Air Traffic Control
October 20th, 2008
Air traffic control (ATC) is a service provided by ground-based controllers who direct aircraft on the ground and in the air. A controller’s primary task is to separate certain aircraft — to prevent them from coming too close to each other horizontally or vertically. Secondary tasks include ensuring orderly and expeditious flow of traffic and providing information to pilots, such as RADAR traffic advisories, weather advisories, flight following, and navigation information.
In many countries, ATC services are provided throughout the majority of airspace, and its services are available to all users (private, military, and commercial). When controllers are responsible for separating some or all aircraft, such airspace is called “controlled airspace” in contrast to “uncontrolled airspace.” Depending on the type of flight and the class of airspace, ATC may issue instructions that pilots are required to follow, or merely flight information (in some countries known as advisories) to assist pilots operating in the airspace. In all cases, however, the pilot has final responsibility for the safety of the flight, and may deviate from ATC instructions in an emergency.
Air traffic control services can be divided into two major subspecialties, terminal or airport control, and en-route or area control.
Terminal or Airport control includes the control of traffic (aircraft and vehicles) on the airport surface and airborne aircraft within the immediate airport environment. Generally, this is approximately a 30 to 50 nautical mile (56 to 93 km) radius of the airport, from the surface to about 10,000 ft (about 3,050 m). Terminal controllers work in facilities called control towers and terminal control centers (called Terminal Radar Approach Control, or TRACON, facilities in the U.S.). At some locations, controllers are shared between tower control and the terminal control center, while at others the tower and the terminal control center are completely separate entities. For example, Philadelphia International Airport is served by a combined (“up/down”) facility, while Chicago’s O’Hare Airport is served by a control tower at the airport, and a remote TRACON located at Elgin, Illinois.
En-route controllers, also called Center or Area controllers, control the traffic between the terminals. They can also control traffic in and out of airports where the traffic volume does not warrant the establishment of a terminal ATC operation or during periods when a terminal operation is closed (e.g., midnight to 6:00 am). En-route controllers work at facilities called Area Control Centers or Air Route Traffic Control Centers (ARTCCs).
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