Page 128 - Planning and Design of Airports

Air Traf fic Management 97

The Organizational Hierarchy of Air Traffic Management
in the United States
In general, aircraft operate in what is known as the National Airspace
System (NAS). The NAS is defined by a series of air routes, airspace
classifications, and navigational aids. Aircraft operate within the NAS
under varying levels of air traffic control, based primarily on the
weather conditions and the type and amount of flight activity within
the area. In areas with very low volumes of flight activity during
excellent visibility conditions, aircraft may operate in the complete
absence of air traffic control, whereas in the busiest airspace or when
visibility is limited, aircraft may be under full “positive” control, only
being able to change speed, course, or altitude by direct orders from
an air traffic controller.
The NAS is operated and managed by a hierarchical organization
of air traffic control facilities. The specific purpose of the air traffic
control service is to prevent collisions between aircraft and on the
maneuvering area between aircraft and obstructions, to expedite and
maintain an orderly flow of air traffic [3].

The Air Traffic Control System Command Center
In the United States, air traffic control is managed on a macro level at
the air traffic control system command center (ATCSCC) in Herndon,
Virginia. In 2007, ATCSCC monitored an average of 25,000 flights per
day, with an average of 6000 flights airborne during peak periods. In
addition, ATCSCC manages flights planned 6 to 12 h in the future,
with the purpose of planning for limiting congestion within the
nation’s airspace. In doing so, ATCSCC has the authority to imple-
ment ground delay programs by dictating certain aircraft to remain at
their airports of departure to prevent further congestion in points of
the airspace or at airports suffering from delays due to weather or
heavy traffic volumes.

Air Route Traffic Control Centers
Air route traffic control centers (ARTCCs) have the responsibility of
controlling the movement of en route aircraft along the airways and jet
routes, and in other parts of the airspace. Each of the 21 air traffic
control centers within the United States has control of a defined geo-
2
graphical area which may be greater than 100,000 mi in size. At the
boundary point, which marks the limits of the control area of the
center, control of aircraft may be transferred to an adjacent center or an
approach control facility, or radar service may be terminated and VFR
aircraft are free to contact the next center. Air traffic control centers are
normally not located at airports. Air traffic control centers can also
provide approach control service to nontowered airports and to
nonterminal radar approach control airports.