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ground obstacles, mountains, etc.; or screened by high
terrain features.
3. Primary radar energy that strikes dense objects will be
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 will not be 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 will
determine the size of the radar return. Therefore, a small
light airplane or a sleek jet fighter will be more difficult
to see on primary radar than a large commercial jet or
military bomber.
7-51
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.
7-52
8-1
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 satellitebased
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 8
8-2
Airspace Classification
Airspace in the United States [Figure 8-1] 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, that 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 will normally be 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
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