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Air traffic control

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Air Traffic Control Towers (ATCTs) at Schiphol Airport

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 weather 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).

Terminal Control

The primary method of controlling the immediate airport environment is visual observation from the control tower. The tower is a tall, windowed structure located on the airport grounds. Aerodrome or 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 to 5 nautical miles (4 to 9 km) depending on the airport procedures.

Radar displays are also available to controllers at some airports. Controllers may use a radar system called Secondary Surveillance Radar also known as Airport Surveillance Radar for airborne traffic approaching and departing. These displays include a map of the area, the position of various aircraft, and data tags that include aircraft identification, speed, heading, and other information described in local procedures.

The areas of responsibility for tower controllers fall into three general operational disciplines; Ground Control, Local or Air Control, and Clearance Delivery -- other categories, such as Apron Control or Ground Movement Planner, may exist at extremely busy airports. While each tower's procedures will vary and while there may be multiple teams in larger towers that control multiple runways, the following provides a general concept of the delegation of responsibilities within the tower environment.

Ground Control

Ground Control Tower (left) at Chicago's O'hare.

Ground Control (sometimes known as Ground Movement Control abbreviated to GMC) is responsible for the airport "maneuvering" areas, or areas not released to the airlines or other users. This generally includes all taxiways, holding areas, and some transitional aprons or intersections where aircraft arrive having vacated the runway and departure gates. Exact areas and control responsibilities are clearly defined in local documents and agreements at each airport. Any aircraft, vehicle, or person walking or working in these areas is required to have clearance from the ground controller. This is normally done via VHF radio, but there may be special cases where other processes are used. Most aircraft and airside vehicles have radios. Aircraft or vehicles without radios will communicate with the tower via aviation light signals or will be led by vehicles with radios. People working on the airport surface normally have a communications link through which they can reach or be reached by ground control, commonly either by handheld radio or even cell phone. Ground control is vital to the smooth operation of the airport because this position might constrain the order in which the aircraft will be sequenced to depart, which can affect the safety and efficiency of the airport's operation.

Some busier airports have Surface Movement Radar (SMR), such as, ASDE-3, AMASS or ASDE-X, designed to display aircraft and vehicles on the ground. These are used by the ground controller as an additional tool to control ground traffic, particularly at night or in poor visibility. 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 include the capability to display higher quality mapping, radar target, data blocks, and safety alerts.

Local or Air Control

Local or Air Control (most often referred to as the generic "Tower" control, although Tower control can also refer to a combination of the local, ground and clearance delivery positions) is responsible for the active runway surfaces. Local control clears 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 nautical miles (4 to 9 km) of airspace around the airport, allowing them to give the clearances necessary for airport safety. If the local controller detects any unsafe condition, a landing aircraft will be told to "go-around" and will be re-sequenced into the landing pattern by the approach or terminal area 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 approach radar controllers to ensure "holes" or "gaps" in the arrival traffic are created (where necessary) to allow taxiing traffic to cross runways and to allow departures aircraft to take off. Crew Resource Management procedures are often used to ensure this communication process is efficient and clear.

Clearance Delivery

Clearance delivery is the position that coordinates with the national command center and the en-route 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 "slot delays"), the airport may go "IFR", 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 ("slot time"). This information is also coordinated with the en-route center and the ground controller in order to ensure the aircraft reaches the runway in time to meet the release time provided by the command center. At some airports the clearance delivery controller also plans aircraft pushbacks and engine starts and is known as Ground Movement Planner (GMP): this position is particularly important at heavily congested airports to prevent taxiway and apron gridlock.

Approach and Terminal Control

Inside the Potomac TRACON
Main article: Terminal Control Center

Many airports have a radar control facility that is associated with the airport. In most countries, this is referred to as Approach or Terminal Control; in the U.S., it is often still referred to as a TRACON or Terminal Radar Approach CONtrol facility. While every airport varies, terminal controllers usually handle traffic in a 30 to 50 nautical mile (56 to 187 km) radius from the airport and from the surface up to 10,000 feet. Where there are many busy airports in close proximity, one single terminal control may service all the airports. The actual airspace boundaries and altitudes assigned to a terminal control are based on factors such as traffic flows, neighboring airports and terrain, and vary widely from airport to airport: a large and complex example is the London Terminal Control Centre which controls traffic for 5 main London airports up to 20,000 feet and out to 100 nautical miles.

Terminal controllers are responsible for providing all ATC services within their airspace. Traffic flow is broadly divided into departures, arrivals, overflights, and VFR aircraft. As aircraft move in and out of the terminal airspace, they are handed off to the next appropriate control facility (a control tower, an en-route control facility, or a bordering terminal or approach control). Terminal control is responsible for ensuring that aircraft are at an appropriate altitude when they are handed off, and that aircraft arrive at a suitable rate for landing.

Not all airports have a radar approach or terminal control available. In this case, the en-route center or a neighboring terminal or approach control will co-ordinate directly with the tower on the airport and vector inbound aircraft to a position from where they can land visually. At some of these airports, the tower may provide a non-radar procedural approach service to arriving aircraft handed over from a radar unit before they are visual to land.

En-route, Center, or Area Control

Controllers at work at the Washington Air Route Traffic Control Center.
Main article: Area Control Center

ATC provides services to aircraft in flight between airports as well. The level of service is dependent on the type of flight the aircraft falls under (IFR or VFR), the type of airspace the aircraft is in and the services requested by the pilots.

En-route air traffic controllers issue clearances and instructions for airborne aircraft, and pilots are required to comply with these instructions. En-route controllers also provide air traffic control services to many smaller airports around the country, including clearance off of the ground and clearance for approach to an airport. Controllers adhere to a set of separation standards that define the minimum distance allowed between aircraft. These distances vary depending on the equipment and procedures used in providing ATC services.

Pilots fly under one of two sets of rules for separation: Visual Flight Rules (VFR) or Instrument Flight Rules (IFR). Air traffic controllers have different responsibilities to aircraft operating under the different sets of rules.

General characteristics

En-route air traffic controllers work in facilities called Area Control Centers, each of which is commonly referred to as a "Center". The United States uses the equivalent term Air Route Traffic Control Center (ARTCC). Each center is responsible for many thousands of square miles of airspace (known as a Flight Information Region) and for the airports within that airspace. Centers control IFR aircraft from the time they depart an airport or terminal area's airspace to the time they arrive at another airport or terminal area's airspace. Centers may also "pick up" VFR aircraft that are already airborne and integrate them into the IFR system. These aircraft must, however, remain VFR until the Center provides a clearance.

Center controllers are responsible for climbing the aircraft to their requested altitude while, at the same time, ensuring that the aircraft is properly separated from all other aircraft in the immediate area. Additionally, the aircraft must be placed in a flow consistent with the aircraft's route of flight. This effort is complicated by crossing traffic, severe weather, special missions that require large airspace allocations, and traffic density. When the aircraft approaches its destination, the center is responsible for meeting altitude restrictions by specific points, as well as providing many destination airports with a traffic flow, which prohibits all of the arrivals being "bunched together". These "flow restrictions" often begin in the middle of the route, as controllers will position aircraft landing in the same destination so that when the aircraft are close to their destination they are sequenced.

As an aircraft reaches the boundary of a Center's control area it is "handed off" or "handed over" to the next Area Control Center. In some cases this "hand-off" process involves a transfer of identification and details between controllers so that air traffic control services can be provided in a seamless manner; in other cases local agreements may allow "silent handovers" such that the receiving center does not require any co-ordination if traffic is presented in an agreed manner. After the hand-off, the aircraft is given a frequency change and begins talking to the next controller. This process continues until the aircraft is handed off to a terminal controller ("approach").

Radar Coverage

Since centers control a large airspace area, they will typically use long range radar that has the capability to see aircraft within 200 nautical miles (370 km) of the radar antenna. They may also use TRACON radar data to control when it provides a better "picture" of the traffic or when it can fill in a portion of the area not covered by the long range radar.

In the U.S. system, at higher altitudes, over 90% of the U.S. airspace is covered by radar and often by multiple radar systems; however, coverage may be inconsistent at lower altitudes used by unpressurized aircraft due to high terrain or distance from radar facilities. A center may require numerous radar systems to cover the airspace assigned to them, and may also rely on pilot position reports from aircraft flying below the floor of radar coverage. This results in a large amount of data being available to the controller. To address this, automation systems have been designed that consolidate the radar data for the controller. This consolidation includes eliminating duplicate radar returns, ensuring the best radar for each geographical area is providing the data, and displaying the data in an effective format.

Centers also exercise control over traffic travelling over the world's ocean areas. These areas are also FIRs. Due to the fact that there are no radar systems available for oceanic control, oceanic controllers provide ATC services using procedural control. These procedures use aircraft position reports, time, altitude, distance, and speed to ensure separation. Controllers record information on flight progress strips and in specially developed oceanic computer systems as aircraft report positions. This process requires that aircraft be separated by greater distances, which reduces the overall capacity for any given route.

Some Air Navigation Service Providers (e.g Airservices Australia, Alaska Center, etc.) are implementing Automatic dependent Surveillance - Broadcast (ADS-B) as part of their surveillance capability. This new technology reverses the radar concept. Instead of radar "finding" a target by interrogating the transponder, ADS transmits the aircraft's position several times a second. ADS also has other modes such as the "contract" mode where the aircraft reports a position based on a pre-determined time interval. This is significant because it can be used where it is not possible to locate the infrastructure for a radar system (e.g. over water). Computerised radar displays are now being designed to accept ADS inputs as part of the display. As this technology develops, oceanic ATC procedures will be modernised to take advantage of the benefits this technology provides.

Problems

Traffic

For more information see Air traffic flow management.

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. Several factors 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 at least one and up to four minutes for each aircraft. Allowing for departures between arrivals, each runway can thus handle about 30 arrivals per hour. A typical large airport with two arrival runways can 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. Up until the 1990s, holding, which has signifincant environmental and cost implications, was a routine occurrence at many airports. Advances in computers now allow the sequencing of planes hours in advance. Thus, planes may be delayed before they even take off (by being given a "slot"), or may reduce power in flight and proceed more slowly thus significantly reducing the amount of holding.

Weather

Beyond runway capacity issues, weather is a major factor in traffic capacity. 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 ACCs, a major weather problem is thunderstorms, which present a variety of hazards to aircraft. Aircraft will deviate around storms, reducing the capacity of the en-route 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. However, at some Area Control Centers, 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 upgrading away from paper flight strips.

Call signs

A prerequisite to safe air traffic separation is the assignment and use of distinctive call signs. These are permanently allocated by ICAO on request usually to scheduled flights and some air forces for military flights. They are written callsigns with 3-letter combination like KLM, TWA, SWA , BAW , DLH followed by the flight number, like TWA872, BAW018. As such they appear on flight plans and ATC radar labels. There are also the audio or Radio-telephony callsigns used on the radio contact between pilots and Air Traffic Control not always identical with the written ones. For example BAW stands for British Airways but on the radio you will only hear the word Speedbird instead. By default, the callsign for any other flight is the registration number (tail number) of the aircraft, such as "N12345" or "CG-ABC". The term tail number is due to the fact that a registration number is usually painted somewhere on the tail of a plane , yet this is not a rule. Registration numbers may appear on the engines or anywhere on the fuselage - additionaly large registration numbers are painted on the wings. The short Radio-telephony callsigns for these tail numbers is the first letter followed by the last two, like C-BC spoken as Charlie-Bravo-Charlie for CG-ABC or the last 3 letters only like ABC spoken Alpha-Bravo-Charlie for CG-ABC or the last 3 numbers like 345 spoken as tree-fower-fife for N12345.

The flight number part is decided by the aircraft operator. 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 the final digit from the outbound flight. Generally, airline flight numbers are even if eastbound, and odd if westbound. In order to reduce the possibility of two callsigns on one frequency at any time sounding too similar, a number of airlines, particularly in Europe, have started using alphanumeric callsigns that are not based on flight numbers. For example DLH23LG, spoken as lufthansa-too-tree-lima-golf. Additionally it is the right of the air traffic controller to change the 'audio' callsign for the period the flight is in his sector if there is a risk of confusion, usually choosing the tail number instead.

Before around 1980 IATA and ICAO were using the same 2-letter callsigns. Due to the larger number of new airlines after deregulation ICAO established the 3-letter callsigns as mentioned above. The IATA callsigns are currently used in aerodromes on the announcement tables but never used any longer in Air Traffic Control. For example, TW is the IATA callsign for the ICAO - ATC equivalent TWA. Other examples include LY/ELY for El Al, AA/AAL for American Airlines, LH/DLH for Lufthansa etc.

Technology

Many 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 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 to data from other radars, are correlated to build the air situation. Some basic processing occurs on the radar tracks, such as calculating ground speed and magnetic headings.

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

Some tools are available in different domains to help the controller further:

  • Conflict Alert (CA): a tool that checks possible conflicting trajectories and alerts the controller. The most common used is the STCA (Short Term CA) that is activated about 2 minutes prior the loss of separation. The algorithms used may also provide in some systems a possible vectoring solution, that is, the way to turn or descend/climb the aircraft in order to avoid infringing the minimum safety distance or altitude clearance.
  • Minimum Safe Altitude Warning (MSAW): a tool that alerts the controller if an aircraft appears to be flying too low to the ground or will impact terrain based on its current altitude and heading.
  • 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 takeoff and landing of aircraft.
  • Converging Runway Display Aid (CRDA) enables Approach controllers to run two final approaches that intersect and make sure that go arounds are minimized
  • Final Approach Spacing Tool (FAST) gives aircraft a runway assignment that the Approach Controller will give to the aircraft. FAST can also suggest vectors for downwind and base with the correct timing. In Europe the equivalent system is known under the term metering system and predicts the future spacing of approaching aircraft on the runway
  • User Request Evaluation Tool (URET) takes paper strips out of the equation for En Route controllers at ARTCCs By providing a display that shows all aircraft in or entering the sector. URET provides conflict resolution up to 30 minutes in advance.
  • Mode S: provides a data downlink of flight parameters via Secondary Surveillance Radars allowing radar processing systems and therefore controllers to see various data on a flight, including airframe unique id, indicated airspeed and flight director selected level, amongst others.
  • CPDLC: Controller Pilot Datalink Communications - allows digital messages to be sent between controllers and pilots, avoiding the need to use radiotelephony. It is especially useful in areas where difficult-to-use HF radiotelephony was previously used for communication with aircraft, e.g oceans. This is currently in use in various parts of the world including the Atlantic and Pacific oceans.
  • ADS-B: Automatic Dependent Surveillance - provides a data downlink of various flight parameters to air traffic control systems via the Transponder (1090 MHZ )and reception of those data by other aircraft in the vicinity. The most important is the aircraft's latitude, longitude and level: such data can be utilised to create a radar-like display of aircraft for controllers and thus allows a form of pseudo-radar control to be done in areas where the installation of radar is either prohibitive on the grounds of low traffic levels, or techinically not feasible (e.g. oceans). This is currently in use in Australia and parts of the Pacific Ocean.

Major accidents

Failures in the system have caused delays or even, in rare cases, 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 despite an instruction from the on-board automatic Traffic Collision Avoidance System software 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. It is worth noting that currently air traffic controllers have no way of knowing if or when the TCAS system is issuing resolution advisories to pilots. They also do not know what the advisory is telling the pilots. Therefore, pilots are supposed to immediately report TCAS resolution advisories and follow them as soon as possible. Consequently, they should ignore ATC instructions until they have reported to the ground that they are clear of the conflict.

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" (air proximity) report can be filed with the air traffic control authority concerned. The worst fatal collision between airliners actually took place on the ground, on March 27, 1977, in what is known as the Tenerife disaster.

The FAA has spent over USD$3 billion on software, but a fully-automated system is still over the horizon. In 2002 the UK brought a new area 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.

Air Navigation Service Providers (ANSPs)

In the United States, the Federal Aviation Administration (FAA) provides this service to all aircraft in the National Airspace System (NAS). The FAA is responsible for all aspects of U.S. Air Traffic Control including hiring and training controllers, although there are contract towers located in many parts of the country. A contract tower is an Airport Traffic Control Tower (ATCT) that performs the same function as an FAA-run ATCT but is staffed by employees of a private company (Martin State Airport in Maryland is an example). In Canada, Air Traffic Control is provided by NAV CANADA, a private, non-share capital corporation that operates Canada's civil air navigation service.

See also

Official Air Traffic Control Organisations and Research Institutes

Controllori Navigazione Aerea - Italian Guild of ATS

Next Generation ATC - Research

History of Air Traffic Control

Internet services

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