Air traffic control

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Air Traffic Control Towers (ATCTs) at Schiphol Airport

Air traffic control (ATC) services are provided by ground based controllers responsible for directing aircraft on the ground and in the air to ensure safe and efficient traffic flow is maintained. For airborne aircraft, "Air traffic controllers" specify the flight path, altitude, and speed of aircraft that request ATC services, and pilots are required to comply with these instructions. At airports, controllers direct all vehicles and aircraft in the areas where they have jurisdiction. Generally, these areas include all runways, taxiways, and ramp areas that are clear of gates or loading areas. Controllers are required to adhere to a set of separation standards that define the minimum distance allowed between aircraft receiving ATC services. These distances vary depending on the equipment and procedures used in providing ATC services.

Within the US system, pilots will fly under one of two sets of rules for separation; Visual Flight Rules (VFR) or Instrument Flight Rules (IFR). Air Traffic Controllers have different responsibilites for each of these flight environments.

Aircraft flying VFR must assume responsibility for their separation from all other aircraft (IFR & VFR) and have no controlling agency. They fly on their own using a "see and be seen" separation criteria. In the US, VFR aircraft are required to have an "altitude encoding transponder" when operating in certain types of airspace. This will show the controller the position and altitude of the aircraft so the controller can warn IFR aircraft of any potential conflict with a VFR aircraft. The FAA provides specific rules concerning visibility, distance from clouds, altitude, etc. to which VFR pilots must adhere. As they approach terminal areas, there will be increased restrictions on where VFR aircraft can fly without being under the control of an air traffic controller. This is done for safety near busy airports to ensure safety in densly populated areas and during critical phases of flight.

VFR pilots can request, and ATC can elect to provide "VFR Advisory Services", traffic permitting. Under this environment, the controller will radar identify the VFR aircraft and provide traffic information and weather advisory services for the VFR pilot. Controllers do not provide any instructions concerning direction of flight, altitude, or speed to the VFR pilot receiving advisory services, and they do not provide separation services. This is an optional service and may be discontinued by ATC or the pilot at any time.

In the US system, pilots flying under IFR, must file their flight plan information with ATC, accept any revision ATC provides to their route or altitude, and comply with all directions from controllers. In return, controllers will ensure that pilots flying IFR are separated from all other IFR aircraft and terrain by the appropriate minimum separation. The IFR pilot, however, must maintain a close watch for VFR aircraft since ATC has no control over these aircraft. For this reason, VFR aircraft are restricted to altitudes below 17,500 in more remote areas and, again, must have an operating transponder that transmits altitude information. Once the IFR aircraft is above 18,000 ft (Flight Level 180) the aircraft is considered in "Positive Control Airspace" where only IFR aircraft are allowed.

Within the US ATC system, ATC services are provided throughout a vast majority of its airspace. Services are provided for all users (private pilot, military, and commercial), usually beginning at altitudes that provide adequate separation from terrain. Outside the US, air traffic control is not usually implemented in remote areas, rather it is reserved for high traffic airspace such as that around airports and major cities. Such airspace is called "controlled airspace" in contrast to "uncontrolled airspace". By law, pilots must obey the directions of air traffic controllers when they are in controlled airspace. As pilots transition from one controller's airspace to another, they must notify each controller.

Air traffic control services 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 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.

Enroute controllers control the traffic between the terminals. They also control traffic in and out of airports where the 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.

Contents

1 Tower Control

1.1 Ground Control
1.2 Local Control
1.3 Clearance Delivery
1.4 TRACON Control

2 Enroute Control

3 Facts and known mishaps

4 See also

5 External links

Tower Control

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Controllers survey the field at Misawa Air Base, Japan.

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. 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. 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 position of various aircraft, and data tags that include aircraft identification, speed, heading, 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 that control multiple runways, the following provides a general concept of the delegation of responsibilities within the tower environment.

Ground Control

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, vehicle, or person walking or working in these areas are 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 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 will establish the order in which the aircraft will be lined up to depart, which can affect the efficiency of the airport operation.

Local Control

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 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 sequenced in 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 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 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 aircraft 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 control tower. In the U.S., this 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 (56 to 93 km) radius from the airport and from the surface up to 10,000 feet. The actual airspace boundaries and altitudes assigned to a TRACON are based on factors such as traffic flows and terrain, and vary widely from airport to airport.

TRACONs normally have their own radar system, a short range radar that has a maximum range of approximately 50 nautical miles (93 km). This radar scans faster than en-route radar (4.7 seconds for a sweep vs 12 seconds). These frequent updates help controllers see the result of direction changes quickly. Most 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 information 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 "hands-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, placing the departing aircraft on a track and in a geographical location (sometimes referred to as a "gate") 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 vicinity 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 airspace and for the airports within that airspace. Centers control IFR aircraft from the time the aircraft leaves the TRACON's airspace or departs an airport until the aircraft approaches the airspace controlled by a TRACON or if the airport does not have a TRACON, until the aircraft lands. Centers may also "pick up" aircraft that are airborne and integrate them into the IFR system. These aircraft must, however, remain VFR until the Center provides a clearance.

Centers 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 cross traffic, severe weather, special missions that require large airspace allocations, and traffic density.

In the US system and many non-US systems, an aircraft that is reaching the boundary of a center or an internal sector within a center is "handed-off" to the next controller. This "hand-off" process is simply a transfer of identification between controllers so that air traffic control services can be provided in a seamless manner. Once the "hand-off" is complete, 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 TRACON.

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, over 90% of the U.S. airspace is covered by radar and often by multiple radar systems. A center may require numerous radar systems to cover the airspace assigned to them. 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 referred to as FIRs. Due to the fact that there are no radar systems available for oceanic contro, oceanic controllers provide ATC services using "non-radar" procedures. These procedures use aircraft positon reports, time, alitiude, distance, and speed to ensure separation. Controllers record information on flight progress strips and in specially developed oceanic computer systems as aircraft report positons. This process requires that aircraft be separated by greater distances, which reduces the overall capacity for any given route.

Some ATC service providers (e.g Air Services Australia, Alaska Center, etc.) are implementing Automatic Dependant Surviellance - 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 Latitude/Longitude positon 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). Computerized radar dispalys are now being designed to accept ADS inputs as part of the dispaly. As this technology develops, oceanic ATC procedures will be modernized to take advantage of the benefits this technology provides.

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 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. Up until the 1990s, holding was a common occurrence at airports. Advances in computers now allow controllers to predict transit times and sequence planes hours in advance. Thus, planes may be delayed before they even take off, or may reduce power in flight and proceed more slowly in order to fit perfectly into a landing sequence without holding.

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 ARTCCs, a major weather problem is thunderstorms. Thunderstorms present a variety of hazards to aircraft, and pilots are extremely reluctant to operate in or near them. Aircraft will 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. However, 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 upgrading 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 the final digit, 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 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 and alerts the controller.
Minimum Safe Altitude Warning (MSAW): a tool that alerts the controller if an aircraft appears to be flying too low to the ground.
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 planes

Facts and known mishaps

Occasionally, 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.

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.