of the PAPI typically provides
safe obstruction clearance within plus or minus 10 degrees of the extended runway centerline and to 3.4 NM from
the runway threshold. Descent, using the PAPI, should not be initiated until the aircraft is visually aligned with
the runway. The row of light units is normally installed on the left side of the runway and the glide path indications
are as depicted. Lateral course guidance is provided by the runway or runway lights. In certain circumstances,
the safe obstruction clearance area may be reduced by narrowing the beam width or shortening the usable
distance due to local limitations, or the PAPI may be offset from the extended runway centerline. This will be
noted in the Chart Supplement and/or applicable NOTAMs. (See FIG 2−1−5.)
FIG 2−1−5
Precision Approach Path Indicator (PAPI)
High Slightly High On Glide Path Slightly Low Low
(More Than (3.2 Degrees) (3 Degrees) (2.8 Degrees) (Less Than
3,5 Degrees) 2.5 Degrees)
White
Red
c. Tri−color Systems. Tri−color visual approach slope indicators normally consist of a single light unit
projecting a three−color visual approach path into the final approach area of the runway upon which the indicator
is installed. The below glide path indication is red, the above glide path indication is amber, and the on glide path
indication is green. These types of indicators have a useful range of approximately one−half to one mile during
the day and up to five miles at night depending upon the visibility conditions. (See FIG 2−1−6.)
FIG 2−1−6
Tri−Color Visual Approach Slope Indicator
Amber
Above Glide PathOn Glide Path
Below Glide Path
Amber
Green
Red
NOTE−
1. Since the tri−color VASI consists of a single light source which could possibly be confused with other light sources, pilots
should exercise care to properly locate and identify the light signal.
2. When the aircraft descends from green to red, the pilot may see a dark amber color during the transition from green to
red.
Airport Lighting Aids2−1−4
2/20/25 AIM
FIG 2−1−7
Pulsating Visual Approach Slope Indicator
NOTE−
Since the PVASI consists of a single light source which could possibly be confused with other light sources, pilots should
exercise care to properly locate and identify the light signal.
FIG 2−1−8
Alignment of Elements
Above Glide Path On Glide Path Below Glide Path
d. Pulsating Systems. Pulsating visual approach slope indicators normally consist of a single light unit
projecting a two−color visual approach path into the final approach area of the runway upon which the indicator
is installed. The on glide path indication may be a steady white light or alternating RED and WHITE light. The
slightly below glide path indication is a steady red light. If the aircraft descends further below the glide path, the
red light starts to pulsate. The above glide path indication is a pulsating white light. The pulsating rate increases
as the aircraft gets further above or below the desired glide slope. The useful range of the system is about four
miles during the day and up to ten miles at night. (See FIG 2−1−7.)
e. Alignment of Elements Systems. Alignment of elements systems are installed on some small general
aviation airports and are a low−cost system consisting of painted plywood panels, normally black and white or
fluorescent orange. Some of these systems are lighted for night use. The useful range of these systems is
approximately three−quarter miles. To use the system the pilot positions the aircraft so the elements are in
alignment. The glide path indications are shown in FIG 2−1−8.
Airport Lighting Aids 2−1−5
AIM 2/20/25
2−1−3. Runway End Identifier Lights (REIL)
REILs are installed at many airfields to provide rapid and positive identification of the approach end of a
particular runway. The system consists of a pair of synchronized flashing lights located laterally on each side of
the runway threshold. REILs may be either omnidirectional or unidirectional facing the approach area. They are
effective for:
a. Identification of a runway surrounded by a preponderance of other lighting.
b. Identification of a runway which lacks contrast with surrounding terrain.
c. Identification of a runway during reduced visibility.
2−1−4. Runway Edge Light Systems
a. Runway edge lights are used to outline the edges of runways during periods of darkness or restricted
visibility conditions. These light systems are classified according to the intensity or brightness they are capable
of producing: they are the High Intensity Runway Lights (HIRL), Medium Intensity Runway Lights (MIRL),
and the Low Intensity Runway Lights (LIRL). The HIRL and MIRL systems have variable intensity controls,
whereas the LIRLs normally have one intensity setting.
b. The runway edge lights are white, except on instrument runways yellow replaces white on the last 2,000
feet or half the runway length, whichever is less, to form a caution zone for landings.
c. The lights marking the ends of the runway emit red light toward the runway to indicate the end of runway
to a departing aircraft and emit green outward from the runway end to indicate the threshold to landing aircraft.
2−1−5. In −runway Lighting
a. Runway Centerline Lighting System (RCLS). Runway centerline lights are installed on some precision
approach runways to facilitate landing under adverse visibility conditions. They are located along the runway
centerline and are spaced at 50−foot intervals. When viewed from the landing threshold, the runway centerline
lights are white until the last 3,000 feet of the runway. The white lights begin to alternate with red for the next
2,000 feet, and for the last 1,000 feet of the runway, all centerline lights are red.
b. Touchdown Zone Lights (TDZL). Touchdown zone lights are installed on some precision approach
runways to indicate the touchdown zone when landing under adverse visibility conditions. They consist of two
rows of transverse light bars disposed symmetrically about the runway centerline. The system consists of
steady−burning white lights which start 100 feet beyond the landing threshold and extend to 3,000 feet beyond
the landing threshold or to the midpoint of the runway, whichever is less.
c. Taxiway Centerline Lead−Off Lights. Taxiway centerline lead−off lights provide visual guidance to
persons exiting the runway. They are color−coded to warn pilots and vehicle drivers that they are within the
runway environment or instrument landing system (ILS) critical area, whichever is more restrictive. Alternate
green and yellow lights are installed, beginning with green, from the runway centerline to one centerline light
position beyond the runway holding position or ILS critical area holding position.
d. Taxiway Centerline Lead −On Lights. Taxiway centerline lead −on lights provide visual guidance to
persons entering the runway. These “lead−on” lights are also color−coded with the same color pattern as lead−off
lights to warn pilots and vehicle drivers that they are within the runway environment or instrument landing
system (ILS) critical area, whichever is more conservative. The fixtures used for lead−on lights are bidirectional,
i.e., one side emits light for the lead−on function while the other side emits light for the lead−off function. Any
fixture that emits yellow light for the lead−off function must also emit yellow light for the lead−on function.
(See FIG 2−1−12.)
e. Land and Hold Short Lights. Land and hold short lights are used to indicate the hold short point on certain
runways which are approved for Land and Hold Short Operations (LAHSO). Land and hold short lights consist
of a row of pulsing white lights installed across the runway at the hold short point. Where installed, the lights
will be on anytime LAHSO is in effect. These lights will be off when LAHSO is not in effect.
2−1−6 Airport Lighting Aids
2/20/25 AIM
REFERENCE−
AIM, Para 4−3−1 1, Pilot Responsibilities When Conducting Land and Hold Short Operations (LAHSO).
2−1−6. Runway Status Light (RWSL) System
a. Introduction. RWSL is a fully automated system that provides runway status information to pilots and
surface vehicle operators to clearly indicate when it is unsafe to enter, cross, takeoff from, or land on a runway.
The RWSL system processes information from surveillance systems and activates Runway Entrance Lights
(REL) and Takeoff Hold Lights (THL), in accordance with the position and velocity of the detected surface traffic
and approach traffic. REL and THL are in−pavement light fixtures that are directly visible to pilots and surface
vehicle operators. RWSL is an independent safety enhancement that does not substitute for or convey an ATC
clearance. Clearance to enter, cross, takeoff from, land on, or operate on a runway must still be received from
ATC. Although ATC has limited control over the system, personnel do not directly use and may not be able to
view light fixture activations and deactivations during the conduct of daily ATC operations.
b. Runway Entrance Lights (REL): The REL system is composed of flush mounted, in-pavement,
unidirectional light fixtures that are parallel to and focused along the taxiway centerline and directed toward the
pilot at the hold line. An array of REL lights include the first light at the hold line followed by a series of evenly
spaced lights to the runway edge; one additional light at the runway centerline is in line with the last two lights
before the runway edge (see FIG 2−1−9 and FIG 2−1−10). When activated, the red lights indicate that there is
high speed traffic on the runway or there is an aircraft on final approach within the activation area.
1. REL Operating Characteristics − Departing Aircraft: When a departing aircraft reaches a site adaptable
speed of approximately 30 knots, all taxiway intersections with REL arrays along the runway ahead of the aircraft
will illuminate (see FIG 2−1−9). As the aircraft approaches an REL equipped taxiway intersection, the lights at
that intersection extinguish approximately 3 to 4 seconds before the aircraft reaches it. This allows controllers
to apply “anticipated separation” to permit ATC to move traffic more expeditiously without compromising
safety. After the aircraft is declared “airborne” by the system, all REL lights associated with this runway will
extinguish.
2. REL Operating Characteristics − Arriving Aircraft: When an aircraft on final approach is approximately
1 mile from the runway threshold, all sets of taxiway REL light arrays that intersect the runway illuminate. The
distance is adjustable and can be configured for specific operations at particular airports. Lights extinguish at
each equipped taxiway intersection appr oximately 3 to 4 seconds before the aircraft reaches it to apply
anticipated separation until the aircraft has slowed to approximately 80 knots (site adjustable parameter). Below
80 knots, all arrays that are not within 30 seconds of the aircraft’s forward path are extinguished. Once the arriving
aircraft slows to approximately 34 knots (site adjustable parameter), it is declared to be in a taxi state, and all
lights extinguish.
3. What a pilot would observe: A pilot at or approaching the hold line to a runway will observe RELs
illuminate and extinguish in reaction to an aircraft or vehicle operating on the runway, or an arriving aircraft
operating less than 1 mile from the runway threshold.
4. When a pilot observes the red lights of the REL, that pilot will stop at the hold line or remain stopped.
The pilot will then contact ATC for resolution if the clearance is in conflict with the lights. Should pilots note
illuminated lights under circumstances when remaining clear of the runway is impractical for safety reasons (for
example, aircraft is already on the runway), the crew should proceed according to their best judgment while
understanding the illuminated lights indicate the runway is unsafe to enter or cross. Contact ATC at the earliest
possible opportunity.
Airport Lighting Aids 2−1−7
AIM 2/20/25
FIG 2−1−9
Runway Status Light System
c. Takeoff Hold Lights (THL) : The THL system is composed of flush mounted, in-pavement, unidirectional
light fixtures in a double longitudinal row aligned either side of the runway centerline lighting. Fixtures are
focused toward the arrival end of the runway at the “line up and wait” point. THLs extend for 1,500 feet in front
of the holding aircraft starting at a point 375 feet from the departure threshold (see FIG 2−1−11). Illuminated red
lights provide a signal, to an aircraft in position for takeoff or rolling, that it is unsafe to takeoff because the
runway is occupied or about to be occupied by another aircraft or ground vehicle. Two aircraft, or a surface
vehicle and an aircraft, are required for the lights to illuminate. The departing aircraft must be in position for
takeoff or beginning takeoff roll. Another aircraft or a surface vehicle must be on or about to cross the runway.
1. THL Operating Characteristics − Departing Aircraft: THLs will illuminate for an aircraft in position for
departure or departing when there is another aircraft or vehicle on the runway or about to enter the runway (see
FIG 2−1−9.) Once that aircraft or vehicle exits the runway, the THLs extinguish. A pilot may notice lights
extinguish prior to the downfield aircraft or vehicle being completely clear of the runway but still moving. Like
RELs, THLs have an “anticipated separation” feature.
NOTE−
When the THLs extinguish, this is not clearance to begin a takeoff roll. All takeoff clearances will be issued by ATC.
2. What a pilot would observe: A pilot in position to depart from a runway, or has begun takeoff roll, will
observe THLs illuminate in reaction to an aircraft or vehicle on the runway or entering or crossing it. Lights will
extinguish when the runway is clear. A pilot may observe several cycles of illumination and extinguishing
depending on the amount of crossing traffic.
3. When a pilot observes the red light of the THLs, the pilot should safely stop if it’s feasible or remain
stopped. The pilot must contact ATC for resolution if any clearance is in conflict with the lights. Should pilots
note illuminated lights while in takeoff roll and under circumstances when stopping is impractical for safety
2−1−8 Airport Lighting Aids
2/20/25 AIM
reasons, the crew should proceed according to their best judgment while understanding the illuminated lights
indicate that continuing the takeoff is unsafe. Contact ATC at the earliest possible opportunity.
d. Pilot Actions:
1. When operating at airports with RWSL, pilots will operate with the transponder/ADS −B “On” when
departing the gate or parking area until it is shut down upon arrival at the gate or parking area. This ensures
interaction with the FAA surveillance systems such as ASDE-X/Airport Surface Surveillance Capability (ASSC)
which provide information to the RWSL system.
2. Pilots must always inform the ATCT when they have stopped due to an RWSL indication that is in conflict
with ATC instructions. Pilots must request clarification of the taxi or takeoff clearance.
3. Never cross over illuminated red lights. Under normal circumstances, RWSL will confirm the pilot’s taxi
or takeoff clearance previously issued by ATC. If RWSL indicates that it is unsafe to takeoff from, land on, cross,
or enter a runway, immediately notify ATC of the conflict and re-confirm the clearance.
4. Do not proceed when lights have extinguished without an ATC clearance. RWSL verifies an ATC
clearance; it does not substitute for an ATC clearance.
e. ATC Control of RWSL System:
1. Controllers can set in−pavement lights to one of five (5) brightness levels to assure maximum conspicuity
under all visibility and lighting conditions. REL and THL subsystems may be independently set.
2. System lights can be disabled should RWSL operations impact the efficient movement of air traffic or
contribute, in the opinion of the assigned ATC Manager, to unsafe operations. REL and THL light fixtures may
be disabled separately. Whenever the system or a component is disabled, a NOTAM must be issued, and the
Automatic Terminal Information System (ATIS) must be updated.
2−1−7. Control of Lighting Systems
a. Operation of approach light systems and runway lighting is controlled by the control tower (ATCT). At
some locations the FSS may control the lights where there is no control tower in operation.
b. Pilots may request that lights be turned on or off. Runway edge lights, in−pavement lights and approach
lights also have intensity controls which may be varied to meet the pilots request. Sequenced flashing lights
(SFL) may be turned on and off. Some sequenced flashing light systems also have intensity control.
2−1−8. Pilot Control of Airport Lighting
Radio control of lighting is available at selected airports to provide airborne control of lights by keying the
aircraft’s microphone. Control of lighting systems is often available at locations without specified hours for
lighting and where there is no control tower or FSS or when the tower or FSS is closed (locations with a part−time
tower or FSS) or specified hours. All lighting systems which are radio controlled at an airport, whether on a single
runway or multiple runways, operate on the same radio frequency. (See TBL 2−1−1 and TBL 2−1−2.)
Airport Lighting Aids 2−1−9
AIM 2/20/25
FIG 2−1−10
Runway Entrance Lights
FIG 2−1−11
Takeoff Hold Lights
2−1−10 Airport Lighting Aids
2/20/25 AIM
FIG 2−1−12
Taxiway Lead−On Light Configuration
TBL 2−1−1
Runways With Approach Lights
Lighting System No. of Int.
Steps
Status During
Nonuse Period
Intensity Step Selected Per No. of Mike Clicks
3 Clicks 5 Clicks 7 Clicks
Approach Lights (Med. Int.) 2 Off Low Low High
Approach Lights (Med. Int.) 3 Off Low Med High
MIRL 3 Off or Low
HIRL 5 Off or Low
V ASI 2 Off
NOTES: Predetermined intensity step.
Low intensity for night use. High intensity for day use as determined by photocell control.
TBL 2−1−2
Runways Without Approach Lights
Lighting System No. of Int.
Steps
Status During
Nonuse Period
Intensity Step Selected Per No. of Mike Clicks
3 Clicks 5 Clicks 7 Clicks
MIRL 3 Off or Low Low Med. High
HIRL 5 Off or Low Step 1 or 2 Step 3 Step 5
LIRL 1 Off On On On
V ASI 2 Off
REIL 1 Off Off On/Off On
REIL 3 Off Low Med. High
NOTES: Low intensity for night use. High intensity for day use as determined by photocell control.
The control of V ASI and/or REIL may be independent of other lighting systems.
Airport Lighting Aids 2−1−11
AIM 2/20/25
a. With FAA approved systems, various combinations of medium intensity approach lights, runway lights,
taxiway lights, V ASI and/or REIL may be activated by radio control. On runways with both approach lighting
and runway lighting (runway edge lights, taxiway lights, etc.) systems, the approach lighting system takes
precedence for air−to−ground radio control over the runway lighting system which is set at a predetermined
intensity step, based on expected visibility conditions. Runways without approach lighting may provide radio
controlled intensity adjustments of runway edge lights. Other lighting systems, including V ASI, REIL, and
taxiway lights may be either controlled with the runway edge lights or controlled independently of the runway
edge lights.
b. The control system consists of a 3−step control responsive to 7, 5, and/or 3 microphone clicks. This 3−step
control will turn on lighting facilities capable of either 3−step, 2−step or 1−step operation. The 3−step and 2−step
lighting facilities can be altered in intensity, while the 1−step cannot. All lighting is illuminated for a period of
15 minutes from the most recent time of activation and may not be extinguished prior to end of the 15 minute
period (except for 1−step and 2−step REILs which may be turned off when desired by keying the mike 5 or 3
times respectively).
c. Suggested use is to always initially key the mike 7 times; this assures that all controlled lights are turned
on to the maximum available intensity. If desired, adjustment can then be made, where the capability is provided,
to a lower intensity (or the REIL turned off) by keying 5 and/or 3 times. Due to the close proximity of airports
using the same frequency, radio controlled lighting receivers may be set at a low sensitivity requiring the aircraft
to be relatively close to activate the system. Consequently, even when lights are on, always key mike as directed
when overflying an airport of intended landing or just prior to entering the final segment of an approach. This
will assure the aircraft is close enough to activate the system and a full 15 minutes lighting duration is available.
Approved lighting systems may be activated by keying the mike (within 5 seconds) as indicated in TBL 2−1−3.
TBL 2−1−3
Radio Control System
Key Mike Function
7 times within 5 seconds Highest intensity available
5 times within 5 seconds Medium or lower intensity
(Lower REIL or REIL−off)
3 times within 5 seconds Lowest intensity available
(Lower REIL or REIL−off)
d. For all public use airports with FAA standard systems the Chart Supplement contains the types of lighting,
runway and the frequency that is used to activate the system. Airports with IAPs include data on the approach
chart identifying the light system, the runway on which they are installed, and the frequency that is used to
activate the system.
NOTE−
Although the CTAF is used to activate the lights at many airports, other frequencies may also be used. The appropriate
frequency for activating the lights on the airport is provided in the Chart Supplement and the standard instrument approach
procedures publications. It is not identified on the sectional charts.
e. Where the airport is not served by an IAP, it may have either the standard FAA approved control system
or an independent type system of different specification installed by the airport sponsor. The Chart Supplement
contains descriptions of pilot controlled lighting systems for each airport having other than FAA approved
systems, and explains the type lights, method of control, and operating frequency in clear text.
2−1−9. Airport/Heliport Beacons
a. Airport and heliport beacons have a vertical light distribution to make them most effective from one to ten
degrees above the horizon; however, they can be seen well above and below this peak spread. The beacon may
be an omnidirectional capacitor−discharge device, or it may rotate at a constant speed which produces the visual
effect of flashes at regular intervals. Flashes may be one or two colors alternately. The total number of flashes
are:
2−1−12 Airport Lighting Aids
2/20/25 AIM
1. 24 to 30 per minute for beacons marking airports, landmarks, and points on Federal airways.
2. 30 to 45 per minute for beacons marking heliports.
b. The colors and color combinations of beacons are:
1. White and Green− Lighted land airport.
2. *Green alone− Lighted land airport.
3. White and Yellow− Lighted water airport.
4. *Yellow alone− Lighted water airport.
5. Green, Yellow, and White− Lighted heliport.
NOTE−
*Green alone or yellow alone is used only in connection with a white −and−green or white−and−yellow beacon display,
respectively.
c. Military airport beacons flash alternately white and green, but are differentiated from civil beacons by
dualpeaked (two quick) white flashes between the green flashes.
d. In Class B, Class C, Class D and Class E surface areas, operation of the airport beacon during the hours
of daylight often indicates that the ground visibility is less than 3 miles and/or the ceiling is less than 1,000 feet.
ATC clearance in accordance with 14 CFR part 91 is required for landing, takeoff and flight in the traffic pattern.
Pilots should not rely solely on the operation of the airport beacon to indicate if weather conditions are IFR or
VFR. At some locations with operating control towers, ATC personnel turn the beacon on or off when controls
are in the tower. At many airports the airport beacon is turned on by a photoelectric cell or time clocks and ATC
personnel cannot control them. There is no regulatory requirement for daylight operation and it is the pilot’s
responsibility to comply with proper preflight planning as required by 14 CFR section 91.103.
2−1−10. Taxiway Lights
a. Taxiway Edge Lights. Taxiway edge lights are used to outline the edges of taxiways during periods of
darkness or restricted visibility conditions. These fixtures emit blue light.
NOTE−
At most major airports these lights have variable intensity settings and may be adjusted at pilot request or when deemed
necessary by the controller .
b. Taxiway Centerline Lights. Taxiway centerline lights are used to facilitate ground traffic under low
visibility conditions. They are located along the taxiway centerline in a straight line on straight portions, on the
centerline of curved portions, and along designated taxiing paths in portions of runways, ramp, and apron areas.
Taxiway centerline lights are steady burning and emit green light.
c. Clearance Bar Lights. Clearance bar lights are installed at holding positions on taxiways in order to
increase the conspicuity of the holding position in low visibility conditions. They may also be installed to indicate
the location of an intersecting taxiway during periods of darkness. Clearance bars consist of three in−pavement
steady−burning yellow lights.
d. Runway Guard Lights. Runway guard lights are installed at taxiway/runway intersections. They are
primarily used to enhance the conspicuity of taxiway/runway intersections during low visibility conditions, but
may be used in all weather conditions. Runway guard lights consist of either a pair of elevated flashing yellow
lights installed on either side of the taxiway, or a row of in−pavement yellow lights installed across the entire
taxiway, at the runway holding position marking.
NOTE−
Some airports may have a row of three or five in−pavement yellow lights installed at taxiway/runway intersections. They
should not be confused with clearance bar lights described in paragraph 2−1−10c, Clearance Bar Lights.
e. Stop Bar Lights. Stop bar lights, when installed, are used to confirm the ATC clearance to enter or cross
the active runway in low visibility conditions (below 1,200 ft Runway Visual Range). A stop bar consists of a
Airport Lighting Aids 2−1−13
AIM 2/20/25
row of red, unidirectional, steady−burning in−pavement lights installed across the entire taxiway at the runway
holding position, and elevated steady −burning red lights on each side. A controlled stop bar is operated in
conjunction with the taxiway centerline lead −on lights which extend from the stop bar toward the runway.
Following the ATC clearance to proceed, the stop bar is turned off and the lead−on lights are turned on. The stop
bar and lead−on lights are automatically reset by a sensor or backup timer.
CAUTION−
Pilots should never cross a red illuminated stop bar, even if an ATC clearance has been given to proceed onto or across
the runway.
NOTE−
If after crossing a stop bar, the taxiway centerline lead−on lights inadvertently extinguish, pilots should hold their position
and contact ATC for further instructions.
2−1−14 Airport Lighting Aids
2/20/25 AIM
Section 2. Air Navigation and Obstruction Lighting
2−2−1. Aeronautical Light Beacons
a. An aeronautical light beacon is a visual NA VAID displaying flashes of white and/or colored light to indicate
the location of an airport, a heliport, a landmark, a certain point of a Federal airway in mountainous terrain, or
an obstruction. The light used may be a rotating beacon or one or more flashing lights. The flashing lights may
be supplemented by steady burning lights of lesser intensity.
b. The color or color combination displayed by a particular beacon and/or its auxiliary lights tell whether the
beacon is indicating a landing place, landmark, point of the Federal airways, or an obstruction. Coded flashes
of the auxiliary lights, if employed, further identify the beacon site.
2−2−2. Code Beacons and Course Lights
a. Code Beacons. The code beacon, which can be seen from all directions, is used to identify airports and
landmarks. The code beacon flashes the three or four character airport identifier in International Morse Code six
to eight times per minute. Green flashes are displayed for land airports while yellow flashes indicate water
airports.
b. Course Lights. The course light, which can be seen clearly from only one direction, is used only with
rotating beacons of the Federal Airway System: two course lights, back to back, direct coded flashing beams of
light in either direction along the course of airway.
NOTE−
Airway beacons are remnants of the “lighted” airways which antedated the present electronically equipped federal airways
system. Only a few of these beacons exist today to mark airway segments in remote mountain areas. Flashes in Morse code
identify the beacon site.
2−2−3. Obstruction Lights
a. Obstructions are marked/lighted to warn airmen of their presence during daytime and nighttime conditions.
They may be marked/lighted in any of the following combinations:
1. Aviation Red Obstruction Lights. Flashing aviation red beacons (20 to 40 flashes per minute) and
steady burning aviation red lights during nighttime operation. Aviation orange and white paint is used for daytime
marking.
2. Medium Intensity Flashing White Obstruction Lights. Medium intensity flashing white obstruction
lights may be used during daytime and twilight with automatically selected reduced intensity for nighttime
operation. When this system is used on structures 500 feet (153m) AGL or less in height, other methods of
marking and lighting the structure may be omitted. Aviation orange and white paint is always required for
daytime marking on structures exceeding 500 feet (153m) AGL. This system is not normally installed on
structures less than 200 feet (61m) AGL.
3. High Intensity White Obstruction Lights. Flashing high intensity white lights during daytime with
reduced intensity for twilight and nighttime operation. When this type system is used, the marking of structures
with red obstruction lights and aviation orange and white paint may be omitted.
4. Dual Lighting. A combination of flashing aviation red beacons and steady burning aviation red lights
for nighttime operation and flashing high intensity white lights for daytime operation. Aviation orange and white
paint may be omitted.
5. Catenary Lighting. Lighted markers are available for increased night conspicuity of high −voltage
(69KV or higher) transmission line catenary wires. Lighted markers provide conspicuity both day and night.
Air Navigation and Obstruction Lighting 2−2−1
AIM 2/20/25
b. Medium intensity omnidirectional flashing white lighting system provides conspicuity both day and night
on catenary support structures. The unique sequential/simultaneous flashing light system alerts pilots of the
associated catenary wires.
c. High intensity flashing white lights are being used to identify some supporting structures of overhead
transmission lines located across rivers, chasms, gorges, etc. These lights flash in a middle, top, lower light
sequence at approximately 60 flashes per minute. The top light is normally installed near the top of the supporting
structure, while the lower light indicates the approximate lower portion of the wire span. The lights are beamed
towards the companion structure and identify the area of the wire span.
d. High intensity flashing white lights are also employed to identify tall structures, such as chimneys and
towers, as obstructions to air navigation. The lights provide a 360 degree coverage about the structure at 40
flashes per minute and consist of from one to seven levels of lights depending upon the height of the structure.
Where more than one level is used the vertical banks flash simultaneously.
2−2−4. LED Lighting Systems
Certain light− emitting diode (LED) lighting systems fall outside the combined visible and near −infrared
spectrum of night vision goggles (NVGs) and thus will not be visible to a flightcrew using NVGs.
The FAA changed specifications for LED −based red obstruction lights to make them visible to pilots using
certain NVG systems, however, other colors may not be visible.
It is recommended that air carriers/operators—includi ng part 91 operators—who use NVGs incorporate
procedures into manuals and/or standard operating procedures (SOPs) requiring periodic, unaided scanning
when operating at low altitudes and when performing a reconnaissance of landing areas.
2−2−2 Air Navigation and Obstruction Lighting
2/20/25 AIM
2−2−3
2/20/25 AIM
Section 3. Airport Marking Aids and Signs
2−3−1. General
a. Airport pavement markings and signs provide information that is useful to a pilot during takeoff, landing,
and taxiing.
b. Uniformity in airport markings and signs from one airport to another enhances safety and improves
efficiency. Pilots are encouraged to work with the operators of the airports they use to achieve the marking and
sign standards described in this section.
c. Pilots who encounter ineffective, incorrect, or confusing markings or signs on an airport should make the
operator of the airport aware of the problem. These situations may also be reported under the Aviation Safety
Reporting Program as described in paragraph 7−7−1, Aviation Safety Reporting Program. Pilots may also report
these situations to the FAA regional airports division.
d. The markings and signs described in this section of the AIM reflect the current FAA recommended
standards.
REFERENCE−
AC 150/5340−1, Standards for Airport Markings.
AC 150/5340−18, Standards for Airport Sign Systems.
2−3−2. Airport Pavement Markings
a. General. For the purpose of this section, the airport pavement markings have been grouped into four areas:
1. Runway Markings.
2. Taxiway Markings.
3. Holding Position Markings.
4. Other Markings.
b. Marking Colors. Markings for runways are white. Markings defining the landing area on a heliport are
also white except for hospital heliports which use a red “H” on a white cross. Markings for taxiways, areas not
intended for use by aircraft (closed and hazardous areas), and holding positions (even if they are on a runway)
are yellow.
2−3−3. Runway Markings
a. General. There are three types of markings for runways: visual, nonprecision instrument, and precision
instrument. TBL 2−3−1 identifies the marking elements for each type of runway and TBL 2 −3−2 identifies
runway threshold markings.
TBL 2−3−1
Runway Marking Elements
Marking Element Visual Runway
Nonprecision
Instrument
Runway
Precision
Instrument
Runway
Designation X X X
Centerline X X X
Threshold X1 X X
Aiming Point X2 X X
Touchdown Zone X
Side Stripes X
1 On runways used, or intended to be used, by international commercial transports.
2 On runways 4,000 feet (1200 m) or longer used by jet aircraft.
Airport Marking Aids and Signs 2−3−1
AIM 2/20/25
FIG 2−3−1
Precision Instrument Runway Markings
b. Runway Designators. Runway numbers and letters are determined from the approach direction. The
runway number is the whole number nearest one-tenth the magnetic azimuth of the centerline of the runway,
measured clockwise from the magnetic north. The letters, differentiate between left (L), right (R), or center (C)
parallel runways, as applicable:
1. For two parallel runways “L” “R.”
2. For three parallel runways “L” “C” “R.”
c. Runway Centerline Marking. The runway centerline identifies the center of the runway and provides
alignment guidance during takeoff and landings. The centerline consists of a line of uniformly spaced stripes and
gaps.
d. Runway Aiming Point Marking. The aiming point marking serves as a visual aiming point for a landing
aircraft. These two rectangular markings consist of a broad white stripe located on each side of the runway
centerline and approximately 1,000 feet from the landing threshold, as shown in FIG 2−3−1, Precision
Instrument Runway Markings.
e. Runway Touchdown Zone Markers. The touchdown zone markings identify the touchdown zone for
landing operations and are coded to provide distance information in 500 feet (150m) increments. These markings
consist of groups of one, two, and three rectangular bars symmetrically arranged in pairs about the runway
centerline, as shown in FIG 2−3−1. For runways having touchdown zone markings on both ends, those pairs of
markings which extend to within 900 feet (270 m) of the midpoint between the thresholds are eliminated.
2−3−2 Airport Marking Aids and Signs
2/20/25 AIM
FIG 2−3−2
Nonprecision Instrument Runway and Visual Runway Markings
AIMING POINT
MARKING
THRESHOLD THRESHOLD
MARKINGS
DESIGNATION
MARKING
PAVEMENT EDGE
AIMING POINT
MARKING
PAVEMENT EDGE
DESIGNATION MARKING
THRESHOLD
NONPRECISION INSTRUMENT RUNWAY MARKINGS
VISUAL RUNWAY MARKINGS
f. Runway Side Stripe Marking. Runway side stripes delineate the edges of the runway. They provide a
visual contrast between runway and the abutting terrain or shoulders. Side stripes consist of continuous white
stripes located on each side of the runway as shown in FIG 2−3−4.
g. Runway Shoulder Markings. Runway shoulder stripes may be used to supplement runway side stripes
to identify pavement areas contiguous to the runway sides that are not intended for use by aircraft. Runway
shoulder stripes are yellow. (See FIG 2−3−5.)
h. Runway Threshold Markings. Runway threshold markings come in two configurations. They either
consist of eight longitudinal stripes of uniform dimensions disposed symmetrically about the runway centerline
(as shown in FIG 2−3−1) or the number of stripes is related to the runway width as indicated in TBL 2−3−2. A
threshold marking helps identify the beginning of the runway that is available for landing. In some instances,
the landing threshold may be relocated or displaced.
TBL 2−3−2
Number of Runway Threshold Stripes
Runway Width Number of Stripes
60 feet (18 m) 4
75 feet (23 m) 6
100 feet (30 m) 8
150 feet (45 m) 12
200 feet (60 m) 16
Airport Marking Aids and Signs 2−3−3
AIM 2/20/25
1. Relocation of a Threshold. Sometimes construction, maintenance, or other activities require the
threshold to be relocated towards the rollout end of the runway. (See FIG 2−3−3.) When a threshold is relocated,
it closes not only a set portion of the approach end of a runway, but also shortens the length of the opposite
direction runway. In these cases, a NOTAM should be issued by the airport operator identifying the portion of
the runway that is closed (for example, 10/28 W 900 CLSD). Because the duration of the relocation can vary from
a few hours to several months, methods identifying the new threshold may vary. One common practice is to use
a ten feet wide white threshold bar across the width of the runway. Although the runway lights in the area between
the old threshold and new threshold will not be illuminated, the runway markings in this area may or may not
be obliterated, removed, or covered.
2. Displaced Threshold. A displaced threshold is a threshold located at a point on the runway other than
the designated beginning of the runway. Displacement of a threshold reduces the length of runway available for
landings. The portion of runway behind a displaced threshold is available for takeoffs in either direction and
landings from the opposite direction. A ten feet wide white threshold bar is located across the width of the runway
at the displaced threshold. White arrows are located along the centerline in the area between the beginning of
the runway and displaced threshold. White arrow heads are located across the width of the runway just prior to
the threshold bar, as shown in FIG 2−3−4.
NOTE−
Airport operator. When reporting the relocation or displacement of a threshold, the airport operator should avoid language
which confuses the two.
i. Demarcation Bar. A demarcation bar delineates a runway with a displaced threshold from a blast pad,
stopway, or taxiway that precedes the runway. A demarcation bar is 3 feet (1m) wide and yellow, since it is not
located on the runway, as shown in FIG 2−3−6.
1. Chevrons. These markings are used to show pavement areas aligned with the runway that are unusable
for landing, takeoff, and taxiing. Chevrons are yellow. (See FIG 2−3−7.)
j. Runway Threshold Bar. A threshold bar delineates the beginning of the runway that is available for
landing when the threshold has been relocated or displaced. A threshold bar is 10 feet (3m) in width and extends
across the width of the runway, as shown in FIG 2−3−4.
2−3−4 Airport Marking Aids and Signs
2/20/25 AIM
FIG 2−3−3
Relocation of a Threshold with Markings for Taxiway Aligned with Runway
Airport Marking Aids and Signs 2−3−5
AIM 2/20/25
FIG 2−3−4
Displaced Threshold Markings
2−3−6 Airport Marking Aids and Signs
2/20/25 AIM
FIG 2−3−5
Runway Shoulder Markings
RUNWAY THRESHOLD
MIDPOINT OF
RUNWAY
SHOULDER SHOULDERRUNWAY
45° 45°
45° 45°
2−3−4. Taxiway Markings
a. General. All taxiways should have centerline markings and runway holding position markings whenever
they intersect a runway. Taxiway edge markings are present whenever there is a need to separate the taxiway from
a pavement that is not intended for aircraft use or to delineate the edge of the taxiway. Taxiways may also have
shoulder markings and holding position markings for Instrument Landing System (ILS) critical areas and
taxiway/taxiway intersection markings.
REFERENCE−
AIM, Para 2−3−5, Holding Position Markings.
b. Taxiway Centerline.
1. Normal Centerline. The taxiway centerline is a single continuous yellow line, 6 inches (15 cm) to 12
inches (30 cm) in width. This provides a visual cue to permit taxiing along a designated path. Ideally, the aircraft
should be kept centered over this line during taxi. However, being centered on the taxiway centerline does not
guarantee wingtip clearance with other aircraft or other objects.
2. Enhanced Centerline. At some airports, mostly the larger commercial service airports, an enhanced
taxiway centerline will be used. The enhanced taxiway centerline marking consists of a parallel line of yellow
dashes on either side of the normal taxiway centerline. The taxiway centerlines are enhanced for a maximum of
150 feet prior to a runway holding position marking. The purpose of this enhancement is to warn the pilot that
he/she is approaching a runway holding position marking and should prepare to stop unless he/she has been
cleared onto or across the runway by ATC. (See FIG 2−3−8.)
c. Taxiway Edge Markings. Taxiway edge markings are used to define the edge of the taxiway. They are
primarily used when the taxiway edge does not correspond with the edge of the pavement. There are two types
of markings depending upon whether the aircraft is supposed to cross the taxiway edge:
Airport Marking Aids and Signs 2−3−7
AIM 2/20/25
1. Continuous Markings. These consist of a continuous double yellow line, with each line being at least
6 inches (15 cm) in width spaced 6 inches (15 cm) apart. They are used to define the taxiway edge from the
shoulder or some other abutting paved surface not intended for use by aircraft.
2. Dashed Markings. These markings are used when there is an operational need to define the edge of a
taxiway or taxilane on a paved surface where the adjoining pavement to the taxiway edge is intended for use by
aircraft (for example, an apron). Dashed taxiway edge markings consist of a broken double yellow line, with each
line being at least 6 inches (15 cm) in width, spaced 6 inches (15 cm) apart (edge to edge). These lines are 15
feet (4.5 m) in length with 25 foot (7.5 m) gaps. (See FIG 2−3−9.)
d. Taxi Shoulder Markings. Taxiways, holding bays, and aprons are sometimes provided with paved
shoulders to prevent blast and water erosion. Although shoulders may have the appearance of full strength
pavement, they are not intended for use by aircraft and may be unable to support an aircraft. Usually the taxiway
edge marking will define this area. Where conditions exist such as islands or taxiway curves that may cause
confusion as to which side of the edge stripe is for use by aircraft, taxiway shoulder markings may be used to
indicate the pavement is unusable. Taxiway shoulder markings are yellow. (See FIG 2−3−10.)
2−3−8 Airport Marking Aids and Signs
2/20/25 AIM
FIG 2−3−6
Markings for Blast Pad or Stopway or Taxiway Preceding a Displaced Threshold
Airport Marking Aids and Signs 2−3−9
AIM 2/20/25
FIG 2−3−7
Markings for Blast Pads and Stopways
2−3−10 Airport Marking Aids and Signs
2/20/25 AIM
FIG 2−3−8
Enhanced Taxiway Centerline
FIG 2−3−9
Dashed Markings
DOUBLE
YELLOW
LINES
TAXIWAY EDGE TAXIWAY EDGE
MARKINGS MARKINGS
CONTINUOUS DASHED
e. Surface Painted Taxiway Direction Signs. Surface painted taxiway direction signs have a yellow
background with a black inscription, and are provided when it is not possible to provide taxiway direction signs
at intersections, or when necessary to supplement such signs. These markings are located adjacent to the
centerline with signs indicating turns to the left being on the left side of the taxiway centerline, and signs
indicating turns to the right being on the right side of the centerline. (See FIG 2−3−11.)
Airport Marking Aids and Signs 2−3−11
AIM 2/20/25
FIG 2−3−10
Taxi Shoulder Markings
RUNWAY
YELLOW STRIPES
PAVEMENT EDGE
TAXIWAY EDGE
MARKINGS
f. Surface Painted Location Signs. Surface painted location signs have a black background with a yellow
inscription. When necessary, these markings are used to supplement location signs located along side the taxiway
and assist the pilot in confirming the designation of the taxiway on which the aircraft is located. These markings
are located on the right side of the centerline. (See FIG 2−3−11.)
g. Geographic Position Markings. These markings are located at points along low visibility taxi routes
designated in the airport’s Surface Movement Guidance Control System (SMGCS) plan. They are used to
identify the location of taxiing aircraft during low visibility operations. Low visibility operations are those that
occur when the runway visible range (RVR) is below 1200 feet (360m). They are positioned to the left of the
taxiway centerline in the direction of taxiing. (See FIG 2−3−12.) The geographic position marking is a circle
comprised of an outer black ring contiguous to a white ring with a pink circle in the middle. When installed on
asphalt or other dark-colored pavements, the white ring and the black ring are reversed (i.e., the white ring
becomes the outer ring and the black ring becomes the inner ring). It is designated with either a number or a
number and letter. The number corresponds to the consecutive position of the marking on the route.
Airport Marking Aids and Signs2−3−12
2/20/25 AIM
FIG 2−3−11
Surface Painted Signs
Airport Marking Aids and Signs 2−3−13
AIM 2/20/25
2−3−5. Holding Position Markings
a. Runway Holding Position Markings. For runways, these markings indicate where aircraft MUST STOP
when approaching a runway. They consist of four yellow lines, two solid and two dashed, spaced six or twelve
inches apart, and extending across the width of the taxiway or runway. The solid lines are always on the side
where the aircraft must hold. There are three locations where runway holding position markings are encountered.
1. Runway Holding Position Markings on Taxiways. These markings identify the locations on a taxiway
where aircraft MUST STOP when a clearance has not been issued to proceed onto the runway. Generally, runway
holding position markings also identify the boundary of the runway safety area (RSA) for aircraft exiting the
runway. Runway holding position markings are shown in FIG 2−3−13 and FIG 2−3−16. When instructed by
ATC, “Hold short of Runway XX,” the pilot MUST STOP so that no part of the aircraft extends beyond the runway
holding position marking. When approaching runways at airports with an operating control tower, pilots must
not cross the runway holding position marking without ATC clearance. Pilots approaching runways at airports
without an operating control tower must ensure adequate separation from other aircraft, vehicles, and pedestrians
prior to crossing the holding position markings. An aircraft exiting a runway is not clear of the runway until all
parts of the aircraft have crossed the applicable holding position marking.
NOTE−
Runway holding position markings identify the beginning of an RSA, and a pilot MUST STOP to get clearance before
crossing (at airports with operating control towers).
REFERENCE−
AIM, Para 4−3−21, Exiting the Runway After Landing.
2. Runway Holding Position Markings on Runways. These markings identify the locations on runways
where aircraft MUST STOP. These markings are located on runways used by ATC for Land And Hold Short
Operations (for example, see FIG 4−3−8) and Taxiing operations. For taxiing operations, the pilot MUST STOP
prior to the holding position markings unless explicitly authorized to cross by ATC. A sign with a white
inscription on a red background is located adjacent to these holding position markings. (See FIG 2−3−14.) The
holding position markings are placed on runways prior to the intersection with another runway, or some
designated point. Pilots receiving and accepting instructions “Cleared to land Runway XX, hold short of Runway
YY” from ATC must either exit Runway XX prior to the holding position markings, or stop at the holding position
markings prior to Runway YY . Otherwise, pilots are authorized to use the entire landing length of the runway
and disregard the holding position markings.
3. Holding Position Markings on Taxiways Located in Runway Approach Areas. These markings are
used at some airports where it is necessary to hold an aircraft on a taxiway located in the approach or departure
area of a runway so that the aircraft does not interfere with the operations on that runway. This marking is
collocated with the runway approach/departure area holding position sign. When specifically instructed by ATC,
“Hold short of Runway XX approach or Runway XX departure area,” the pilot MUST STOP so that no part of
the aircraft extends beyond the holding position marking. (See Subparagraph 2−3−8b2, Runway Approach Area
Holding Position Sign, and FIG 2−3−15.)
b. Holding Position Markings for Instrument Landing System (ILS). Holding position markings for ILS
critical areas consist of two yellow solid lines spaced two feet apart connected by pairs of solid lines spaced ten
feet apart extending across the width of the taxiway as shown. (See FIG 2−3−16.) A sign with an inscription in
white on a red background is located adjacent to these hold position markings. When instructed by ATC to hold
short of the ILS critical area, pilots MUST STOP so that no part of the aircraft extends beyond the holding
position marking. When approaching the holding position marking, pilots must not cross the marking without
ATC clearance. The ILS critical area is not clear until all parts of the aircraft have crossed the applicable holding
position marking.
REFERENCE−
AIM, Para 1−1−9, Instrument Landing System (ILS).
c. Holding Position Markings for Intersecting Taxiways Holding position markings for intersecting
taxiways consist of a single dashed line extending across the width of the taxiway as shown. (See FIG 2−3−17.)
2−3−14 Airport Marking Aids and Signs
2/20/25 AIM
They are located on taxiways where ATC holds aircraft short of a taxiway intersection. When instructed by ATC,
“Hold short of Taxiway XX,” the pilot MUST STOP so that no part of the aircraft extends beyond the holding
position marking. When the marking is not present, the pilot MUST STOP the aircraft at a point which provides
adequate clearance from an aircraft on the intersecting taxiway.
d. Surface Painted Holding Position Signs. Surface painted holding position signs have a red background
with a white inscription and supplement the signs located at the holding position. This type of marking is
normally used where the width of the holding position on the taxiway is greater than 200 feet (60 m). It is located
to the left side of the taxiway centerline on the holding side and prior to the holding position marking. (See
FIG 2−3−11.)
FIG 2−3−12
Geographic Position Markings
Airport Marking Aids and Signs 2−3−15
AIM 2/20/25
FIG 2−3−13
Runway Holding Position Markings on Taxiway
RUNWAY TAXIWAY/RUNWAY
HOLDING POSITION
MARKINGS
HOLDING
BAY TAXIWAY
EXAMPLE OF HOLDING POSITION MARKINGS
EXTENDED ACROSS HOLDING BAY
Airport Marking Aids and Signs2−3−16
2/20/25 AIM
FIG 2−3−14
Runway Holding Position Markings on Runways
Airport Marking Aids and Signs 2−3−17
AIM 2/20/25
FIG 2−3−15
Taxiways Located in Runway Approach and Departure Areas
2−3−18 Airport Marking Aids and Signs
2/20/25 AIM
NOTE−
1. Refer to Advisory Circular 150/5300−13 for additional information on obstruction surfaces.
2. Because Taxiway C does not enter the departure area of Runway 33, the sign on Taxiway C does not include the “33 DEP”
legend.
3. The location of a holding position is relative to the point on the aircraft that infringes the surface; for inclining surfaces
such as an approach surface, the location of the holdline position may differ from the location of the infringement point.
FIG 2−3−16
Holding Position Markings: ILS Critical Area
2−3−6. Other Markings
a. Vehicle Roadway Markings. The vehicle roadway markings are used when necessary to define a pathway
for vehicle operations on or crossing areas that are also intended for aircraft. These markings consist of a white
solid line to delineate each edge of the roadway and a dashed line to separate lanes within the edges of the
roadway. In lieu of the solid lines, zipper markings may be used to delineate the edges of the vehicle roadway.
(See FIG 2−3−18.) Details of the zipper markings are shown in FIG 2−3−19.
b. VOR Receiver Checkpoint Markings. The VOR receiver checkpoint marking allows the pilot to check
aircraft instruments with navigational aid signals. It consists of a painted circle with an arrow in the middle; the
arrow is aligned in the direction of the checkpoint azimuth. This marking, and an associated sign, is located on
Airport Marking Aids and Signs 2−3−19
AIM 2/20/25
the airport apron or taxiway at a point selected for easy access by aircraft but where other airport traffic is not
to be unduly obstructed. (See FIG 2−3−20.)
NOTE−
The associated sign contains the VOR station identification letter and course selected (published) for the check, the words
“VOR check course,” and DME data (when applicable). The color of the letters and numerals are black on a yellow
background.
EXAMPLE−
DCA 176−356
VOR check course
DME XXX
FIG 2−3−17
Holding Position Markings: Taxiway/Taxiway Intersections
TAXIWAY HOLDING
POSITION MARKINGS,
YELLOW, SEE
DETAIL 1
DETAIL 1
2−3−20 Airport Marking Aids and Signs
2/20/25 AIM
FIG 2−3−18
Vehicle Roadway Markings
Airport Marking Aids and Signs 2−3−21
AIM 2/20/25
FIG 2−3−19
Roadway Edge Stripes, White, Zipper Style
c. Nonmovement Area Boundary Markings. These markings delineate the movement area (i.e., area under
ATC). These markings are yellow and located on the boundary between the movement and nonmovement area.
The nonmovement area boundary markings consist of two yellow lines (one solid and one dashed) 6 inches
(15cm) in width. The solid line is located on the nonmovement area side, while the dashed yellow line is located
on the movement area side. The nonmovement boundary marking area is shown in FIG 2−3−21.
2−3−22 Airport Marking Aids and Signs
2/20/25 AIM
FIG 2−3−20
Ground Receiver Checkpoint Markings
1. WHITE
2. YELLOW
3. YELLOW ARROW ALIGNED TOWARD THE FACILITY
4. INTERIOR OF CIRCLE BLACK (CONCRETE SURFACE ONLY)
5. CIRCLE MAY BE BORDERED ON INSIDE AND OUTSIDE WITH
6" BLACK BAND IF NECESSARY FOR CONTRAST
FIG 2−3−21
Nonmovement Area Boundary Markings
DASHED LINE ON
MOVEMENT SIDE BOTH LINES
ARE YELLOW
SOLID LINE ON
NONMOVEMENT
SIDE
FIG 2−3−22
Closed or Temporarily Closed Runway and Taxiway Markings
d. Marking and Lighting of Permanently Closed Runways and Taxiways. For runways and taxiways
which are permanently closed, the lighting circuits will be disconnected. The runway threshold, runway
designation, and touchdown markings are obliterated and yellow crosses are placed at each end of the runway
and at 1,000 foot intervals. (See FIG 2−3−22.)
Airport Marking Aids and Signs 2−3−23
AIM 2/20/25
FIG 2−3−23
Helicopter Landing Areas
e. Temporarily Closed Runways and Taxiways. To provide a visual indication to pilots that a runway is
temporarily closed, crosses are placed on the runway only at each end of the runway. The crosses are yellow in
color. (See FIG 2−3−22.)
1. A raised lighted yellow cross may be placed on each runway end in lieu of the markings described in
Subparagraph e,Temporarily Closed Runways and Taxiways, to indicate the runway is closed.
2. A visual indication may not be present depending on the reason for the closure, duration of the closure,
airfield configuration, and the existence and the hours of operation of an airport traffic control tower. Pilots
should check NOTAMs and the Automated Terminal Information System (ATIS) for local runway and taxiway
closure information.
3. Temporarily closed taxiways are usually treated as hazardous areas, in which no part of an aircraft may
enter, and are blocked with barricades. However, as an alternative, a yellow cross may be installed at each
entrance to the taxiway.
f. Helicopter Landing Areas. The markings illustrated in FIG 2−3−23 are used to identify the landing and
takeoff area at a public use heliport and hospital heliport. The letter “H” in the markings is oriented to align with
the intended direction of approach. FIG 2−3−23 also depicts the markings for a closed airport.
2−3−7. Airport Signs
There are six types of signs installed on airfields: mandatory instruction signs, location signs, direction signs,
destination signs, information signs, and runway distance remaining signs. The characteristics and use of these
signs are discussed in paragraph 2 −3−8, Mandatory Instruction Signs, through paragraph 2 −3−13, Runway
Distance Remaining Signs.
REFERENCE−
AC150/5340−18, Standards for Airport Sign Systems for Detailed Information on Airport Signs.
2−3−24 Airport Marking Aids and Signs
2/20/25 AIM
FIG 2−3−24
Runway Holding Position Sign
FIG 2−3−25
Holding Position Sign at Beginning of Takeoff Runway
2−3−8. Mandatory Instruction Signs
a. These signs have a red background with a white inscription and are used to denote:
1. An entrance to a runway or critical area; and
2. Areas where an aircraft is prohibited from entering.
b. Typical mandatory signs and applications are:
1. Runway Holding Position Sign. This sign is located at the holding position on taxiways that intersect
a runway or on runways that intersect other runways. The inscription on the sign contains the designation of the
intersecting runway, as shown in FIG 2−3−24. The runway numbers on the sign are arranged to correspond to
the respective runway threshold. For example, “15−33” indicates that the threshold for Runway 15 is to the left
and the threshold for Runway 33 is to the right.
(a) On taxiways that intersect the beginning of the takeoff runway, only the designation of the takeoff
runway may appear on the sign (as shown in FIG 2−3−25), while all other signs will have the designation of both
runway directions.
Airport Marking Aids and Signs 2−3−25
AIM 2/20/25
FIG 2−3−26
Holding Position Sign for a Taxiway that Intersects the Intersection of Two Runways
FIG 2−3−27
Holding Position Sign for Runway Approach and Departure Areas
(b) If the sign is located on a taxiway that intersects the intersection of two runways, the designations for
both runways will be shown on the sign along with arrows showing the approximate alignment of each runway,
as shown in FIG 2−3−26. In addition to showing the approximate runway alignment, the arrow indicates the
direction to the threshold of the runway whose designation is immediately next to the arrow.
(c) A runway holding position sign on a taxiway will be installed adjacent to holding position markings
on the taxiway pavement. On runways, holding position markings will be located only on the runway pavement
adjacent to the sign, if the runway is normally used by ATC for “Land, Hold Short” operations or as a taxiway.
The holding position markings are described in paragraph 2−3−5, Holding Position Markings.
2. Runway Approach Area Holding Position Sign. At some airports, it is necessary to hold an aircraft
on a taxiway located in the approach or departure area for a runway so that the aircraft does not interfere with
operations on that runway. FIG 2−3−15 depicts common situations. A sign with the runway designation(s) and
the protected area(s) will be located at applicable holding positions on the taxiway. For locations protecting only
the approach area, the holding position on the taxiway includes a sign identifying the approach end runway
designation (e.g., 15) followed by a dash (−) and the letters “APCH”. For locations protecting both the approach
and departure areas, the holding position on the taxiway includes a sign with the approach end runway
2−3−26 Airport Marking Aids and Signs
2/20/25 AIM
designation and letters “APCH” followed by a dash (−), the departure end runway designation and the letters
“DEP”. The arrangement of the runway designations and protected areas legend on the sign reflects the
orientation of the runway as viewed from the holding position. Holding position markings in accordance with
paragraph 2−3−5, Holding Position Markings, are co−located on the taxiway pavement in line with the sign.
Examples of these signs are shown in FIG 2−3−27.
FIG 2−3−28
Holding Position Sign for ILS Critical Area
FIG 2−3−29
Sign Prohibiting Aircraft Entry into an Area
3. ILS Critical Area Holding Position Sign. At some airports, when the instrument landing system is
being used, it is necessary to hold an aircraft on a taxiway at a location other than the holding position described
in Paragraph 2 −3−5, Holding Position Markings. In these situ ations, the holding position sign for these
operations will have the inscription “ILS” and be located adjacent to the holding position marking on the taxiway
described in paragraph 2−3−5. An example of this sign is shown in FIG 2−3−28.
4. No Entry Sign. This sign, shown in FIG 2−3−29, prohibits an aircraft from entering an area. Typically,
this sign would be located on a taxiway intended to be used in only one direction or at the intersection of vehicle
roadways with runways, taxiways, or aprons where the roadway may be mistaken as a taxiway or other aircraft
movement surface.
NOTE−
Holding position signs provide the pilot with a visual cue as to the location of the holding position marking.
REFERENCE−
AIM, Para 2−3−5, Holding Position Markings.
Airport Marking Aids and Signs 2−3−27
AIM 2/20/25
FIG 2−3−30
Taxiway Location Sign
FIG 2−3−31
Taxiway Location Sign Collocated with Runway Holding Position Sign
2−3−9. Location Signs
a. Location signs are used to identify either a taxiway or runway on which the aircraft is located. Other location
signs provide a visual cue to pilots to assist them in determining when they have exited an area. The various
location signs are described below.
1. Taxiway Location Sign. This sign has a black background with a yellow inscription and yellow border,
as shown in FIG 2−3−30. The inscription is the designation of the taxiway on which the aircraft is located. These
signs are installed along taxiways either by themselves or in conjunction with direction signs or runway holding
position signs. (See FIG 2−3−35 and FIG 2−3−31.)
2−3−28 Airport Marking Aids and Signs
2/20/25 AIM
FIG 2−3−32
Runway Location Sign
FIG 2−3−33
Runway Boundary Sign
2. Runway Location Sign. This sign has a black background with a yellow inscription and yellow border,
as shown in FIG 2−3−32. The inscription is the designation of the runway on which the aircraft is located. These
signs are intended to complement the information available to pilots through their magnetic compass and
typically are installed where the proximity of two or more runways to one another could cause pilots to be
confused as to which runway they are on.
3. Runway Boundary Sign. This sign has a yellow background with a black inscription with a graphic
depicting the pavement holding position marking, as shown in FIG 2−3−33. This sign, which faces the runway
and is visible to the pilot exiting the runway, is located adjacent to the holding position marking on the pavement.
The sign is intended to provide pilots with another visual cue which they can use as a guide in deciding when
they are “clear of the runway.”
Airport Marking Aids and Signs 2−3−29
AIM 2/20/25
FIG 2−3−34
ILS Critical Area Boundary Sign
4. ILS Critical Area Boundary Sign. This sign has a yellow background with a black inscription with a
graphic depicting the ILS pavement holding position marking as shown in FIG 2−3−34. This sign is located
adjacent to the ILS holding position marking on the pavement and can be seen by pilots leaving the critical area.
The sign is intended to provide pilots with another visual cue which they can use as a guide in deciding when
they are “clear of the ILS critical area.”
2−3−10. Direction Signs
a. Direction signs have a yellow background with a black inscription. The inscription identifies the
designation(s) of the intersecting taxiway(s) leading out of the intersection that a pilot would normally be
expected to turn onto or hold short of. Each designation is accompanied by an arrow indicating the direction of
the turn.
b. Except as noted in subparagraph e, each taxiway designation shown on the sign is accompanied by only
one arrow. When more than one taxiway designation is shown on the sign, each designation and its associated
arrow is separated from the other taxiway designations by either a vertical message divider or a taxiway location
sign as shown in FIG 2−3−35.
c. Direction signs are normally located on the left prior to the intersection. When used on a runway to indicate
an exit, the sign is located on the same side of the runway as the exit. FIG 2−3−36 shows a direction sign used
to indicate a runway exit.
d. The taxiway designations and their associated arrows on the sign are arranged clockwise starting from the
first taxiway on the pilot’s left. (See FIG 2−3−35.)
e. If a location sign is located with the direction signs, it is placed so that the designations for all turns to the
left will be to the left of the location sign; the designations for continuing straight ahead or for all turns to the
right would be located to the right of the location sign. (See FIG 2−3−35.)
f. When the intersection is comprised of only one crossing taxiway, it is permissible to have two arrows
associated with the crossing taxiway, as shown in FIG 2−3−37. In this case, the location sign is located to the left
of the direction sign.
2−3−30 Airport Marking Aids and Signs
2/20/25 AIM
FIG 2−3−35
Direction Sign Array with Location Sign on Far Side of Intersection
FIG 2−3−36
Direction Sign for Runway Exit
Airport Marking Aids and Signs 2−3−31
AIM 2/20/253/15/077110.65R CHG 2AIM 8/7/25
FIG 2−3−37
Direction Sign Array for Simple Intersection
2−3−11. Destination Signs
a. Destination signs have a yellow background with a black inscription indicating a taxi route to a destination
on the airport. These signs supplement standard taxiway direction signs to optimize taxi paths to specific areas
of the airport.
b. Destination signs always have an arrow showing the direction of the taxi route to the destination indicated
on the sign. Where the destination sign arrow indicates a turn, the sign location is prior to the intersection. The
sign may reside on the opposite side of an inters ection for straight ahead paths and for ending taxiway
intersections.
c. Inbound destination signs identify a taxi path to specific areas of the airport. Sign legends are typically short
descriptions or abbreviations of the destination. FIG 2−3−38 shows examples of typical inbound destination
signs. Common sign legends include:
1. APRON. General parking, servicing, and loading areas
(a) FBO Apron. An apron where itinerant general aviation operators can park their aircraft and expect
to have access to traditional Fixed Base Operator services subject to terms and conditions.
(b) GA Transient Apron. An apron where itinerant general aviation operators can park their aircraft
without FBO services and subject to terms and conditions.
(c) GA Tenant Apron. An area designated for parking of based general aviation aircraft, e.g., tie down
area.
(d) North/South/East/West Apron. An apron designation describing relative location on the airport.
2. CARGO. Areas set aside for cargo handling.
2−3−32 Airport Marking Aids and Signs
AIM2/20/258/7/25 AIM
3. CIVIL. Areas set aside for civil aircraft.
4. FUEL. Areas where aircraft receive fuel or related services.
5. INTL. Areas set aside for handling international flights.
6. MIL. Areas set aside for military aircraft.
(a) ANG. Area reserved for Air National Guard
(b) USN. Area reserved for U.S. Navy
7. PARKING. Alternative name for apron area.
8. PAX. Areas set aside for passenger handling.
9. RAMP. Name synonymous with APRON.
10. TERM. Gate positions at which aircraft load or unload passengers and cargo.
d. Outbound destination signs identify the general direction to departure runways. The sign legend consists
of direction arrow(s) and the applicable runway designations. FIG 2−3−39 is an example of a typical outbound
destination sign.
e. When a sign indicates the inscription for two or more destinations having a common taxi route, a “dot” (•)
separates the destinations and one arrow indicates the direction of the taxi path, as shown in FIG 2−3−39.
f. When a sign shows the inscription for two or more destinations having different taxiing routes, each
destination will have its own arrow to indicate the taxi direction. A vertical black message divider separates each
destination, as shown in FIG 2−3−40.
Airport Marking Aids and Signs 2−3−33
AIM 2/20/253/15/077110.65R CHG 2AIM 8/7/25
FIG 2−3−38
Inbound Destination Sign Example
FIG 2−3−39
Outbound Destination Sign for Common Taxi Route to Two Separate Runways
2−3−34 Airport Marking Aids and Signs
AIM2/20/258/7/25 AIM
FIG 2−3−40
Destination Sign for Different Taxiing Routes to Two Runways
2−3−12. Information Signs
Information signs have a yellow background with a black inscription. They are used to provide the pilot with
information on such things as areas that cannot be seen from the control tower, applicable radio frequencies, and
noise abatement procedures. The airport operator determines the need, size, and location for these signs.
2−3−13. Runway Distance Remaining Signs
Runway distance remaining signs have a black background with a white numeral inscription and may be installed
along one or both side(s) of the runway. The number on the signs indicates the distance (in thousands of feet)
of landing runway remaining. The last sign (i.e., the sign with the numeral “1”) will be located at least 950 feet
from the runway end. FIG 2−3−41 shows an example of a runway distance remaining sign.
FIG 2−3−41
Runway Distance Remaining Sign Indicating 3,000 feet of Runway Remaining
2−3−14. Aircraft Arresting Systems
a. Certain airports are equipped with a means of rapidly stopping military aircraft on a runway. This
equipment, normally referred to as EMERGENCY ARRESTING GEAR, generally consists of pendant cables
Airport Marking Aids and Signs 2−3−35
AIM 2/20/253/15/077110.65R CHG 2AIM 8/7/25
supported over the runway surface by rubber “donuts.” Although most devices are located in the overrun areas,
a few of these arresting systems have cables stretched over the operational areas near the ends of a runway.
b. Arresting cables which cross over a runway require special markings on the runway to identify the cable
location. These markings consist of 10 feet diameter solid circles painted “identification yellow,” 30 feet on
center, perpendicular to the runway centerline across the entire runway width. Additional details are contained
in AC 150/5220−9, Aircraft Arresting Systems for Joint Civil/Military Airports.
NOTE−
Aircraft operations on the runway are not restricted by the installation of aircraft arresting devices.
c. Engineered Materials Arresting Systems (EMAS) . EMAS, which is constructed of high
energy−absorbing materials of selected strength, is located in the safety area beyond the end of the runway.
EMAS will be marked with yellow chevrons. EMAS is designed to crush under the weight of commercial aircraft
and will exert deceleration forces on the landing gear. These systems do not affect the normal landing and takeoff
of airplanes. More information concerning EMAS is in AC 150/5220−22, Engineered Materials Arresting
Systems (EMAS) for Aircraft Overruns.
NOTE−
EMAS may be located as close as 35 feet beyond the end of the runway. Aircraft and ground vehicles should never taxi or
drive across the EMAS or beyond the end of the runway if EMAS is present.
FIG 2−3−42
Engineered Materials Arresting System (EMAS)
2−3−15. Security Identification Display Area (SIDA)
a. Security Identification Display Areas (SIDA) are limited access areas that require a badge issued in
accordance with procedures in 49 CFR part 1542. A SIDA can include the Air Operations Area (AOA), e.g.,
aircraft movement area or parking area, or a Secured Area, such as where commercial passengers enplane. The
AOA may not be a SIDA, but a Secured Area is always a SIDA. Movement through or into a SIDA is prohibited
without authorization and proper identification being displayed. If you are unsure of the location of a SIDA,
contact the airport authority for additional information. Airports that have a SIDA will have a description and
map detailing boundaries and pertinent features available.
2−3−36 Airport Marking Aids and Signs