E N C Y C L O P E D I A

Air Traffic Control

The air traffic control system gives guidance to aircraft, to prevent collisions and manage efficient traffic flow. See also air traffic controller.

Much money has been spent on creating software to streamline this process. Yet at some air route traffic control centers (ARTCCs), air traffic controllers still record data for each flight on strips of paper and personally coordinate their paths. In newer sites, the flight strips have been replaced by computer screens. As new equipment is brought in, more and more sites are getting away from paper flight strips. A prerequisite to safe air traffic separation is the assignment and use of distinctive airline call signs that usually include up to four digits (the flight number) prefaced by a company-specific airline call sign. In this arrangement, an identical call sign might well be used for the same scheduled journey each day it is operated, even if the departure time varies a little across different days of the week. The call sign of the return flight often differs only by one digit, the final number, from the outbound flight. In air traffic control terminology, a block of airspace of predetermined size assigned to a radar air traffic controller is called a "sector." Depending on various factors (traffic density, etc.), a controller may be responsible for one or more sectors at any given time.

Many interesting technologies are used in air traffic control systems. Primary and secondary radar are used to enhance a controller's "situational awareness" within his assigned airspace -- all types of aircraft send back primary echoes of varying sizes to controllers' screens as radar energy is bounced off their (usually) metallic skins, and transponder equipped aircraft reply to secondary radar interrogations by giving an ID (mode A), an altitude (mode C) and/or a unique callsign (mode S). Certain types of weather may also register on the radar screen.

These inputs, added perhaps to data from other radars are correlated to build the air situation. Some basic processing happens on the radar tracks like calculating ground speed and magnetic headings.

Other correlations with electronic flight plans are also available to controllers on modern operational display systems.

At last, some tools are available in different domains to help the controller further, like

Conflict Alert (CA): a tool that checks possible conflicting trajectories to alert the controller.
Minimum Safe Altitude Warning (MSAW) is another warning system available for controller to warn pilots about altitude problems.
System Coordination (SYSCO) to enable controller to negotiate the release of flights from one sector to another.
Area Penetration Warning (APW) to inform a controller that a flight will penetrate a restricted area.
Arrival and Departure manager to help sequence the takeoffs and landing of planes

Facts and known mishaps

Occasionally, failures in the system have caused delays (or more rarely, crashes). On 1st July 2002 a Tupolev Tu-154 and Boeing 757 collided above Überlingen near the boundary between German and Swiss-controlled airspace when a Skyguide-employed controller apparently gave instructions to the southbound Tupolev to descend whereas on-board automatic Collision Avoidance software had instructed the crew to climb. The northbound Boeing, equipped with similar avionics, was already descending due to a software prompt. All passengers and crew died in the resultant collision. Skyguide company publicity had previously acknowledged that the relatively small size of Swiss airspace makes real-time cross-boundary liaison with adjoining authorities particularly important.

Other fatal collisions between airliners have occurred over India and Zagreb in Yugoslavia. When a risk of collision is identified by aircrew or ground controllers an "air miss"or "air prox" report can be filed with the air traffic control authority concerned.

The FAA has spent over $3 billion on software, but a fully-automated system is still over the horizon. The UK has recently brought a new control centre into service at Swanwick, in Hampshire, relieving a busy suburban centre at West Drayton in Middlesex, north of London Heathrow Airport. Software from Lockheed-Martin predominates at Swanwick.