Introduction to Air Traffic Control

Air Traffic Control comprises the various aircraft navigation and communication systems that use computers, radar, radios, and other instruments and devices to provide guidance to flying aircraft.

Trained personnel working as air traffic controllers at stations on the ground constantly monitor these systems and track the locations and speeds of individual aircraft. Controllers can warn aircraft should they come too close to each other. Air traffic control is also used for the safe coordination of landings and takeoffs at airports.

The goal of air traffic control is to minimise the risk of aircraft collisions while maximising the number of aircraft that can fly safely at the same time. Aircraft pilots and their onboard flight crews work closely with controllers to manage air traffic. Air traffic control systems also provide updated weather information to airports around the country, so aircraft can take off and land safely. This information is important not only to airline passengers but also to industries that rely on aviation for the timely transport of goods, materials, and personnel.

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Elements of Air Traffic Control

Air traffic control is a combination of three general elements. The first element is the basic set of flying rules that pilots follow in the air. These are much like the traffic rules that motorists must obey. The second element is the multitude of electronic navigation systems and instruments that pilots use to remain on course. The third element is made up of air traffic controllers and the computer systems they use to track aircraft during takeoff, flight, and landing. These three elements work together to keep aircraft safely separated in the air and to avoid collisions.

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Flight Rules

The basic system of air traffic control relies on the ability of pilots to provide their own navigation in order to see and visually avoid other aircraft. This system is known as Visual Flight Rules (VFR). Under VFR pilots navigate using charts that display terrain features, airports, and landmarks. VFR pilots also may use radio beacons or other ground-based navigational aids to monitor their flight path. To avoid other aircraft, pilots fly at specified altitudes reserved for their general direction of flight. Pilots also simply keep a constant lookout for other aircraft. VFR works well where visibility is good, aircraft speeds are fairly low, and air traffic is sparse. VFR pilots must remain clear of clouds and have a range of visibility of at least 5 km (3 mi).

Control Screen

When any of the VFR conditions cannot be met, or if a pilot is operating in a busy area, aircraft must be operated under Instrument Flight Rules (IFR). IFR is a more complex set of rules, and pilots flying under IFR must have an instrument pilot certificate. IFR requires that pilots notify the airport control tower of their intended route before takeoff, a procedure known as filing a flight plan. Once the tower gives clearance, the pilot may take off. The pilot must also maintain radio contact with air traffic controllers during the flight. IFR is required whenever flight visibility is less than 5 km (3 mi), when pilots must fly through clouds, or when pilots are flying in congested areas. Airlines and larger aircraft normally operate under IFR at all times.

Air Traffic Control Tower

The flight crew of an aircraft, made up of the pilot and any other personnel that fly or navigate the aircraft, use various instruments when flying under IFR. These instruments are designed to work in any weather condition, day or night, and tell the pilot the direction and speed of the aircraft. The altimeter indicates altitude, and the airspeed indicator shows how fast the aircraft is moving. The attitude indicator shows how the aircraft is tilted in flight. Other instruments indicate direction.

The flight crew also uses radio to stay in contact with air traffic controllers. Flight crews file flight plans with the control tower by radio, and ask for clearance before taking off or landing at an airport. Another communications instrument used by aircraft is an automatic device called a transponder. A transponder sends an electronic identification signal to air traffic control centres on the ground. Controllers use transponder signals to identify individual aircraft and track their positions by computer.

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Navigation Systems

Navigation systems assist pilots in flying from one airport to another. These systems help both pilots and air traffic controllers determine an aircraft's position relative to the ground and to other aircraft. At high altitudes, or during bad weather, navigation systems are essential for safe aircraft flight. Navigation systems have developed from fairly inaccurate ground-based radio transmitters to sophisticated space-based systems.

The earliest navigational aids were simple radio beacons, in use since 1924. Radio beacons provided the pilot with only the ability to head toward the beacon. Although fairly inaccurate, beacons were inexpensive to install and were at one time fairly numerous.

The basic electronic navigation system in use is the VHF omnidirectional range (VOR) system. VOR consists of a series of radio stations that beam direction information to aircraft. Most VOR stations also have distance-measuring equipment (DME). A display indicator in the aircraft reads the signals and tells the pilots if they are on course and how far they are from the station. VOR-DME systems are limited in range to 260 km (160 mi) and can only provide direct courses to or from a given station.

Satellites provide a better system of area navigation than ground-based radio stations. In the 1980s the U.S. Department of Defense developed a highly accurate satellite-based navigation system known as the Global Positioning System, or GPS. GPS and other satellite navigation systems provide highly accurate positioning information to anyone using an appropriate receiver.

GPS-type systems are so accurate that satellite navigation is becoming the standard for international aviation navigation. Satellite navigation provides adequate accuracy for in-flight navigation.

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Landing systems

The navigation instruments that pilots use to land aircraft during foul weather are more sensitive than those used to navigate during flight. The systems mentioned above only guide aircraft to within 2 km (1 mi) of the end of an airport runway.

Landing Systems

To guide aircraft to a safe landing, many runways have been equipped with the Instrument Landing Systems (ILS). An ILS uses two transmitters to guide aircraft to within 800 m (0.5 mi) of the runway. One transmitter provides altitude information as the aircraft approaches the runway, and the other transmitter alerts the pilot if the aircraft drifts to the left or right of the runway path. More sophisticated versions of the ILS guide aircraft to within 400 meters (0.25 mi) of the runway, or in some airports around the world even to the runway itself for an automatic landing.

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How Air Traffic Control Works

Before departure, IFR pilots file a flight plan and contact the clearance delivery controller to receive their clearance to fly. A clearance includes the route and flight altitude, the frequencies for radio and transponder use, and departure instructions. At airports with a control tower, both IFR and VFR pilots contact the ground controller to receive taxi instructions, which tell the pilot which runway to use, and when to proceed. When ready for departure, the pilot contacts the local controller. When the local controller is confident that the runway and all intersections are clear of traffic, the airplane is cleared for takeoff. Once airborne, IFR pilots contact the departure controller to receive heading and altitude instructions, guiding the airplane to the appropriate airway. VFR pilots usually navigate visually to their destination airport.

In most cases, airliners and business aircraft file IFR flight plans and use the ATC system during their entire flight, even if the weather is suitable for visual navigation. This is a safety requirement of both the CAA and the airlines, since these flights occur at high speed and in congested areas. Small, privately owned aircraft usually operate under VFR once they have left the immediate vicinity of the airport. Since VFR pilots operate at low altitudes where airliners do not typically fly, and at much slower speeds, it is easier for them to see and avoid other aircraft. If they operate exclusively from small airports, they may never need to contact a controller at all. But once within the vicinity of a large airport, they are required to make contact with a controller so that separation of all aircraft can be provided.

Air traffic controllers watch radar displays that show the locations of individual aircraft. These displays also predict future positions and altitudes of aircraft. If the computer detects that two aircraft might come too close to each other or that one aircraft might descend to an inappropriately low altitude, it will sound an alert and the controller will tell the pilots to change course. A similar computer system installed in most airliners is called the traffic alert/collision avoidance system, or TCAS. TCAS independently monitors the positions of nearby aircraft and determines whether a potential for collision exists. If TCAS predicts a potential problem, it alerts the pilots automatically and issues course and altitude changes to avoid a collision.
Once an aircraft has flown 50 km (30 mi) from the airport, the departure controller transfers, or hands off, the tracking signal to a succession of ARTCC controllers. ARTCC controllers monitor the aircraft's progress, separate it from other aircraft, and issue route or speed changes when needed to avoid bad weather or to keep the aircraft in the proper flow of traffic. As an aircraft flies out of range and toward another ARTCC, the tracking controller hands off the signal to a controller at the next ARTCC, who monitors the aircraft as it continues on its journey.

Once the aircraft is close to its destination, the controller issues arrival instructions to the pilot, and then hands the aircraft off to the approach controller at the airport. VFR pilots usually contact approach control 50 km (30 mi) from the destination airport. Approach control is responsible for lining inbound aircraft up for the runway. Once aircraft are properly spaced, local control takes over and issues landing instructions. If there is a delay in landing, an aerial traffic jam can develop. To avoid this, aircraft waiting to land are directed to a holding area away from the runway. At the holding area the waiting aircraft circle a radio beacon at different altitudes, forming a stack of aircraft. When a runway becomes available, an airplane at the bottom of the stack is instructed to land, and the waiting aircraft spiral down one layer. After the aircraft has landed and taxied off the runway, ground control issues taxi instructions that direct the aircraft to parking.

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