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Aircraft Traffic Management

Aircraft Traffic Management

In the early days of aviation, airplanes were mostly used as joy rides, in air shows, and for just general "barnstorming." Even into the 1920s, debate occurred regarding whether the airplane had a useful purpose, and one of the main concerns was the inability of planes to fly in bad weather. With the development of radio navigation aids, which would eventually lead to the flight instruments of today, radio landing aids were installed at larger airports. These airports became the first hubs for successful air travel and were connected to each other via airways, which are similar to highways that connect large cities. However, just as highways funnel traffic into a city, causing traffic congestion and possible collisions, airways also experience similar problems near airports, making methods of controlling air traffic necessary.

In the beginning, air traffic control (ATC) was mostly implemented via such tools as colored lights and flags. However, while lights and flags work well on the ground and therefore seem obvious choices for use at airports, they are only good for operations very close to airports and then only when the aircraft has a clear view of the airport. A system was needed to control the entire airway.

Early air traffic control systems were based on ensuring safe separationthat is, an aircraft departing an airport reported its position by radio. If the airplane was flying under poor visibility conditions, it would be difficult to determine its position with the crude radio navigation equipment of the time; the aircraft's altitude, which could be measured with reasonable accuracy even in poor visibility, was an important part of position reports. Other departing aircraft would then be held at the airport until the previous plane was sufficiently far from the airport to ensure that no collisions would occur. The key person in such ATC was the "air traffic controller," who was in constant radio contact with the aircraft's crew. The problem with this type of ATC is that the required separation is so great, often more than 37 kilometers (20 nautical miles), that the capacity of the airport is diminished.

Primary and Secondary Radar

With the advent of radar , aircraft could be located from the ground even in clouds, and accurate position reports were not required from the aircraft. The required amount of separation was reduced significantly and the airport was used at a higher capacity. However, radar alone was not enough to ensure flight safety. The controller saw only anonymous green "blips" on the radar screen, denoting the aircraft. Secondly, the radar screen, which is called a plan position indicator, or PPI, is a two-dimensional display and aircraft are flying in a three-dimensional world. In order to control planes more accurately, a way was needed to identify the blips and to show their altitude. The system used is called a "secondary radar" system, a communications system that operates in concert with the radar.

With these two tools, primary and secondary radar, air traffic controllers could monitor more aircraft in less airspace and increase the amount of aircraft on the airways. The air traffic controllers noted the pertinent information about an aircraft such as its flight number, aircraft ID number, assigned altitude, and destination on a small card. This card was placed in a holder called a "shrimp boat" because of its shape; it held the card so that the data were easily visible to the controller. These data were a part of the flight plan, which was filed with ATC before the flight. As with the first ATC systems, constant communication with the aircraft was vital for safe separation.

Even with long distance radar, the controller can only handle a limited area of airspace, called a sector. When an aircraft goes from one controller's sector into another, the shrimp boat is pushed along a track to the next sector's controller. This process is called "handing off."

Computers in ATC

ATC involves a large number of radar sites, information from flight plans, and controller-entered information. One of the first uses of large computers was gathering and displaying these data. Radar sites are connected to an en route air traffic control center, which is responsible for tens of thousands of square kilometers (or miles) of air traffic. Smaller centers serve airports and are called "terminal radar control centers" or TRACONs.

Radar data require reliable high-speed data links . Data from several radar sites are combined and distributed to the sector displays. Even though air traffic control is accomplished with many controllers using data obtained from many radar sites, the entire system needs to be "seamless"; that is, there can be no gaps or areas not covered by ATC.

Historically the controller's PPI display would update in synchronism with the radar in real time. The image was not stored in a computer only displayed on the screen. The PPI cathode ray tube (CRT) had a very long "persistence," meaning the image would fade away very slowly, allowing the controller to view it for a few seconds. Before the image faded completely, the radar would make another sweep and refresh the image. Because the image grew dim as it faded, it was necessary to make the radar room dark.

Modern air traffic control systems store the images digitally. The display is much brighter, can contain color, can be zoomed in or out, and can be scanned with a trackball or mouse. This improves the controllers' working conditions.

In the early part of the twenty-first century, air traffic control began a massive change to a totally different philosophy called "free flight." The existing air traffic control system funnels aircraft onto airways and then controls them from the ground. If aircraft were to choose their flight plan without using airways, the preferred route would be a direct route from departure to destination; separation would be automatic. Of course there is a chance that routes would cross and cause a collision. In a free flight system, aircraft choose a direct flight path and enter that flight path into a computer. A software algorithm called a "conflict probe" compares the flight plan to those of other aircraft, looking for potential collisions. If a potential collision does exist, the flight plan will be rejected and the aircraft will be requested to make a change.

The computer also receives data from the air traffic radars and predicts future paths of aircraft already airborne. If the path of any aircraft projects a collision, the air traffic manager contacts the aircraft in question and requests a change of course. With this system, air traffic control becomes air traffic management or ATM.

The key to this system is a massive networked computer system with data from flight plans, en route radar data, and aircraft. Just as important as the computer is a network of data links, including air-ground-air links. A much smaller staff of air traffic managers would be able to ensure a higher level of safety for future air traffic.

see also Aircraft Flight Control; Global Positioning Systems.

Albert D. Helfrick


Illman, Paul E. The Pilot's Air Traffic Control Handbook. New York: McGraw-Hill, 1999.

Jane's Air Traffic Control. Alexandria, VA: Jane's Information Group, 1994.

Nolan, Michael S. Fundamentals of Air Traffic Control. Pacific Grove, CA: Brooks/Cole, Wadsworth, 1999.

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