Introduction
The National Airspace System (NAS) is the network of
United States airspace: air navigation facilities, equipment,
services, airports or landing areas, aeronautical charts,
information/services, rules, regulations, procedures, technical
information, manpower, and material. Included are system
components shared jointly with the military. The system’s
present configuration is a reflection of the technological
advances concerning the speed and altitude capability of jet
aircraft, as well as the complexity of microchip and satellite-
based navigation equipment. To conform to international
aviation standards, the United States adopted the primary
elements of the classification system developed by the
International Civil Aviation Organization (ICAO).
This chapter is a general discussion of airspace classification;
en route, terminal, and approach procedures; and operations
within the NAS. Detailed information on the classification
of airspace, operating procedures, and restrictions is found
in the Aeronautical Information Manual (AIM).
The National
Airspace System
Chapter 1
Airspace Classification
Airspace in the United States [Figure 1-1] is designated
as follows:
1. Class A. Generally, airspace from 18,000 feet mean
sea level (MSL) up to and including flight level (FL)
600, including the airspace overlying the waters
within 12 nautical miles (NM) of the coast of the
48 contiguous states and Alaska. Unless otherwise
authorized, all pilots must operate their aircraft under
instrument flight rules (IFR).
2. Class B. Generally, airspace from the surface to 10,000
feet MSL surrounding the nation’s busiest airports in
terms of airport operations or passenger enplanements.
The configuration of each Class B airspace area is
individually tailored, consists of a surface area and two
or more layers (some Class B airspace areas resemble
upside-down wedding cakes), and is designed to
contain all published instrument procedures once
an aircraft enters the airspace. An air traffic control
(ATC) clearance is required for all aircraft to operate
in the area, and all aircraft that are so cleared receive
separation services within the airspace.
3. Class C. Generally, airspace from the surface to 4,000
feet above the airport elevation (charted in MSL)
surrounding those airports that have an operational
control tower are serviced by a radar approach control
and have a certain number of IFR operations or
passenger enplanements. Although the configuration
of each Class C area is individually tailored, the
airspace usually consists of a surface area with a
5 NM radius, an outer circle with a 10 NM radius
that extends from 1,200 feet to 4,000 feet above the
airport elevation and an outer area. Each aircraft
must establish two-way radio communications with
the ATC facility providing air traffic services prior
to entering the airspace and thereafter maintain those
communications while within the airspace.
4. Class D. Generally, airspace from the surface to 2,500
feet above the airport elevation (charted in MSL)
surrounding those airports that have an operational
control tower. The configuration of each Class D
airspace area is individually tailored and, when
instrument procedures are published, the airspace
normally designed to contain the procedures. Arrival
extensions for instrument approach procedures (IAPs)
may be Class D or Class E airspace. Unless otherwise
authorized, each aircraft must establish two-way radio
communications with the ATC facility providing
air traffic services prior to entering the airspace and
thereafter maintain those communications while in
the airspace.
5. Class E. Generally, if the airspace is not Class A, B,
C, or D, and is controlled airspace, then it is Class E
airspace. Class E airspace extends upward from either
the surface or a designated altitude to the overlying
or adjacent controlled airspace. When designated as a
surface area, the airspace is configured to contain all
instrument procedures. Also in this class are federal
airways, airspace beginning at either 700 or 1,200 feet
above ground level (AGL) used to transition to and
from the terminal or en route environment, and en
route domestic and offshore airspace areas designated
below 18,000 feet MSL. Unless designated at a lower
altitude, Class E airspace begins at 14,500 MSL over
the United States, including that airspace overlying the
waters within 12 NM of the coast of the 48 contiguous
states and Alaska, up to but not including 18,000 feet
MSL, and the airspace above FL 600.
6. Class G. Airspace not designated as Class A, B, C,
D, or E. Class G airspace is essentially uncontrolled
by ATC except when associated with a temporary
control tower.
Special Use Airspace
Special use airspace is the designation for airspace in which
certain activities must be confined or where limitations may
be imposed on aircraft operations that are not part of those
activities. Certain special use airspace areas can create
limitations on the mixed use of airspace. The special use
airspace depicted on instrument charts includes the area name
or number, effective altitude, time and weather conditions
of operation, the controlling agency, and the chart panel
location. On National Aeronautical Navigation Products
(AeroNav Products) en route charts, this information is
available on one of the end panels.
Prohibited areas contain airspace of defined dimensions
within which the flight of aircraft is prohibited. Such areas
are established for security or other reasons associated with
the national welfare. These areas are published in the Federal
Register and are depicted on aeronautical charts. The area is
charted as a “P” followed by a number (e.g., “P-123”).
Restricted areas are areas where operations are hazardous to
nonparticipating aircraft and contain airspace within which
the flight of aircraft, while not wholly prohibited, is subject
to restrictions. Activities within these areas must be confined
because of their nature, or limitations may be imposed upon
aircraft operations that are not a part of those activities, or
both. Restricted areas denote the existence of unusual, often
invisible, hazards to aircraft (e.g., artillery firing, aerial
gunnery, or guided missiles). IFR flights may be authorized
to transit the airspace and are routed accordingly. Penetration
14,500' MSL
Nontowered
airport with
no instrument
approach
Nontowered
airport with
instrument
approach
700'
AGL700'
AGL
18,000' MSL
FL 600
1,200'
AGL
700'
AGL
1,200'
AGL
1,200'
AGL
(Not to scale)
Class A
Class B
Class C
Class D
Class E
Class G
Class G Class G Class G
*Exception: temporary tower or control tower present
**True only below 10,000 feet
True only during day at or below 1,200 feet AGL (see 14 CFR part 91)
Class A Class B Class C Class D Class E Class G
ATC clearance
Instrument
Rating
Yes
No
N/A
N/A
N/A
Yes
N/A
ATC clearance
Private or Student
certification—
local restrictions
apply.
Yes
Yes
3 statute miles
Clear of clouds
All
Yes
Radar
Instrument
approaches
Weather
Control tower
High density
Prior two-way
communications
Student
certificate
Yes
Yes
3 statute miles
500' below,
1,000' above,
2,000' horizontal
IFR aircraft
Yes
Radar
Instrument
approaches
Weather
Control tower
Prior two-way
communications
Student
certificate
Yes
Yes
3 statute miles
500' below,
1,000' above,
2,000' horizontal
Runway
operations
Workload
permitting
Instrument
approaches
Weather
Control tower
Prior two-way
communications*
Student
certificate
Yes, under IFR
flight plan*
Yes
3 statute miles**
500' below,**
1,000' above,
2,000' horizontal
None
Workload
permitting
Instrument
approaches
Weather
None
Student
certificate
None
N/A
1 statute mile†
Clear of clouds†
None
Workload
permitting
Control tower
AGL—above ground level
FL—flight level
MSL—mean sea level
Figure 1-1. Airspace classifications.
of restricted areas without authorization from the using
or controlling agency may be extremely hazardous to the
aircraft and its occupants. ATC facilities apply the following
procedures when aircraft are operating on an IFR clearance
(including those cleared by ATC to maintain visual flight
rules (VFR)-On-Top) via a route that lies within joint-use
restricted airspace:
1. If the restricted area is not active and has been released
to the Federal Aviation Administration (FAA), the
ATC facility will allow the aircraft to operate in the
restricted airspace without issuing specific clearance
for it to do so.
2. If the restricted area is active and has not been
released to the FAA, the ATC facility will issue a
clearance that will ensure the aircraft avoids the
restricted airspace.
Restricted areas are charted with an “R” followed by a
number (e.g., “R-5701”) and are depicted on the en route
chart appropriate for use at the altitude or FL being flown.
Warning areas are similar in nature to restricted areas;
however, the U.S. Government does not have sole jurisdiction
over the airspace. A warning area is airspace of defined
dimensions, extending from 12 NM outward from the coast of
the United States, containing activity that may be hazardous
to nonparticipating aircraft. The purpose of such areas is
to warn nonparticipating pilots of the potential danger. A
warning area may be located over domestic or international
waters or both. The airspace is designated with a “W”
followed by a number (e.g., “W-123”).
Military operations areas (MOAs) consist of airspace with
defined vertical and lateral limits established for the purpose
of separating certain military training activities from IFR
traffic. Whenever an MOA is being used, nonparticipating
IFR traffic may be cleared through an MOA if IFR separation
can be provided by ATC. Otherwise, ATC will reroute or
restrict nonparticipating IFR traffic. MOAs are depicted on
sectional, VFR terminal area, and en route low altitude charts
and are not numbered (e.g., “Boardman MOA”).
Alert areas are depicted on aeronautical charts with an
“A” followed by a number (e.g., “A-123”) to inform
nonparticipating pilots of areas that may contain a high
volume of pilot training or an unusual type of aerial activity.
Pilots should exercise caution in alert areas. All activity
within an alert area shall be conducted in accordance with
regulations, without waiver, and pilots of participating
aircraft, as well as pilots transiting the area, shall be equally
responsible for collision avoidance.
Military Training Routes (MTRs) are routes used by military
aircraft to maintain proficiency in tactical flying. These routes
are usually established below 10,000 feet MSL for operations
at speeds in excess of 250 knots. Some route segments may
be defined at higher altitudes for purposes of route continuity.
Routes are identified as IFR (IR) and VFR (VR) followed by
a number. MTRs with no segment above 1,500 feet AGL are
identified by four number characters (e.g., IR1206, VR1207).
MTRs that include one or more segments above 1,500 feet
AGL are identified by three number characters (e.g., IR206,
VR207). IFR low altitude en route charts depict all IR routes
and all VR routes that accommodate operations above 1,500
feet AGL. IR routes are conducted in accordance with IFR
regardless of weather conditions.
Temporary flight restrictions (TFRs) are put into effect
when traffic in the airspace would endanger or hamper air
or ground activities in the designated area. For example, a
forest fire, chemical accident, flood, or disaster-relief effort
could warrant a TFR, which would be issued as a Notice to
Airmen (NOTAM).
National Security Areas (NSAs) consist of airspace with
defined vertical and lateral dimensions established at
locations where there is a requirement for increased security
and safety of ground facilities. Flight in NSAs may be
temporarily prohibited by regulation under the provisions of
Title 14 of the Code of Federal Regulations (14 CFR) part 99
and prohibitions will be disseminated via NOTAM.
Federal Airways
The primary means for routing aircraft operating under
IFR is the Federal Airways System. Each Federal airway is
based on a centerline that extends from one navigational aid
(NAVAID)/waypoint/fix/intersection to another NAVAID/
waypoint/fix/intersection specified for that airway. A Federal
airway includes the airspace within parallel boundary lines
4 NM to each side of the centerline. As in all instrument
flight, courses are magnetic, and distances are in NM. The
airspace of a Federal airway has a floor of 1,200 feet AGL,
unless otherwise specified. A Federal airway does not include
the airspace of a prohibited area.
Victor airways include the airspace extending from 1,200
feet AGL up to, but not including 18,000 feet MSL. The
airways are designated on sectional and IFR low altitude en
route charts with the letter “V” followed by a number (e.g.,
“V23”). Typically, Victor airways are given odd numbers
when oriented north/south and even numbers when oriented
east/west. If more than one airway coincides on a route
segment, the numbers are listed serially (e.g., “V287-495-
500”). [Figure 1-2]
Figure 8-2. Victor airways, and charted IFR altitudes.
Altitude change
V287-495-500
confluence of airways
V520 (even)
oriented east/west
Victor Airway V23
V595 MRA 9300
No altitude change
V287 MOCA *3400
V165 (odd)
oriented north/south
Altitude change
Figure 1-2. Victor airways and charted IFR altitudes.
Jet routes exist only in Class A airspace, from 18,000 feet MSL
to FL 450, and are depicted on high-altitude en route charts.
The letter “J” precedes a number to label the airway (e.g., J12).
Area navigation (RNAV) routes have been established in
both the low-altitude and the high-altitude structures in recent
years and are depicted on the en route low and high chart
series. High altitude RNAV routes are identified with a “Q”
prefix (except the Q-routes in the Gulf of Mexico) and low
altitude RNAV routes are identified with a “T” prefix. RNAV
routes and data are depicted in aeronautical blue.
In addition to the published routes, a random RNAV route
may be flown under IFR if it is approved by ATC. Random
RNAV routes are direct routes, based on RNAV capability,
between waypoints defined in terms of latitude/longitude
coordinates, degree-distance fixes, or offsets from established
routes/airways at a specified distance and direction.
Radar monitoring by ATC is required on all random
RNAV routes. These routes can only be approved in a
radar environment. Factors that are considered by ATC
in approving random RNAV routes include the capability
to provide radar monitoring and compatibility with traffic
volume and flow. ATC will radar monitor each flight;
however, navigation on the random RNAV route is the
responsibility of the pilot.
Other Routing
Preferred IFR routes have been established between
major terminals to guide pilots in planning their routes of
flight, minimizing route changes, and aiding in the orderly
management of air traffic on Federal airways. Low and high
altitude preferred routes are listed in the Airport/Facility
Directory (A/FD). To use a preferred route, reference the
departure and arrival airports; if a routing exists for your
flight, then airway instructions are listed.
Tower En Route Control (TEC) is an ATC program that
uses overlapping approach control radar services to provide
IFR clearances. By using TEC, a pilot is routed by airport
control towers. Some advantages include abbreviated filing
procedures and reduced traffic separation requirements. TEC
is dependent upon the ATC’s workload, and the procedure
varies among locales.
The latest version of Advisory Circular (AC) 90-91, North
American Route Program (NRP), provides guidance to users
of the NAS for participation in the NRP. All flights operating
at or above FL 290 within the conterminous United States
and Canada are eligible to participate in the NRP, the primary
purpose of which is to allow operators to plan minimum time/
cost routes that may be off the prescribed route structure. NRP
aircraft are not subject to route-limiting restrictions (e.g.,
published preferred IFR routes) beyond a 200 NM radius of
their point of departure or destination.
IFR En Route Charts
The objective of IFR en route flight is to navigate within the
lateral limits of a designated airway at an altitude consistent
with the ATC clearance. Your ability to fly instruments
safely and competently in the system is greatly enhanced by
understanding the vast array of data available to the pilot on
instrument charts. AeroNav Products maintains and produces
the charts for the U.S. Government.
En route high-altitude charts provide aeronautical information
for en route instrument navigation at or above 18,000 feet
MSL. Information includes the portrayal of Jet and RNAV
routes, identification and frequencies of radio aids, selected
airports, distances, time zones, special use airspace, and
related information. Established jet routes from 18,000 feet
MSL to FL 450 use NAVAIDs not more than 260 NM apart.
The charts are revised every 56 days.
To effectively depart from one airport and navigate en route
under instrument conditions, a pilot needs the appropriate
IFR en route low-altitude chart(s). The IFR low altitude en
route chart is the instrument equivalent of the sectional chart.
When folded, the cover of the AeroNav Products en route
chart displays an index map of the United States showing the
coverage areas. Cities near congested airspace are shown in
black type and their associated area chart is listed in the box
in the lower left-hand corner of the map coverage box. Also
noted is an explanation of the off-route obstruction clearance
altitude (OROCA). The effective date of the chart is printed
on the other side of the folded chart. Information concerning
MTRs is also included on the chart cover. The en route charts
are revised every 56 days.
When the AeroNav Products en route chart is unfolded, the
legend is displayed and provides information concerning
airports, NAVAIDs, communications, air traffic services,
and airspace.
Airport Information
Airport information is provided in the legend, and the
symbols used for the airport name, elevation, and runway
length are similar to the sectional chart presentation.
Associated city names are shown for public airports only.
FAA identifiers are shown for all airports. ICAO identifiers
are also shown for airports outside of the contiguous United
States. Instrument approaches can be found at airports with
blue or green symbols, while the brown airport symbol
denotes airports that do not have instrument approaches.
Stars are used to indicate the part-time nature of tower
operations, Automatic Terminal Information Service (ATIS)
frequencies, part-time or on request lighting facilities, and
part-time airspace classifications. A box after an airport name
with a “C” or “D” inside (e.g., ) indicates Class C and D
airspace, respectively, per Figure 1-3.
Charted IFR Altitudes
The minimum en route altitude (MEA) ensures a navigation
signal strong enough for adequate reception by the aircraft
navigation (NAV) receiver and obstacle clearance along the
airway. Communication is not necessarily guaranteed with
MEA compliance. The obstacle clearance, within the limits of
the airway, is typically 1,000 feet in non-mountainous areas
and 2,000 feet in designated mountainous areas. MEAs can
be authorized with breaks in the signal coverage; if this is
the case, the AeroNav Products en route chart notes “MEA
GAP” parallel to the affected airway. MEAs are usually
bidirectional; however, they can be single-directional. Arrows
are used to indicate the direction to which the MEA applies.
The minimum obstruction clearance altitude (MOCA), as the
name suggests, provides the same obstruction clearance as
an MEA; however, the NAV signal reception is ensured only
within 22 NM of the closest NAVAID defining the route. The
MOCA is listed below the MEA and indicated on AeroNav
Products charts by a leading asterisk (e.g., “*3400”—see
Figure 1-2, V287 at bottom left).
The minimum reception altitude (MRA) identifies the lowest
altitude at which an intersection can be determined from
an off-course NAVAID. If the reception is line-of-sight
based, signal coverage only extends to the MRA or above.
However, if the aircraft is equipped with distance measuring
equipment (DME) and the chart indicates the intersection can
be identified with such equipment, the pilot could define the
Figure 8-3. En route airport legend. Figure 1-3. En route airport legend.
fix without attaining the MRA. On AeroNav Products charts,
the MRA is indicated by the symbol and the altitude
preceded by “MRA” (e.g., “MRA 9300”). [Figure 1-2]
The minimum crossing altitude (MCA) is charted when
a higher MEA route segment is approached. The MCA is
usually indicated when a pilot is approaching steeply rising
terrain and obstacle clearance and/or signal reception is
compromised. In this case, the pilot is required to initiate a
climb so the MCA is reached by the time the intersection is
crossed. On AeroNav Products charts, the MCA is indicated
by the symbol , and the Victor airway number, altitude,
and the direction to which it applies (e.g. “V24 8000 SE”).
The maximum authorized altitude (MAA) is the highest
altitude at which the airway can be flown with assurance
of receiving adequate navigation signals. Chart depictions
appear as “MAA-15000.”
When an MEA, MOCA, and/or MAA change on a segment
other than at a NAVAID, a sideways “T” ( ) is depicted
on the chart. If there is an airway break without the symbol,
one can assume the altitudes have not changed (see the upper
left area of Figure 1-2). When a change of MEA to a higher
MEA is required, the climb may commence at the break,
ensuring obstacle clearance. [Figure 1-4]
Navigation Features
Types of NAVAIDs
Very high frequency omnidirectional ranges (VORs) are the
principal NAVAIDs that support the Victor and Jet airways.
Many other navigation tools are also available to the pilot.
For example, nondirectional beacons (NDBs) can broadcast
signals accurate enough to provide stand-alone approaches,
and DME allows the pilot to pinpoint a reporting point on the
airway. Though primarily navigation tools, these NAVAIDs
can also transmit voice broadcasts.
Tactical air navigation (TACAN) channels are represented
as the two- or three-digit numbers following the three-letter
identifier in the NAVAID boxes. The AeroNav Products
terminal procedures provide a frequency-pairing table for
the TACAN-only sites. On AeroNav Products charts, very-
high frequencies and ultra-high frequencies (VHF/UHF)
NAVAIDs (e.g., VORs) are depicted in black, while low
frequencies and medium frequencies (LF/MF) are depicted
as brown. [Figure 1-5]
Identifying Intersections
Intersections along the airway route are established by a variety
of NAVAIDs. An open triangle indicates the location of an
ATC reporting point at an intersection. If the triangle is solid
, a report is compulsory. [Figure 1-4] NDBs, localizers,
Figure 8-4b. Legend from en route low altitude chart.
Figure 1-4. Legend from en route low attitude chart, air traffic services and airspace information section.