The air traffic control (ATC) system ensures a minimum standard of separation is maintained between aircraft it is controlling. It also provides traffic information services to prevent collisions with aircraft not under its control, and it manages the traffic to ensure the most 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 includes the control of traffic (aircraft and vehicles) on the airport proper and and airborne aircraft within the immediate airport environment. Generally, this is approximately a 30 to 50 nautical mile radius of the airport.
The second subspeciality is referred to as "enroute" control. These facilities control all traffic between the terminals and traffic in and out of airports where traffic volume does not warrant the establishment of a terminal ATC operation.
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.
Tower Control
The primary method of controlling the immediate airport environment is visual observation from the tower. The tower is a tall, windowed structure located on the airport. Tower controllers are responsible for the separation and efficient movement of aircraft and vehicles operating on the taxiways and runways of the airport itself, and aircraft in the air near the airport (generally 2-5 nautical miles depending on the airport procedures).
Radar displays are also available to display traffic for tower controllers at some airports. Local control (described below) may use a radar display for airborne traffic on final approach and for departing traffic once they are airborne. These displays include a map of the area, the aircraft position, and data tags that include the aircraft identification, speed, and other information described in local procedures. This radar system is normally a part of the radar system used by terminal radar controllers.
Some airports also have radar designed to display aircraft and vehicles on the ground. This is used by the ground controller as an additional tool to control ground traffic. There are a wide range of capabilities on these systems as they are being modernized. Older systems will display a map of the airport and the target. Newer systems may include the capability to display higher quality mapping, radar target, data blocks, safety alerts, etc. Local and national procedures govern the use of these systems for each tower.
The areas of responsibility for tower controllers fall into three general operational disciplines; Ground Control, Local Control, and Clearance Delivery. While each tower's procedures will vary and while there may be multiple teams in larger towers needed to control multiple runways, the following provides a general concept of the delegation of responsibilities within the tower environment.
Ground Control is responsible for the airport "movement" areas, or areas not released to the airlines or other users. This generally includes all taxiways, holding areas, and some transition areas where aircraft transition between their local gate control and the FAA's ground control. (Exact areas and control responsibilities are clearly defined in local documents and agreements at each airport.) Any aircraft, any vehicle, and any person walking or working on these area are required to have clearance from the ground controller. Normally, this is done via VHF radio, however, there may special cases where other processes are used. All aircraft are required to have radios, most all vehicles have radios or are lead by vehicles with radios, and those working on the airport surface normally have a communications link through which they can reach or be reached by ground conrol. Ground control is vital to the smooth operation of the airport because this position will establish the order in which the aircraft will be lined up to depart and that can affect the efficiency of the airport operation.
Local Control is responsible for the runway(s) as defined in the local procedures. Local control is the position that clears the aircraft for take off or landing and ensures the runway is clear for these aircraft. To accomplish this, local control controllers are normally given 2 to 5 nm of airspace around the airport, allowing them to give the clearances necessary for airport safety. If the local controller detects any unsafe condition, the landing aircraft will be told to "go around" and will be sequenced in the the pattern by the TRACON controller.
Within the tower, a highly disciplined communications process between local and ground control is an absolute necessity. Ground control must request and gain approval from local control to cross any runway with any aircraft or vehicle. Likewise, local control must ensure ground control is aware of any operations that impact the taxiways and must work with the arrival radar controllers to ensure "holes" in the arrival traffic are created (where necessary) to allow taxiing traffic to cross the runways and to allow departures aircraft to take off.
Clearance delivery is the position that coordinates with the national command center and the enroute center to obtain releases for aircraft. Under normal conditions, this is more or less automatic. When weather or extremely high demand for a certain airport become a factor, there may be ground "stops" (or delays), or re-routes to ensure the system does not get overloaded. The primary responsibility of the clearance delivery position is to ensure that the aircraft have the proper route and release time. This information is also coordinated with the enroute center and the ground controller in order to ensure the aircrat reaches the runway in time to meet the release time provided by the command center.
TRACON Control
Larger airports have a radar control facility that is associated with the tower. This radar facility in the U.S. ATC system is referred to as a TRACON or Terminal Radar Approach CONtrol facility. While every airport varies, TRACONs usually control traffic in a 30 to 50 nautical mile radius from the airport and from the surface to 10,000 feet. The actual airspace assigned to a TRACON varies widely from airport to airport in both airspace boundaries and alititudes under a TRACON's jurisdiction. The actual geographic boundaries and altitude are based on traffic flows, terrain, etc.
TRACONs normally have their own radar system. It is a short range radar that has a maximum range of approximately 50 nautical miles. This radar turns faster than en-route radar (4.7 seconds for a sweep vs 12 seconds). This frequent updates help controllers see the result of turns quickly. Most all U.S. TRACONS have long range radar to back up the normal short range radar if it fails or requires maintenance. Expanded separation minimums are normally required when in this mode.
TRACON control positions usually include a radar controller and a coordinator who generally stands behind the radar position. The radar controller is responsible for ensuring appropriate separation, and issuing traffic and other local aviation informatoin for aircraft under its control. Additionally, the radar controller is responsible for ensuring all required coordination with other controllers in the tower, TRACON, or en-route center is completed, making computer required computer entries, and updating the flight progress strips.
The coordinator provides coordination support for the radar controller. He/she will provide inter/intra faciity coordination when required for the radar controller and make computer entries.
Some TRACONs have the ability to staff a second position at the radar console, referred to as a "hand-off" controller. This position is responsible for providing direct support by coordinating for the radar controller, managing flight progress strips, and making computer entries. When this position is staffed, the coordinator duties are greatly reduced, allowing him/her to provide support for a number of positions.
TRACONs are responsible for providing all ATC services within their airspace. Generally, there are four types of traffic flows controlled by TRACON controllers. These are departures, arrivals, overflights, and aircraft operating under Visual Flight Rules (VFR).
Departure aircraft are received from the tower and are generally 1,000 feet to 2,000 feet high, climbing to a pre-determined altitude. The TRACON controller working this traffic is responsible for clearing all other TRACON traffic and, based on the route of flight, placed on a track and in a geographical location (sometimes referred to as "gates") that is pre-determined through agreements for the en-route center controller. This positioning is designed to allow the en-route center to integrate the aircraft into its traffic flow easily.
Arrival aircraft are received from the en-route center in compliance with pre-determined agreements on routing, altitude, speed, spacing, etc. The TRACON controller working this traffic will take control of the aircraft and blend it with other aircraft entering the TRACON from other areas or "gates" into a single file or final for the runway. The spacing is critical to ensure the aircraft can land and clear the runway prior to the next aircraft touching down on the runway. The tower may also request expanded spacing between aircraft to allow aircraft to depart or to cross the runway in use.
Overflight aircraft are aircraft that enter the TRACON airspace at one point and exit the airspace at another without landing at an airport. They must be controlled in a manner that ensures they remain separated from the climbing and descending traffic that is moving in and out of the airport. Their route may be altered to ensure this is possible. Whey are returned to the en-route center, they must be on the original routing unless a change has been coordinated.
VFR aircraft are handled as traffic permits outside Positive Control Areas. Controllers will provide traffic calls and traffic alerts to ensure safety with other aircraft. Controller lack the level of control over these aircraft that he/she has over aircraft on instrument flight plans in non-positive control airpace. Controllers usually provide information for the pilot about traffic in the immediate vacinity and weather reports if applicable. In positive control areas, the aircraft are required to conform to all control instructions until the exit. This ensures separation from Instrument Flight Plan (IFR) aircraft is maintained in the critical flight areas around the airports.
Not all airports have a TRACON available. In this case, the en-route center will coordinate directly with the tower and provide this type of service where radar coverage permits. Generally, however, the separation minimums are greatly increased.
Enroute Control
Enroute air traffic controllers work in facilities called Air Route Traffic Control Centers (ARTCCs ) 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 FIRs .
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, these flight progress strips have been replaced by electronic data presented on 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. Generally, airline flight numbers are even if eastbound, and odd if westbound. 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 July 1, 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 Traffic Collision Avoidance System 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
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