OBS
3 33
2I
I5 I2
CRS 180°HDG 330°
LOC1
330°
As you begin procedure turn,
course needle shows
divergence from course
centerline increasing.
While on the procedure turn
inbound the course needle
becomes directional. Start
the turn to final approach
course when the needle
comes alive (needle moves
away from side) and is
moving toward center.
OBS
3 33
33 30
2I
I5I2
CRS 180°HDG 135°
LOC1
135°
OBS
21 15
NAV
GS
Instrument view is from
the pilot’s perspective,
and the movable card is
reset after each turn
OBS N
3 33
242II5
Maintain course with the
needle centered.
CRS 180°HDG 180°
LOC1
180° D
CRS 180°HDG 350°
LOC1
350°
OBS
3 33
2I I5
As you fly northbound of the
instrument runway (in this case
Localizer 18), with the radio
tuned to the Localizer
frequency (and identified), the
course needle is not directional.
Correct left to center the course
needle.
FAF
Localizer inbound
course is 180°
As you fly a missed approach
along the back course, the
needle, indicating “fly left” is
directional.
In this case fly left to get on
course.
OBS
3 33
242II5
CRS 180°HDG 180°
LOC1
180°
Figure 9-38. Localizer course indications. To follow indications displayed in the aircraft, start from A and proceed through E.
OBS
FR
TO
E S
15 12
NAV 1
Figure 7-35. Glide-slope receiver indications and aircraft displacement.
Outer Marker Beacons
Middle Marker Beacons
OBS
FR
TO
E S
15 12
NAV 1
Figure 9-39. A GS receiver indication and aircraft displacement. An analog system is on the left and the same indication on the Garmin
PFD on the right.
Some marker beacon receivers, to decrease weight and cost,
are designed without their own power supply. These units
utilize a power source from another radio in the avionics
stack, often the ADF. In some aircraft, this requires the ADF
to be turned on in order for the marker beacon receiver to
function, yet no warning placard is required. Another source
of trouble may be the “HIGH/LOW/OFF” three-position
switch, which both activates the receiver and selects receiver
sensitivity. Usually, the “test” feature only tests to see if
the light bulbs in the marker beacon lights are working.
Therefore, in some installations, there is no functional way
for the pilot to ascertain the marker beacon receiver is actually
on except to fly over a marker beacon transmitter and see if
a signal is received and indicated (e.g., audibly, and visually
via marker beacon lights).
Operational Errors
1. Failure to understand the fundamentals of ILS ground
equipment, particularly the differences in course
dimensions. Since the VOR receiver is used on the
localizer course, the assumption is sometimes made
that interception and tracking techniques are identical
when tracking localizer courses and VOR radials.
Remember that the CDI sensing is sharper and faster
on the localizer course.
2. Disorientation during transition to the ILS due to poor
planning and reliance on one receiver instead of on all
available airborne equipment. Use all the assistance
available; a single receiver may fail.
3. Disorientation on the localizer course, due to the first
error noted above.
4. Incorrect localizer interception angles. A large
interception angle usually results in overshooting
and possible disorientation. When intercepting,
if possible, turn to the localizer course heading
immediately upon the first indication of needle
movement. An ADF receiver is an excellent aid to
orient you during an ILS approach if there is a locator
or NDB on the inbound course.
5. Chasing the CDI and glidepath needles, especially
when you have not sufficiently studied the approach
before the flight.
Simplified Directional Facility (SDF)
The simplified directional facility (SDF) provides a final
approach course similar to the ILS localizer. The SDF course
may or may not be aligned with the runway and the course
may be wider than a standard ILS localizer, resulting in less
Figure 7-36. MLS coverage volumes, 3-D representation
40°L 40°R
20,000’
Figure 9-40. MLS coverage volumes, 3-D representation.
precision. Usable off-course indications are limited to 35°
either side of the course centerline. Instrument indications
in the area between 35° and 90° from the course centerline
are not controlled and should be disregarded.
The SDF must provide signals sufficient to allow satisfactory
operation of a typical aircraft installation within a sector
which extends from the center of the SDF antenna system
to distances of 18 NM covering a sector 10° either side of
centerline up to an angle 7° above the horizontal. The angle
of convergence of the final approach course and the extended
runway centerline must not exceed 30°. Pilots should note
this angle since the approach course originates at the antenna
site, and an approach continued beyond the runway threshold
would lead the aircraft to the SDF offset position rather than
along the runway centerline.
The course width of the SDF signal emitted from the
transmitter is fixed at either 6° or 12°, as necessary, to provide
maximum flyability and optimum approach course quality.
A three-letter identifier is transmitted in code on the SDF
frequency; there is no letter “I” (two dots) transmitted before
the station identifier, as there is with the LOC. For example,
the identifier for Lebanon, Missouri, SDF is LBO.
Localizer Type Directional Aid (LDA)
The localizer type directional aid (LDA) is of comparable
utility and accuracy to a localizer but is not part of a
complete ILS. The LDA course width is between 3° and 6°
and thus provides a more precise approach course than an
SDF installation. Some LDAs are equipped with a GS. The
LDA course is not aligned with the runway, but straight-in
minimums may be published where the angle between the
runway centerline and the LDA course does not exceed 30°. If
this angle exceeds 30°, only circling minimums are published.
The identifier is three letters preceded by “I” transmitted in
code on the LDA frequency. For example, the identifier for
Van Nuys, California, LDA is I-BUR.
Microwave Landing System (MLS)
The microwave landing system (MLS) provides precision
navigation guidance for exact alignment and descent of aircraft
on approach to a runway. It provides azimuth, elevation, and
distance. Both lateral and vertical guidance may be displayed
on conventional course deviation indicators or incorporated
into multipurpose flight deck displays. Range information
can be displayed by conventional DME indicators and also
incorporated into multipurpose displays. [Figure 9-40]
The system may be divided into five functions, which are
approach azimuth, back azimuth, approach elevation, range;
and data communications. The standard configuration of
MLS ground equipment includes an azimuth station to
perform functions as indicated above. In addition to providing
azimuth navigation guidance, the station transmits basic data,
which consists of information associated directly with the
operation of the landing system, as well as advisory data on
the performance of the ground equipment.
Approach Azimuth Guidance
The azimuth station transmits MLS angle and data on one
of 200 channels within the frequency range of 5031 to 5091
MHz. The equipment is normally located about 1,000 feet
beyond the stop end of the runway, but there is considerable
flexibility in selecting sites. For example, for heliport
operations the azimuth transmitter can be collocated with
the elevation transmitter. The azimuth coverage extends
laterally at least 40° on either side of the runway centerline
in a standard configuration, in elevation up to an angle of 15°
and to at least 20,000 feet, and in range to at least 20 NM.
MLS requires separate airborne equipment to receive and
process the signals from what is normally installed in general
aviation aircraft today. It has data communications capability,
and can provide audible information about the condition
of the transmitting system and other pertinent data such as
weather, runway status, etc. The MLS transmits an audible
identifier consisting of four letters beginning with the letter
M, in Morse code at a rate of at least six per minute. The
MLS system monitors itself and transmits ground-to-air data
messages about the system’s operational condition. During
periods of routine or emergency maintenance, the coded
identification is missing from the transmissions. At this time
there are only a few systems installed.
Required Navigation Performance
RNP is a navigation system that provides a specified level
of accuracy defined by a lateral area of confined airspace in
which an RNP-certified aircraft operates. The continuing
growth of aviation places increasing demands on airspace
capacity and emphasizes the need for the best use of the
available airspace. These factors, along with the accuracy of
modern aviation navigation systems and the requirement for
increased operational efficiency in terms of direct routings
and track-keeping accuracy, have resulted in the concept
of required navigation performance—a statement of the
navigation performance accuracy necessary for operation
within a defined airspace. RNP can include both performance
and functional requirements and is indicated by the RNP type.
These standards are intended for designers, manufacturers,
and installers of avionics equipment, as well as service
providers and users of these systems for global operations.
The minimum aviation system performance specification
(MASPS) provides guidance for the development of airspace
and operational procedures needed to obtain the benefits of
improved navigation capability. [Figure 9-41]
The RNP type defines the total system error (TSE) that
is allowed in lateral and longitudinal dimensions within
a particular airspace. The TSE, which takes account of
navigation system errors (NSE), computation errors, display
errors and flight technical errors (FTE), must not exceed the
specified RNP value for 95 percent of the flight time on any
part of any single flight. RNP combines the accuracy standards
laid out in the ICAO Manual (Doc 9613) with specific accuracy
requirements, as well as functional and performance standards,
for the RNAV system to realize a system that can meet future
air traffic management requirements. The functional criteria
for RNP address the need for the flightpaths of participating
aircraft to be both predictable and repeatable to the declared
levels of accuracy. More information on RNP is contained in
subsequent chapters.
The term RNP is also applied as a descriptor for airspace,
routes, and procedures (including departures, arrivals,
and IAPs). The descriptor can apply to a unique approach
procedure or to a large region of airspace. RNP applies to
navigation performance within a designated airspace and
includes the capability of both the available infrastructure
(navigation aids) and the aircraft.
RNP type is used to specify navigation requirements for the
airspace. The following are ICAO RNP Types: RNP-1.0,
RNP-4.0, RNP-5.0, and RNP-10.0. The required performance
is obtained through a combination of aircraft capability and
the level of service provided by the corresponding navigation
infrastructure. From a broad perspective:
Aircraft Capability + Level of Service = Access
In this context, aircraft capability refers to the airworthiness
certification and operational approval elements (including
avionics, maintenance, database, human factors, pilot
procedures, training, and other issues). The level of service
element refers to the NAS infrastructure, including published
routes, signal-in-space performance and availability, and air
traffic management. When considered collectively, these
elements result in providing access. Access provides the
desired benefit (airspace, procedures, routes of flight, etc.).
RNP levels are actual distances from the centerline of the
flightpath, which must be maintained for aircraft and obstacle
separation. Although additional FAA-recognized RNP
levels may be used for specific operations, the United States
currently supports three standard RNP levels:
• RNP 0.3 – Approach
• RNP 1.0 – Departure, Terminal
• RNP 2.0 – En route
RNP 0.3 represents a distance of 0.3 NM either side of a
specified flightpath centerline. The specific performance that
is required on the final approach segment of an instrument
approach is an example of this RNP level. At the present
time, a 0.3 RNP level is the lowest level used in normal
RNAV operations. Specific airlines, using special procedures,
are approved to use RNP levels lower than RNP 0.3, but
those levels are used only in accordance with their approved
operations specifications (OpsSpecs). For aircraft equipment to
qualify for a specific RNP type, it must maintain navigational
accuracy at least 95 percent of the total flight time.
Flight Management Systems (FMS)
A flight management system (FMS) is not a navigation
system in itself. Rather, it is a system that automates the
tasks of managing the onboard navigation systems. FMS may
perform other onboard management tasks, but this discussion
is limited to its navigation function.
FMS is an interface between flight crews and flightdeck
systems. FMS can be thought of as a computer with a large
database of airport and NAVAID locations and associated
data, aircraft performance data, airways, intersections,
DPs, and STARs. FMS also has the ability to accept and
store numerous user-defined WPs, flight routes consisting
of departures, WPs, arrivals, approaches, alternates, etc.
FMS can quickly define a desired route from the aircraft’s
current position to any point in the world, perform flight
plan computations, and display the total picture of the flight
route to the crew.
EN ROUTE
TERMINAL
FINAL APPROACH
2.0 NM
2.0 NM
1.0 NM
1.0 NM
0.3 NM
0.3 NM
APPROACH TERMINAL ENROUTE DEPARTURE
RNP 1.0RNP 2.0RNP 1.0RNP 0.3
Figure 9-41. Required navigation performance.
FMS also has the capability of controlling (selecting) VOR,
DME, and LOC NAVAIDs, and then receiving navigational
data from them. INS, LORAN, and GPS navigational data
may also be accepted by the FMS computer. The FMS may
act as the input/output device for the onboard navigation
systems, so that it becomes the “go-between” for the crew
and the navigation systems.
The Universal UNS-1 The Avidyne The Garmin system
Figure 9-42. Typical display and control unit(s) in general aviation. The Universal UNS-1 (left) controls and integrates all other systems. The
Avidyne (center) and Garmin systems (right) illustrate and are typical of completely integrated systems. Although the Universal CDU is not
typically found on smaller general aviation aircraft, the difference in capabilities of the CDUs and stand-alone sytems is diminishing each year.
Function of FMS
At startup, the crew programs the aircraft location, departure
runway, DP (if applicable), WPs defining the route, approach
procedure, approach to be used, and routing to alternate. This
may be entered manually, be in the form of a stored flight
plan, or be a flight plan developed in another computer and
transferred by disk or electronically to the FMS computer.
The crew enters this basic information in the control/display
unit (CDU). [Figure 9-42]
Once airborne, the FMS computer channels the appropriate
NAVAIDs and takes radial/distance information or
channels two NAVAIDs, taking the more accurate distance
information. FMS then indicates position, track, desired
heading, groundspeed, and position relative to desired track.
Position information from the FMS updates the INS. In more
sophisticated aircraft, the FMS provides inputs to the HSI,
RMI, glass flight deck navigation displays, head-up display
(HUD), autopilot, and autothrottle systems.
Head-Up Display (HUD)
The HUD is a display system that provides a projection of
navigation and air data (airspeed in relation to approach
reference speed, altitude, left/right and up/down GS) on a
transparent screen between the pilot and the windshield.
Other information may be displayed, including a runway
target in relation to the nose of the aircraft. This allows the
pilot to see the information necessary to make the approach
while also being able to see out the windshield, which
diminishes the need to shift between looking at the panel to
looking outside. Virtually any information desired can be
displayed on the HUD if it is available in the aircraft’s flight
computer and if the display is user definable. [Figure 9-43]
Radar Navigation (Ground-Based)
Radar works by transmitting a pulse of RF energy in a specific
direction. The return of the echo or bounce of that pulse from
a target is precisely timed. From this, the distance traveled
by the pulse and its echo is determined and displayed on a
radar screen in such a manner that the distance and bearing to
this target can be instantly determined. The radar transmitter
must be capable of delivering extremely high power levels
toward the airspace under surveillance, and the associated
radar receiver must be able to detect extremely small signal
levels of the returning echoes.
The radar display system provides the controller with a map-
like presentation upon which appear all the radar echoes
of aircraft within detection range of the radar facility. By
means of electronically-generated range marks and azimuth-
indicating devices, the controller can locate each radar target
with respect to the radar facility, or can locate one radar target
with respect to another.
Another device, a video-mapping unit, generates an actual
airway or airport map and presents it on the radar display
equipment. Using the video-mapping feature, the air traffic
controller not only can view the aircraft targets, but can see
these targets in relation to runways, navigation aids, and
hazardous ground obstructions in the area. Therefore, radar
becomes a NAVAID, as well as the most significant means
of traffic separation.
In a display presenting perhaps a dozen or more targets,
a primary surveillance radar system cannot identify one
specific radar target, and it may have difficulty “seeing” a
small target at considerable distance—especially if there is
a rain shower or thunderstorm between the radar site and
the aircraft. This problem is solved with the Air Traffic
Control Radar Beacon System (ATCRBS), sometimes
called secondary surveillance radar (SSR), which utilizes
a transponder in the aircraft. The ground equipment is an
interrogating unit, in which the beacon antenna is mounted
so it rotates with the surveillance antenna. The interrogating
Figure 9-43. Example of a head-up display (top) and a head-down
display (bottom). The head-up display presents information in front of
the pilot along his/her normal field of view while a head-down display
may present information beyond the normal head-up field of view.
unit transmits a coded pulse sequence that actuates the aircraft
transponder. The transponder answers the coded sequence
by transmitting a preselected coded sequence back to the
ground equipment, providing a strong return signal and
positive aircraft identification, as well as other special data
such as aircraft altitude.
Functions of Radar Navigation
The radar systems used by ATC are air route surveillance
radar (ARSR), airport surveillance radar (ASR), and
precision approach radar (PAR) and airport surface detection
equipment (ASDE). Surveillance radars scan through 360°
of azimuth and present target information on a radar display
located in a tower or center. This information is used
independently or in conjunction with other navigational aids
in the control of air traffic.
ARSR is a long-range radar system designed primarily to
cover large areas and provide a display of aircraft while en
route between terminal areas. The ARSR enables air route
traffic control center (ARTCC) controllers to provide radar
service when the aircraft are within the ARSR coverage.
In some instances, ARSR may enable ARTCC to provide
terminal radar services similar to but usually more limited
than those provided by a radar approach control.
ASR is designed to provide relatively short-range coverage
in the general vicinity of an airport and to serve as an
expeditious means of handling terminal area traffic through
observation of precise aircraft locations on a radarscope.
Nonprecision instrument approaches are available at airports
that have an approved surveillance radar approach procedure.
ASR provides radar vectors to the final approach course and
then azimuth information to the pilot during the approach.
In addition to range (distance) from the runway, the pilot
is advised of MDA, when to begin descent, and when the
aircraft is at the MDA. If requested, recommended altitudes
are furnished each mile while on final.
PAR is designed to be used as a landing aid displaying range,
azimuth, and elevation information rather than as an aid for
sequencing and spacing aircraft. PAR equipment may be
used as a primary landing aid, or it may be used to monitor
other types of approaches. Two antennas are used in the PAR
array: one scanning a vertical plane and the other scanning
horizontally. Since the range is limited to 10 miles, azimuth
to 20°, and elevation to 7°, only the final approach area is
covered. The controller’s scope is divided into two parts. The
upper half presents altitude and distance information, and the
lower half presents azimuth and distance.
PAR is a system in which a controller provides highly
accurate navigational guidance in azimuth and elevation
to a pilot. Pilots are given headings to fly to direct them to
and keep their aircraft aligned with the extended centerline
of the landing runway. They are told to anticipate glidepath
interception approximately 10–30 seconds before it occurs
and when to start descent. The published decision height (DH)
is given only if the pilot requests it. If the aircraft is observed
to deviate above or below the glidepath, the pilot is given
the relative amount of deviation by use of terms “slightly” or
“well” and is expected to adjust the aircraft’s rate of descent/
ascent to return to the glidepath. Trend information is also
issued with respect to the elevation of the aircraft and may
be modified by the terms “rapidly” and “slowly” (e.g., “well
above glidepath, coming down rapidly”).
Range from touchdown is given at least once each mile. If
an aircraft is observed by the controller to proceed outside
of specified safety zone limits in azimuth and/or elevation
and continue to operate outside these prescribed limits, the
pilot will be directed to execute a missed approach or to fly a
specified course unless the pilot has the runway environment
(runway, approach lights, etc.) in sight. Navigational
guidance in azimuth and elevation is provided to the pilot
until the aircraft reaches the published decision altitude (DA)/
DH. Advisory course and glidepath information is furnished
by the controller until the aircraft passes over the landing
threshold, at which point the pilot is advised of any deviation
from the runway centerline. Radar service is automatically
terminated upon completion of the approach.
Airport Surface Detection Equipment
Radar equipment is specifically designed to detect all
principal features on the surface of an airport, including
aircraft and vehicular traffic, and to present the entire image
on a radar indicator console in the control tower. It is used
to augment visual observation by tower personnel of aircraft
and/or vehicular movements on runways and taxiways.
Radar Limitations
1. It is very important for the aviation community to
recognize the fact that there are limitations to radar
service and that ATC may not always be able to issue
traffic advisories concerning aircraft which are not
under ATC control and cannot be seen on radar.
2. The characteristics of radio waves are such that they
normally travel in a continuous straight line unless
they are “bent” by abnormal atmospheric phenomena
such as temperature inversions; reflected or attenuated
by dense objects such as heavy clouds, precipitation,
ground obstacles, mountains, etc.; or screened by high
terrain features.
3. Primary radar energy that strikes dense objects is
reflected and displayed on the operator’s scope,
thereby blocking out aircraft at the same range and
greatly weakening or completely eliminating the
display of targets at a greater range.
4. Relatively low altitude aircraft are not seen if they are
screened by mountains or are below the radar beam
due to curvature of the Earth.
5. The amount of reflective surface of an aircraft
determines the size of the radar return. Therefore,
a small light airplane or a sleek jet fighter is more
difficult to see on primary radar than a large
commercial jet or military bomber.
6. All ARTCC radar in the conterminous United States
and many ASR have the capability to interrogate Mode
C and display altitude information to the controller
from appropriately-equipped aircraft. However,
a number of ASR do not have Mode C display
capability; therefore, altitude information must be
obtained from the pilot.