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TM is apportioned into traffic management units
(TMUs), which monitor and balance traffic flows
within their areas of responsibility in accordance
with TM directives. TMUs help to ensure system
efficiency and effectiveness without compromising
safety, by providing the ATCSCC with advance
notice of planned outages and runway closures that
will impact the air traffic system, such as NAVAID
and radar shutdowns, runway closures, equipment
and computer malfunctions, and procedural changes.
[Figure 1-7 on page 1-8]
HOW THE SYSTEM COMPONENTS
WORK TOGETHER
The NAS comprises the common network of U.S. airspace, air navigation facilities, equipment, services,
airports and landing areas, aeronautical charts, information and services, rules and regulations, procedures,
technical information, manpower, and material.
Included are system components shared jointly with
the military. The underlying demand for air commerce
is people’s desire to travel for business and pleasure
and to ship cargo by air. This demand grows with the
economy independent of the capacity or performance
of the NAS. As the economy grows, more and more
people want to fly, whether the system can handle it or
Figure 1-6. Approximately 5,000 Aircraft in ATC System at One Time.
1-8
not. Realized demand refers to flight plans filed by the
airlines and other airspace users to access the system.
It is moderated by the airline’s understanding of the
number of flights that can be accommodated without
encountering unacceptable delay, and is limited by
the capacity for the system.
USERS
Despite a drop in air traffic after the September 11 terrorist attacks, air travel returned to 2000 levels within
three years and exceeded them in 2004. Industry forecasts predict growth in airline passenger traffic of
around 4.3 percent per year. Commercial aviation is
expected to exceed one billion passengers by 2015. The
system is nearing the point of saturation, with limited
ability to grow unless major changes are brought about.
Adding to the growth challenge, users of the NAS cover a
wide spectrum in pilot skill and experience, aircraft types,
and air traffic service demands, creating a challenge to the
NAS to provide a variety of services that accommodate all
types of traffic. NAS users range from professional airline,
commuter, and corporate pilots to single-engine piston
pilots, as well as owner-operators of personal jets to military
jet fighter trainees.
AIRLINES
Though commercial air carrier aircraft traditionally
make up less than 5 percent of the civil aviation fleet,
they account for about 30 percent of the instrument
operations flown in civil aviation. Commercial air carriers are the most homogenous category of airspace users,
although there are some differences between U.S. trunk
carriers (major airlines)
and regional airlines
(commuters) in terms
of demand for ATC
services. Generally,
U.S. carriers operate
large, high performance airplanes that
cruise at altitudes
above 18,000 feet.
Conducted exclusively
under IFR, airline
flights follow established schedules and
operate in and out of
larger and betterequipped airports. In
terminal areas, however, they share airspace
and facilities with all
types of traffic and must
compete for airport
access with other users.
Airline pilots are highly
proficient and thoroughly familiar with the rules and procedures under which they must operate.
Some airlines are looking toward the use of larger
aircraft, with the potential to reduce airway and terminal congestion by transporting more people in
fewer aircraft. This is especially valuable at major
hub airports, where the number of operations
exceeds capacity at certain times of day. On the
other hand, the proliferation of larger aircraft also
requires changes to terminals (e.g., double-decker
jetways and better passenger throughput), rethinking
of rescue and fire-fighting strategies, taxiway fillet
changes, and perhaps stronger runways and
taxiways.
Commuter airlines also follow established schedules
and are flown by professional pilots. Commuters
characteristically operate smaller and lower performance aircraft in airspace that must often be shared by
general aviation (GA) aircraft, including visual flight
rules (VFR) traffic. As commuter operations have
grown in volume, they have created extra demands on
the airport and ATC systems. At one end, they use hub
airports along with other commercial carriers, which
contributes to growing congestion at major air traffic
　
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