Updated:11:00 AM May 03, 2012
The FAA provides air traffic control services to pilots in the terminal environment, which is typically an area around a primary airport extending from the surface to about 10,000 feet and laterally approximately 40 miles in any direction. Additional airports may be within the same terminal area. In this environment, aircraft are normally taxiing on the airport surface, departing the airport and climbing to their cruise altitudes, or descending for a landing at one of the airports in the area. General aviation aircraft may also transit through the terminal area without landing or taking off. There are two types of facilities in the terminal environment. Towers are elevated observation buildings at airports where controllers can visually monitor aircraft on the airport surface and in the air within 5 statute miles of the airport. The tower controllers normally have additional surveillance displays, but use the information for situation awareness only, not for separation purposes. The tower controllers deliver route clearances and authorize taxi operations, takeoffs, and landings. They can allow only one aircraft or other vehicle on an active runway at the same time, but multiple vehicles can occupy taxiways and ramp areas. Terminal radar approach control (TRACON) facilities use radar to monitor and direct the movement of aircraft via radio in one or more sectors of the airspace. Under instrument rules, the controllers maintain a minimum horizontal separation of 3 miles between the aircraft unless they are separated by at least 1000 feet vertically or they are on diverging courses. Under visual clearances, the aircraft may get closer but must still maintain adequate separation to avoid wake turbulence from a leading aircraft. Aircraft in the terminal area cannot exceed a speed of 250 nm because of the concentration of aircraft and the amount of maneuvering required. In the national airspace system, there are 352 FAA-operated towers (there are others operated under contract) and 185 TRACONs.