Air Traffic Control

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Air Traffic Control, management of aircraft proceeding along civil airways, including airport arrivals and departures. Different rules of operation apply to pilots proceeding under visual flight rules (VFR) and those under instrument flight rules (IFR).

The minimum instruments required under VFR include an airspeed indicator, altimeter, and magnetic direction indicator. Minimum flying conditions in radar-controlled airspace in transition areas specify a cloud ceiling about 215 m (700 ft) above ground level and 1.6 km (1 mi) visibility. Other VFR requirements for visibility and distance from clouds depend on altitude and whether operation is in controlled or uncontrolled airspace. VFR flight is not permitted in all airspaces, and terminal control areas sometimes require positive (radar) air traffic control. Airport traffic areas typically encompass a radius of 8 km (5 mi) and are extended laterally for the control of instrument-dependent departures and landings. Control zones around airports extend upwards with no limit. Radio communication with the control tower is required during landing and take-off. The aim in the United Kingdom is generally to segregate VFR and IFR flying to different airfields. The remainder of this article will concern itself primarily with aircraft operating under IFR.

Operation and Equipment

At major airports, air traffic control begins with the controller in the airport tower, who guides airliners from the loading ramp, along the taxi strips, to the runway threshold. The controller must consider other aircraft as well as the legions of vehicles, such as fuel tankers, luggage and cargo vehicles, and maintenance vehicles, needed for airport operation. The job continues day and night, in all weather, so that at times of reduced visibility large airports rely on special radar to aid the controller. For take-off, another air traffic controller located in the airport tower may take over, confirming an assigned runway clearance and providing information on wind and weather and other data needed for departure. A departure controller may pass on additional data as the airliner is handed over to the air route traffic control (ARTC), the personnel of which remain in communication with the airliner from one ARTC centre to the next, until the air traffic control tower at the plane's destination takes over. See Airway.

ARTC radar and computer systems represent a major advance in air traffic control, in that controllers are relieved of the accumulation and interpretation of immense amounts of routine material, thereby permitting them more time to assess data relevant to key decisions. In the control room, the controller wears earphones and a microphone for radio communication with aircraft and other controllers. The planes themselves are represented as a data block on a special radar screen in front of the controller. The data block includes a symbol for the individual aircraft, along with the plane's identifying call sign, speed, and altitude. Some radar equipment can display additional information pertaining to a particular flight. Flights are kept at separate altitudes and at specific distances from one another. Flight plans are fed into computers and updated as the flight progresses. Air traffic controllers watch these displayed assignments carefully to prevent midair collisions. Collision-avoidance radar systems for individual aircraft are coming into use. As aircraft converge on airports and begin the descent for landing, aerial congestion can develop. In this case, new arrivals are directed to a holding area in the sky, about 50 km (30 mi) or so from the airfield. Waiting planes in this area repeatedly circle a beacon, so that they create an aerial "stack", maintaining a vertical spacing of 305 m (1,000 ft) between planes. Each time a suitable runway becomes available, a plane is taken from the bottom of the stack, permitting the others to spiral down to the next layer.

Navigational Aids

Navigation between airports relies on ground beacons and on electronic and computerized equipment in the plane. The most widely used ground system is the very-high-frequency omnidirectional range beacon (VOR). VOR stations, which are not always located at an airport, operate on frequencies that are generally free of atmospheric noise and provide an accuracy lacking in previous equipment. Aboard the plane, a visual display indicates the magnetic course the pilot must fly in order to travel directly to or away from the VOR station. Most VOR stations also have distance-measuring equipment (DME), which tells the pilot distances to and from VORs. These VOR/DME stations provide excellent service for both private aircraft and scheduled airliners worldwide.

For intercontinental routes, a radio and electronic system called Omega uses a network of eight global transmission sites that emit powerful long-range signals. A computer on board the aircraft receives the signals, analyses their pattern, and calculates the position of the plane anywhere in the world. A different method, the inertial navigation system (INS), requires no ground stations or radio beams, which might be subject to distortion or interruption. The INS uses a gyroscopically stabilized inertial platform, aligned to true north. Accelerometers associated with the system can determine the direction and speed of the aircraft, and a computerized display indicates this information, along with wind speed, drift, and other data. These systems, when combined with an autopilot, enable large jet transports literally to fly themselves on global routes. Many airliners also carry compact weather radar to detect storm conditions en route. Military equipment uses VOR, Omega, and other systems, including more sophisticated radar. See Also Navigation.

For instrument landings, pilots use an instrument landing system (ILS). Cockpit instruments indicate deviations to either side of the localizer beam leading directly to the runway, and guidance information from the glide-slope beam indicates whether the plane is too high or too low on the approach, which may start some 13 to 16 km (8 to 10 mi) from the airport. Some existing ILS systems can accommodate totally automatic landings, permitting operations in heavy fog. Elsewhere, special radar systems can be used by air traffic controllers to "talk down" aircraft in bad weather.

The ILS system, which is subject to "ground clutter" and occasional distortions, began to be replaced by a microwave landing system (MLS) in the early 1980s. The MLS equipment is more precise, permits curving approaches (unlike the rigidly linear ILS-mediated approach) by multiple aircraft over a broader gateway area, and is cheaper to operate. However, there is now a question as to whether MLS will completely replace ILS or will be superseded by global positioning systems (GPS).

Air Traffic Control Problems

Despite the impressive sophistication of electronic aids and computerization, air traffic control continues to rely heavily on people, whether the planes are moving on the ground, approaching or departing from the airport, or en route. Direct responsibility for people's lives rests with the men and women who control air traffic, and training standards are demanding. The controllers occupy a powerful position when they strike or initiate a work slowdown while bargaining on working conditions, pay scales, and other contract clauses. Such actions have created many problems for passengers and airline management alike, in Europe and elsewhere.

Private aircraft using major airport facilities create additional problems in air traffic control planning, although this is more of a problem in North America than in Europe. Some officials would prefer to ban all but scheduled airline traffic at major air terminals. Even without the presence of private aircraft, increased airline traffic has intensified the concern for passenger safety. For this reason, collision-avoidance radar systems were developed during the 1980s.