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

Air traffic control can be divided into two major subspecialties, terminal control and enroute control. Terminal control involves air traffic on the airport proper and within the immediate airport environment, within about 30 nautical miles, while enroute control handles traffic between major terminals and at locations not busy enough to deserve a dedicated terminal facility.

Terminal air traffic controllers work in facilities called Air Traffic Control Towers (ATCTs) and Terminal Radar Approach Control (TRACON ). At some locations, staff is shared between the ATCT and the TRACON, while at others the tower and the TRACON are completely separate entities. For example, Honolulu International Airport is served by a combined ("up/down") facility, while Chicago O'Hare Airport is served by an ATCT at the airport, and a remote TRACON located at Elgin, Illinois.

Terminal controllers are responsible for the separation and efficient movement of aircraft operating on the taxiways and runways of the airport itself, and those in the air near the airport. The immediate airport environment is contolled primarily visually from a tall, windowed structure (the "tower") located on the airport. Lining up arrival aircraft to land and fanning departures out for handoff to enroute facilities is accomplished by radar in the TRACON.

Enroute air traffic controllers work in facilities called Air Route Traffic Control Centers (ARTCC s) or Area Control Centers (ACCs), commonly referred to as "centers". Each center is responsible for many thousands of square miles of territory. Centers that exercise control over traffic travelling over the world's oceans control immense areas. The Oakland, California ARTCC, for example, controls most of the Pacific Ocean between the U.S. West Coast and Guam. Outside the US, they are called FIR s.

Centers control traffic strictly through the use of long-range radar, or using complex non-radar procedural separation where radar does not exist.

The day-to-day problems faced by the air traffic control system are primarily related to the volume of air traffic demand placed on the system, and weather. Simple physics dictate the amount of traffic that can land at an airport in a given amount of time. Each landing aircraft must touch down, slow, and exit the runway before the next crosses the end of the runway. This process requires between one and up to four minutes for each aircraft (depending mainly on the number of taxiways and the angle they're making with the runway). Allowing for departures between arrivals, each runway can thus handle about 30 arrivals per hour. A typical large airport with two arrival runways can thus handle about 60 arrivals per hour in good weather. Problems begin when airlines schedule more arrivals into an airport than can be physically handled, or when delays elsewhere cause groups of aircraft that would otherwise be separated in time to arrive simultaneously. Aircraft must then be delayed in the air by holding over specified locations until they may be safely sequenced to the runway.

Compounding runway capacity issues is weather. Rain or ice and snow on the runway cause landing aircraft to take longer to slow and exit, thus reducing the safe arrival rate and requiring more space between landing aircraft. This, in turn, increases airborne delay for holding aircraft. If more aircraft are scheduled than can be safely and efficiently held in the air, a ground delay program may be established, delaying aircraft on the ground before departure due to conditions at the arrival airport.

In ARTCCs, the major weather problem is thunderstorms. Thunderstorms present a variety of hazards to aircraft, and their pilots are extremely reluctant to operate in or near them. Aircraft will thus deviate around storms, reducing the capacity of the enroute system by requiring more space per aircraft, or causing congestion as many aircraft try to move through a single hole in a line of thunderstorms. Occasionally weather considerations cause delays to aircraft prior to their departure as routes are closed by thunderstorms.

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. See Bashkirian Airlines Flight 2937 for more on this accident.

Other fatal collisions between airliners have occurred over India and Zagreb in Croatia. 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 USD$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. The Swanwick facility, however, has been troubled by software and communications problems causing delays and occasional shutdowns, paralyzing air traffic in the area..

See also

• Air safety
• Aviation System
• Flight level
• Flight Progress Strip
• Free Flight (Air traffic control)
• IFATCA
• Tenerife disaster
• Controlled airspace
• Global Air Traffic Management
• Automatic Dependent Surveillance-Broadcast (also called ADS-B)

External links

• National Air Traffic Controllers Association
• Federal Aviation Administration
• Air Traffic Control Manual