Air Traffic Control

I

Introduction

Air Traffic Control, 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 minimize the risk of aircraft collisions while maximizing 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.

II

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.

A

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).

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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. In the United States, the Federal Aviation Administration (FAA) is the federal agency that regulates air travel. The FAA requires that all aircraft use IFR when flying near major metropolitan areas or at the high altitudes normally used by commercial airliners.

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 centers on the ground. Controllers use transponder signals to identify individual aircraft and track their positions by computer.

B

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. Advances in navigation technology led the FAA to decommission many of these navigation aids.

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. This limitation compelled the FAA to install thousands of ground stations across the United States and to provide over 8,000 airway segments connecting each VOR-DME station to another.

Researchers have been working since the 1950s to increase the flexibility of the VOR system. Area navigation systems have been developed that permit a pilot to fly directly from one airport to another, bypassing the VOR airways. Loran (long range navigation) is a radio system that automatically calculates an aircraft’s position and provides direct navigation guidance to any location. However, the charged particles in the layer of the atmosphere known as the ionosphere limit the radio range of Loran signals and can sometimes cause interference.

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 the FAA and its international counterpart, the International Civil Aviation Organization (ICAO), have agreed that satellite navigation will become the standard for international aviation navigation. Satellite navigation provides adequate accuracy for in-flight navigation, but will need to be improved if it is to guide aircraft during the more complex landing procedure. Two systems have been developed and are planned for installation by the FAA. One system, called the Wide Area Augmentation System (WAAS), uses a satellite transmitter to send accuracy corrections to all aircraft operating over the continental United States. The other, the Local Area Augmentation System (LAAS), will be installed at airports to provide guidance information that will allow automated aircraft landings in any type of weather.

One type of instrument navigation that does not rely on radio or satellite transmissions is inertial guidance. Inertial guidance uses mechanical or laser gyroscopes to determine precisely an aircraft’s direction of flight. When an inertial guidance system has been programmed correctly, it can provide direction to any point in the world. Although inertial guidance is fairly costly, its biggest advantage is that it is a self-contained system, independent of either ground or space-based transmitters.

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. To guide aircraft to a safe landing, many runways have been equipped with the Instrument Landing System (ILS). The 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 to the runway itself for an automatic landing. The combination of the satellite-based WAAS and LAAS is planned to replace ILS and should provide approaches to the major runways in the United States.

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