Air traffic control

Air traffic control (ATC) uses technology and trained staff to assure safe movement of aircraft in airspace and at airports.

History and Evolution

Air transportation is essential to modern life, and it requires that passengers feel safe during air travel. The features of aviation that distinguish it from other transportation forms are its high speed and vertical operation. Crashes are devastating because of their intense impact, created by the heights from which aircraft can fall and the speeds at which they may be traveling. The potential danger is alarming to consumers, especially because the frequently high death tolls from single crashes make a strong impression in public awareness, while many people are unaware of the great overall safety of air transportation compared to other forms of travel.

The potential for severe injury or death to aircraft passengers has led to air traffic control (ATC) systems that have evolved from early traffic control with signal flags in the 1920’s to the sophisticated systems using advanced technology and specially trained staff of the twenty-first century. Current ATC assures the safe movement of virtually all aircraft operating in airspace and at airports. Its objectives include giving pilots all the data and control services needed to maximize safe, efficient aircraft operation; maximizing safe air traffic at airports; and minimizing unavoidable flight arrival and departure delays. It is ATC, a product of the National Airspace System, that makes air transportation the safest means of mass transportation in the United States.

Commercial airplanes generally travel airways, which are analogous to roads, although they are not physical structures. Airways have fixed widths and defined altitudes, which separate traffic moving in opposite directions. Vertical separation of aircraft allows some flights to pass over airports while other processes occur below. Air travel usually covers long distances, with short periods of intense pilot activity at takeoff and landing and long periods of lower pilot activity while in the air, the portion of the flight known colloquially as the “long haul.” During the long-haul portion of a flight, pilots spend more time assessing aircraft status than searching out nearby planes. This is because collisions between aircraft usually occur in the vicinity of airports, while crashes due to aircraft malfunction tend to occur during long-haul flight.

Flying Rules

The main rule systems governing flights are instrument flight rules (IFR) and visual flight rules (VFR). The minimum instruments needed for VFR are an airspeed indicator, an altimeter, and a magnetic direction indicator. In VFR, pilots fly using visual ground references and a “see and be seen” rule. The minimum requirements for VFR vary, but often include cloud ceilings of 1,000 feet and visibility of three miles.

IFR are used if aircraft operate above 18,000 feet, an area known as Class A airspace. Outside this airspace, any aircraft may use VFR, although only slow-moving, low-flying aircraft or small jets on short flights routinely do so. In some conditions, such as congested airspace around medium and large airports (Class B, C, and D airspace), in poor visibility, and at night, pilots must obtain permission from ATC controllers to fly by VFR and usually are only granted that permission if they are instrument rated and there is at least one mile visibility.

At airports with control towers, all air and ground movements are subject to permission from and instruction by ATC. This is because major airport peak traffic may involve three to four landings or takeoffs per minute. With dense aircraft concentration around airports, maintaining acceptable collision risk is not possible without strict adherence to procedures set out and monitored by ATC.

All pilots wishing to utilize IFR must demonstrate their ability through detailed testing, and all aircraft must have adequate flight instruments. For each flight, a detailed flight plan must be filed with the Flight Service Station, part of the Federal Aviation Administration (FAA); flight clearance must be received from Clearance Delivery or Ground Control (or from Approach Control if the pilot files while in the air); and ATC directions must be followed throughout the flight. Such directions usually depend upon radar surveillance, including the use of airborne radar beacon transponders that allow ATC facilities to identify each aircraft in flight.

Airspace is divided into classes designated A through E. Class A airspace is all airspace above 18,000 feet; Classes B, C, and D are designated around specific airports rated by their size and amount of traffic, and Class E covers all other airspace between 14,500 and 17,999 feet. In addition to vertical airway spacing, horizontal spacing is important. This is created by planned time intervals (often ten minutes) between aircraft on the same track, with lateral spacing of ten miles.

Landing, Takeoff, and En Route Procedures

Air terminal ATC, the element of air traffic control that is most familiar to the public, involves aircraft departures and arrivals. Its procedures include issuing instrument flight rules route clearances and communicating departure runways, taxi instructions, and definition of climb and altitude routes. These operations assure passengers of safe, speedy air traffic patterns.

A departing aircraft enters the taxiway as instructed by the ground controller and the pilot awaits being fitted into the pattern of incoming and outgoing flights. ATC controllers allocate available departure runways to enable safe aircraft separation. Once the aircraft climbs to its initial altitude on an ATC-instructed heading, departure control makes sure that radio contact with the departing pilot is established before allowing a new takeoff. More instructions clear the aircraft for its final climb to the en route segment of the flight and for transferring the pilot to the next control facility.

Air traffic controllers relay descent instructions to incoming aircraft to keep them separated by five-mile intervals. As the aircraft approaches an airport, its speed is adjusted and its flight path altered to maintain an aircraft separation of over three miles within airport boundaries. ATC controllers determine aircraft landing sequences, stacking plans, and takeoff adjustments to handle aircraft flow. To simplify this flow, all commercial aircraft remain at their origin airport until it is confirmed that a landing site will be available at their destination airport at the planned arrival time. Travelers often become frustrated when a pilot cannot obtain a landing slot after leaving the gate at the origin airport, but the practice maximizes safety since flight delays are safer when spent on the ground than in the air.

The last part of descent control transfers the aircraft to the approach controller. Data from radar surveillance determine the final landing directions. Radar monitors optimize landing, and once on a runway, the pilot and the ground controller interact to prevent aircraft movement conflicts on the field. This controller also tells the pilot how to reach the craft’s apron or parking position at the airport.

En route ATC includes monitoring the routes between terminals granted to individual pilots. A flight follows a predetermined path in a defined airway corridor. Effective en route ATC instructs pilots when and how to avoid nearby aircraft. During most flights, a given ATC facility periodically transfers control of each flight within its jurisdiction to the next facility on a flight plan. For this reason, ATC gives pilots radio-frequency changes that occur as they are passed on to the next controller along their flight paths.

The availability of inertial navigation units for commercial aircraft has reduced the need for this communication. In an inertial navigation unit, a computer and gyroscope are oriented to true north, while speed sensors track the aircraft’s direction and the distance to its destination. Although inertial navigational units can fly virtually automatically until the aircraft reaches an airport, en route information is provided for safety and to warn of impending delays or other dangers. As a result, all IFR aircraft are monitored continuously throughout each flight. In addition, pilots must get ATC approval before making any flight path alterations. Required en route progress reports are tracked on air route surveillance radar, which monitors aircraft in each sector of the air route ATC system.

Craft-to-Ground Communication

Navigation within a designated—and desired—airspace requires pilots to identify the exact position of their aircraft and assure that they are in the airway assigned in their flight plan. This depends upon ground beacons and electronic equipment in airplanes. The most widely used ground system is the very high frequency omnidirectional range beacon (VOR). VOR stations operate on noise-free radio frequencies and provide fine accuracy. On airplanes, visual displays indicate the course needed to travel directly to a VOR station.

Most stations have distance-measuring equipment, which gives pilots the distance to VOR stations. VOR and distance-measuring equipment service all aircraft. Other methods used for navigation are Doppler navigators and inertial navigation systems, which do not require ground stations or radio beams. Another navigational tool is the Global Positioning System (GPS). It is composed of GPS satellites and provides worldwide position ability accurate to 350 feet horizontally and 500 feet vertically. GPS is available for all phases of flight.

Pilots and ATC controllers communicate by radio en route and at airports. This helps ATC to make continuous updates of the positions of all planes in its operation area and provides an unambiguous means of instant flight instruction communication. All ATC surveillance of aircraft uses radar. Each radar system possesses a primary beacon that sweeps its coverage area and transmits images of all objects encountered back to a radarscope. The primary beacons are synchronized with a secondary radar system that uses automatic aircraft transponders to identify each flight in a given jurisdiction. Such radar systems are found in many air route traffic control centers and air terminal radar control facilities, providing sophisticated ATC.

The operation of the ATC system utilizes computer-assimilated flight information such as the position, course, airspeed, and transponder number of all craft in a jurisdiction. This enables controllers to determine the safest, most direct flight plan for each aircraft and to make continual updates. ATC also incorporates traffic alert and collision avoidance systems in aircraft. ATC technology in the United States is located at a national air traffic control command center at about twenty-four regional control centers, nearly six hundred terminal control facilities, and 250 flight service stations. All facilities interact to provide a nationwide weather profile, data on all airborne flight positions, and information on airport flight acceptance. Data are analyzed by a national computer and automatically circulated to regional facilities and airports. Regional air traffic control facilities are also computerized and automated.

Recognition of aircraft under IFR is essential at all points in a flight, especially when aircraft enter an airport terminal control area. Recognition is achieved through cross talk between aircraft flight transponders and surveillance radar beacons. For this reason, the FAA requires that all aircraft flight transponders are kept on from takeoff until landing is complete. A flight transponder provides several levels of information to ATC. When turned on, it continually transmits a radar symbol showing the geographic position of the aircraft relative to ATC facilities. The level of information transmitted is determined by a pilot’s responses to queries from ATC facilities. Most flight transponders also alert ATC facilities of radio failure, skyjacking, and other emergencies by pilot input to transmit specific code numbers.

Terminal Control

Terminal ATC is found in most public airports. Control facilities are divided into two parts, an ATC tower (ATCT) and approach and departure control. The tower controls approaching or departing aircraft in the five-mile radius of the airport traffic area around the airport. Approach and departure control covers a radius between five and thirty-five miles from the airport, beginning where tower control stops.

ATC controllers identify and sequence all aircraft in the airport traffic area, expedite aircraft arrivals into and departures from the airport traffic area, control all ground aircraft movement, cancel flight plans, and provide other useful information. In both landing and takeoff control, IFR aircraft may be provided with horizontal and vertical path guidance. In contrast to ATCT control, which regulates the flow of traffic within the airport itself, approach and departure control regulates the safe and efficient flow of air traffic around airports.

This regulation begins as soon as an aircraft reaches the perimeter of the area under approach and departure facility control. At that time air traffic controllers begin to fit the aircraft into the pattern of current air traffic. They define the course, altitude, and speed changes that each aircraft needs for a safe landing within the framework of existing arrivals and departures.

Terminal ATC is based mostly on data obtained from ATC radar beacon systems. Major air terminals are equipped with computerized automated radar terminal systems (ARTS), which accept radar inputs from all surveillance systems. They automatically show individual aircraft on ATC video displays, where a plane is seen as a radar screen data block in front of the controller. The block shows a symbol for each aircraft, along with its identifying call sign, speed, and altitude. The systems also warn of possible collisions or instances where aircraft approach minimum safe altitudes.

Using ARTS, all flights are kept at specific distances, horizontally and vertically, from one another. Flight plans are fed into computers and updated as flights progress. ATC controllers watch carefully to prevent collisions. As aircraft converge on an airport to descend for landing, congestion may occur. In this situation, IFR planes are instructed to circle a specific location designated by an IFR intersection. As more planes arrive, they are held increasingly farther out from the airport.

For instrument landings, pilots use an instrument landing system (ILS) similar to VOR. Cockpit instruments indicate deviations to either side of a localizer beam leading to the runway, and guidance information from a glide slope beam warns if the plane is too high or low on the approach.

The State of Flux in ATC

Air safety is created by the efficient operation and continued updating of ATC. ATC operation modes change quickly and extensively, since the system operates at near capacity, while the total number of operating aircraft is increasing. As a result, the FAA is continuing an automation and modernization program addressing aspects of en route air navigation methods, weather identification and dissemination, flight service stations, and facets of airport terminal ATC. Goals of this National Airspace System Plan include meeting continued increased airspace demands, reducing operational errors, collision risk, and weather-related accidents, and minimizing escalating ATC operation and maintenance costs. ATC’s main need is to increase its efficiency, because while industry demand will grow, major airports do not have the ability to significantly expand, due to physical or environmental constraints. The prospect for construction of new large airports is also dim.


  • Buck, Robert N. Weather Flying. New York: Macmillan, 1988. Describes bad weather flying and related ATC, weather checks and information, equipment needs, VFR, and takeoff and landing in bad weather.
  • Cronin, John. Your Flight Questions Answered: By a Jetliner Pilot. Vergennes, Vt.: Plymouth Press, 1998. Discusses airports and ATC from a pilot’s perspective.
  • Garrison, Kevin. Flying in Congested Airspace: A Private Pilot’s Guide. Blue Ridge Summit, Pa.: Tab Books, 1989. Covers ATC history, U.S. airspace, airport radar service and terminal control areas, VFR and IFR safety techniques, and departure and arrival regulations.
  • Illman, Paul E. The Pilot’s Air Traffic Control Handbook. Blue Ridge Summit, Pa.: Tab Books, 1993. Provides pilot information on ATC, its history, use of airspace, terminal ATC, approach and departure control, flight service stations, and air traffic controllers.
  • Massie, David. Airline Pilot: Let the Pros Show You How to Launch Your Professional Piloting Career. New York: ARCO, 1990. Covers pilot demographics, education, qualifications, occupational ratings, training and experience levels needed, and testing.
  • Mathews, James A. How to Prepare for the Air Traffic Controller Exam. Hauppauge, N.Y.: Barron’s Educational Series, 1997. Gives insights on the air traffic controller’s skills and education needs, as well as qualifying exams.
  • Rowberg, Richard E. Safer Skies with TCAS. Washington, D.C.: U.S. Government Printing Office. Report on Office of Technology Assessment thoughts on traffic alert and collision avoidance systems.

Air traffic controllers spend much of their time tracking the complicatedmovements of flights on radar screens.

(AP/WideWorld Photos)

Top Ten FAA-Operated Air Traffic Control Towers, 1996

Annual Operations

Chicago O’Hare International, Illinois

Dallas/Ft. Worth International, Texas

Atlanta International, Georgia

Los Angeles International, California

Miami International, Florida

Phoenix Sky Harbor International, Arizona

Van Nuys, California

Detroit Metro Wayne County, Michigan

St. Louis International, Missouri

Oakland International, California

Source: FederalAviation Administration, Statistical Handbook ofAviation, 1996.