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History of Air Traffic Control

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Air Traffic Control in the United States (ATC)

The Air Traffic Control (ATC) system is run by the Federal Aviation Administration, an agency of the U.S. Department of Transportation. The government developed the system primarily to maintain safe separation of aircraft flying over the United States and in and out of U.S. airports. Secondarily, it is ATC's job to keep aircraft traffic moving as efficiently as possible throughout the system. In short, ATC is aviation's traffic cop, working to ensure that aircraft do not run into each other and that traffic moves in an orderly fashion with minimum delays.

ATC Facilities

There are several types of ATC facilities. These include the airport towers familiar to most travelers, terminal radar approach control facilities (TRACONs), en route centers and flight service stations.

The airport towers control aircraft while they taxi to and from runways and during takeoffs and landings. The FAA bases its decision to build and operate a tower on the number and type of aircraft operations at a given airport. More than 450 U.S. airports currently have such towers.

TRACONs control the aircraft immediately prior to and after landings and takeoffs, or during the climb and descent phases of flight. There are 236 TRACONs, less than the number of towers because some TRACONs handle more than one airport. For example, a single TRACON handles the traffic approaching and departing from all three New York-area major airports.

The 21 ATC centers cover even broader areas. Their job is to keep track of aircraft while they are en route or during the high-altitude cruise phase of their flights. They are located in Albuquerque, Anchorage, Atlanta, Boston, Chicago (the busiest center), Cleveland, Denver, Fort Worth, Houston, Indianapolis, Jacksonville, Kansas City, Los Angeles, Memphis, Miami, Minneapolis, New York, Oakland, Salt Lake City, Seattle and Washington, D.C.

Flight service stations are information centers for pilots flying in and out of small cities and rural areas. Currently, there are 68 of these stations providing such things as weather reports and route and terrain information. In addition, flight service stations assist in emergency situations, initiating and coordinating searches for missing or overdue aircraft.

Central Flow Control

Another key facility, overseeing the entire ATC system, is FAA's Air Traffic Control System Command Center (ATCSCC), also known as central flow control and located in Herndon, Virginia. Its job is to look for situations that will create bottlenecks or other problems in the system, then respond with a management plan for traffic into and out of the troubled sector. For example, if bad weather develops or a runway is closed for repairs, ATCSCC will manage the number of aircraft operations into and out of the affected area. The objective is to keep traffic levels in the trouble spots manageable for the controllers.

Tracking Systems

ATC primarily uses radar to keep track of aircraft flying over the United States. Radar transmits radio waves of ultra-high frequency that bounce back to their source when they hit something solid. The return signal, or radar echo, is then analyzed by the receiver to determine both the distance and direction of the object hit. In the case of airplanes, a transponder aboard the plane senses the radar signal and responds with an amplified radio signal directed toward the source of the signal received. The return signal not only is stronger, but contains a discreet four-digit code that identifies the aircraft to a ground radar station. A type of transponder known as Mode C, which is used aboard all commercial jets, also is capable of encoding the aircraft's altitude onto the return signal. Controllers on the ground then know how far away the aircraft is, how high it is, the direction it is headed, the airline operating the flight, and the type of aircraft at which they are looking. In the future, satellites are expected to supplant ground-based radar as the primary means for keeping track of airplanes.


Flight crews and air traffic controllers communicate by radio using VHF frequencies between 118 and 136 megahertz. Pilots tune to the frequency of the controller tracking their flight, and switch frequencies, as they move through the system and are handed off from one controller to another.

A Typical Flight

From the standpoint of ATC, all airline flights begin with the flight plan, which spells out the route the flight crew plans to follow, alternative airports the crew would use in the event of an aircraft emergency or a problem at the intended destination, as well as the amount of fuel onboard the aircraft. For some flights, the pilot puts together the flight plan and submits it to ATC, via their dispatcher, just prior to flight. Many airlines which fly the same routes every day, however, keep flight plans stored in the FAA's computer and merely activate them through their dispatch system prior to flight. In any event, a flight plan provides crucial information to ATC about what a particular crew intends to do.

Once the pilots have completed their pre-flight planning and aircraft inspections and have settled into the cockpit, they make their first call to ATC. Typically, this call is made to clearance delivery, which goes over the ATC routes and instructions the crew can expect from takeoff to landing. Ideally, this information matches the route requested in the flight plan, but that is not always the case. ATC sometimes has other ideas, and may give pilots new instructions before and even during a flight.

When a crew is ready to depart, it contacts ground control for permission to leave the gate. Airlines sometimes conduct their own ground control at their hubs, but only in the immediate area of their gates. Once an aircraft leaves a gate area and begins to taxi toward a runway, it comes under the jurisdiction of FAA ground control, which governs the movement of all vehicles around an airfield.

Since aircraft must occasionally taxi across an active runway, ground control coordinates its instructions with tower control, which oversees all movements across or along runways. The tower assumes full control of the aircraft as soon as it reaches the end of the runway it will use for takeoff. When the runway is clear, the tower grants permission for takeoff. It also instructs the crew on the heading, or direction, it should follow immediately after takeoff.

When safely airborne, tower control hands off the aircraft to departure control, which oversees the flight as it climbs away from the airport and enters the en route airspace. Given the speed and climb capabilities of modern jets, this may only take a few minutes. Departure control then turns over the flight to an en route center.

All of these and subsequent handoffs are accomplished by radio. The controller who is handing off the flight instructs the crew to contact the next level of ATC surveillance, and gives them the radio frequency they need to do that. Once contacted, a receiving controller acknowledges radar contact with the plane and issues instructions for heading and altitude.

Depending on where the plane is going, it may be handed off many times, from one en route controller to another, during the course of its flight. En route controllers are assigned to specific geographic areas, and they work to maintain the safe separation of aircraft only in their sector of airspace.

Aircraft separation standards vary according to circumstances. Above 29,000 feet, when aircraft are cruising at high speeds in the en route airspace, the standard is five miles of horizontal radar separation or 2,000 feet of vertical separation. Below 29,000 feet in the en route airspace, the vertical separation is reduced to 1,000 feet while the horizontal radar separation remains at five miles. When aircraft are moving at much slower speeds as they depart or approach an airport, the standard is three miles of horizontal radar separation or 1,000 feet of vertical separation. In certain oceanic airspace, vertical separation has been reduced to 1,000 feet at altitudes above 29,000 feet. Eventually, this reduction will follow in domestic airspace as well.

As a flight crew approaches its destination airport and begins its descent, it is instructed to contact approach control. An approach controller will issue maneuver instructions to the crew to integrate the aircraft into the flow of other aircraft arriving at the airport. As soon as the crew is on its final, straight-in approach, the approach controller hands the aircraft off to the airport tower, which grants final clearance to land and monitors the aircraft until it completes its landing and exits the runway. A ground controller then directs the aircraft to its gate.

While all commercial airline aircraft are controlled every step of the way, the same level of positive control does not always extend to general aviation aircraft. These aircraft can, and often do, fly in uncontrolled airspace, outside the ATC system. In general, these uncontrolled spaces are areas below the cruise lanes used by commercial airline aircraft, and outside the airspace the airlines use on takeoff and approach to landing at the 450 plus airports with FAA control towers. Since aircraft climb and descend at an angle, the controlled airspace above an airport resembles the conical shape of a giant, upside down wedding cake over the airport property.

General aviation aircraft are allowed to fly under visual flight rules, or VFR, when weather and visibility are good. They do not have to file a flight plan, and they do not have to be in touch with air traffic control, unless they choose to operate in or out of an airport with a control tower. Under VFR, pilots are responsible for maintaining adequate separation from other aircraft, which is why these rules sometimes are called the see and be seen rules.

Instrument flight rules, or IFR, on the other hand, are the rules under which general aviation aircraft must fly in bad weather and low visibility. Pilots must be in contact with ATC and must file a flight plan. They also must be instrument rated, meaning they are proficient at navigating and flying their aircraft using cockpit instruments only, without benefit of good visibility out of the cockpit windows. Commercial airline flights always operate under instrument flight rules, regardless of weather, since they operate solely within the ATC system.

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Controller Training

All air traffic controllers work for the FAA and all must go through a screening process and rigorous training before they are certified to control airplanes. Typically, applicants who pass through initial screening, go first to the FAA training academy in Oklahoma City. On completion of that program, they are assigned to an actual ATC station where they receive extensive on-the-job training. Among other things, controllers must master traffic management techniques, communication skills, and knowledge of the specific area they are overseeing.

Airport and Airway Trust Fund

In 1970, Congress created the Aviation Trust Fund to pay for improvements to airports and the ATC system, such as new runways and taxiways, control towers, landing aids, radar systems, etc. In the years since, Congress also has authorized the use of trust fund money for FAA operating costs, such as the salaries of controllers.

The money in the fund comes from taxes and fees paid by users of the aviation system, primarily air travelers and shippers. Congress has raised the taxes several times. By 1994, travelers were paying a tax on all domestic tickets and shippers were paying a tax on their freight bills. Altogether, these and related aviation taxes and fees collected from U.S. airlines in FY99 totaled $21 billion.

Delays, Modernization and Corporatization

Because ATC is involved in the movement of all commercial airline aircraft, the capabilities and efficiencies of ATC has a direct bearing on the schedule performance of the airlines. An equipment glitch or personnel shortage at an ATC facility, for example, usually means that the flights it handles will be delayed because the controllers get behind in their work. They cannot process the flights fast enough to prevent a backup of traffic on the taxiways or at the airport gates. Bad weather, of course, is the primary cause of most back-ups, but deficiencies in the ATC system itself also play a major role in airline delays.

In recent years, there have been an average 900 daily flight delays of 15 minutes or more. The cost of these delays to the airlines and their customers is estimated at more than $5 billion annually. That is the estimated cost of the extra fuel that aircraft have burned waiting their turn to take off, the extra crew costs incurred from delayed flights, the cost of buying or leasing the additional planes needed to maintain service in a congested system that reduces equipment utilization, the cost to passengers of extra nights on the road due to missed connections, and many other costs related to the delay problem.

When air travel and service soared following deregulation, the FAA began a massive modernization effort intended to bring the ATC system up to where it needed to be, in order to handle air traffic efficiently. However, the effort quickly bogged down and remains troubled, with little to show in terms of reducing airline delays.

The concept of a federal corporation to run ATC, more along the lines of a modern business, was advanced by the airlines in the mid-1980s. However, the idea met considerable opposition at that time, and again in 1994, when the Clinton Administration advanced its own version of the concept.

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