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Definition: air traffic control from The Macquarie Dictionary

direction of airborne aircraft movement by ground-based personnel via a radiotelephone link or, in emergencies, by light signals.

Plural: air traffic controls

Summary Article: Air Traffic Control
from Encyclopedia of Transportation

No mode of public transportation has a greater emphasis on safety than the aviation industry. From manufacturing to maintenance requirements, every phase of air travel is closely scrutinized to ensure the safest environment for the air traveler. This same mind-set extends into the role of the professional air traffic controllers who must move the vast number of daily flights and the passengers aboard them. These efforts have resulted in the development of the safest mode of travel available—commercial flight. The thousands of daily flights that involve private general aviation aircraft also factor into this mix.

So how does one constantly mix the largest of jumbo jet aircraft with the smallest of privately owned aircraft and still maintain the level of safety demanded by the flying public? The Federal Aviation Administration (FAA) has structured the National Airspace System (NAS) in a manner to ensure the separation of aircraft from other aircraft that share the crowded skies. Through years of aviation accident investigations and incident reports, the FAA has also constantly developed and revised specific procedures and phraseology that must be adhered to by all air traffic controllers. These procedures and specific ways of communicating with the aircraft, and with other controllers, ensure that the controllers maintain a safe, orderly, and expeditious flow of air traffic.

Preparing for Safety

To be able to comprehend the methods of separating aircraft, it is necessary to be able to visualize airspace as a three-dimensional block when looking at the flat surface of a radar scope. Air traffic controllers are trained to form a mental picture of the constantly changing traffic situations taking place within their airspace of responsibility. They ensure that the traffic is kept apart by applying rules of separation that are broken down into miles, altitudes, airspeeds, and times. Remembering that three-dimensional picture, one must understand that all the aircraft that a particular controller is working are being separated vertically, laterally, longitudinally, or visually. To further complicate the air traffic picture, factor in the differences in aircraft performance capabilities, such as airspeed, climb rate, and wake turbulence situations caused by mixing larger aircraft with smaller ones.

Every procedure and every phrase to be spoken, and how it is to be spoken, is provided by the FAA in great detail in one publication. The FAA Handbook 7110.65 is the procedural bible for air traffic control operations, procedures, and phraseology. The FAA uses this publication to mold every air traffic controller and to guide them in their daily operations for the rest of their careers. This ensures uniformity and a mutual understanding of all of the safety requirements and how they are to be maintained.

Air traffic controllers begin early with the training process. The vast majority of controllers are graduates of Air Traffic–Collegiate Training Initiative (AT-CTI) schools. These are colleges and universities selected by the FAA to provide training to undergraduate students attending these institutions. The training includes classroom lectures, laboratory simulations, and assessments of the students' performance. During their training, the controllers are exposed to as many types of emergencies as possible through the use of simulation. This helps in the development of a fine-tuned response that is mostly human reflex. The students are taught that the first operational priority of air traffic control is to separate aircraft and to issue safety alerts. A safety alert is defined as an aircraft coming within close proximity of basically anything that will make it come to a sudden stop, such as an obstruction, terrain, or another aircraft.

Air traffic controllers at work in a new 11-story control tower at Pope Field, North Carolina, in June 2013. Air traffic controllers must keep aircraft that are at the same altitude, or close to it, at a lateral distance of either 3 or 5 miles from each other, depending on their distance from the radar. If an aircraft is over 40 nautical miles from the radar, the 5-mile lateral separation is required because radar becomes less accurate the farther an object is from the antenna.

Safety Measures in a Radar Environment

Over the years, the FAA has devised a strong and proven system of separating aircraft. Presented here is a brief overview, or explanation, of how that system actually works. The goal of any separation system is to keep the targets separated. This is not rocket science. The first requirement is to establish a minimum distance that must be maintained to separate aircraft. This is called the separation minima. It is quite a logical system. Remember, as previously stated, all aircraft are being separated vertically, laterally, longitudinally, or visually. Approach control works the traffic in the air around the airport and usually out to a distance of approximately 60 miles with varying upper-level ceilings, depending on the airspace structure allocated to them.

For aircraft at the same altitude or less than 1,000 feet apart vertically, the minima is either 3 miles or 5 miles. The exact distance required between the aircraft is determined by how far the aircraft are from the radar antenna. Radar is inherently more inaccurate the farther an object is from the antenna. To allow for this “slop” within the system, the FAA has established a range of 40 nautical miles as the changeover point. If the aircraft to be separated are within 40 miles of the antenna, the controller must maintain 3 miles between the targets. If the aircraft are 40 or more miles from the antenna, the controller must maintain 5 miles between the targets. This is called lateral separation because the range is the lateral distance between the aircraft. Remember, they are at the same altitude.

Most air travelers have landed at a large, high-volume airport with parallel runways and, looking out the window, have seen other aircraft landing at the same airport that were less than 3 miles away. So, how can this be legal after reading and understanding the rules given above for lateral separation? It is really quite simple. When parallel runways are located more than 2,500 feet apart, they are considered to be a single runway and simultaneous operations may be conducted without regard for the lateral separation requirement. This operation is perfectly safe and allows the controllers to expedite the arriving traffic. Imagine how much of a delay there would be if 3 miles had to be maintained between each arriving aircraft at an airport with the traffic volume of Atlanta or Dallas/Ft. Worth airports.

For aircraft that are receiving vectors, or turns, within the arrival traffic pattern to position them for landing on the runways at an airport, vertical separation is also used. When controllers are using vertical separation, they are keeping the aircraft separated by 1,000 feet vertically. Controllers ensure that all arriving aircraft receive the local altimeter setting, or barometric sea-level pressure, as they arrive into the airspace. Therefore, all aircraft under one approach controller’s jurisdiction will be using the same altimeter setting, ensuring a uniform, consistent measurement of distance between each aircraft. Having all aircraft use the local altimeter setting ensures that the vertical distance between aircraft is accurate and the controller is using the aircraft’s exact altitude, since altitude is measured by pressure differential, or variation, from sea level. All aircraft below 18,000 feet operate on local altimeter settings.

All these rules and measurements work fine as long as you have a method to measure them—radar. What do controllers do when the radar is inoperative for some reason? This is a rare occurrence, but it does happen, and the safety requirement does not go away just because the aircraft cannot be seen on radar anymore. Controllers must resort to using position reports provided by the pilots and the distance reported by the instruments in the aircraft that have distance-measuring equipment, or DME. This measurement is combined with time, altitude, and the use of land-based navigation aids to provide the required increased separation between aircraft. Although safe, this process is extremely slow due to the increased radio communications involved with requesting and receiving position reports and the additional distance required between arriving aircraft when working without the advantage of radar.

Safety in Tower Operations

The most visible of air traffic facilities is the control tower. When most people think of air traffic controllers, they usually think of the controllers in the tower. This seems logical since the control tower is usually seen someplace on the airport and the radar controllers are located in some obscure building out of sight. Just as in radar facilities, the control tower must exercise safety measures during aircraft movements as well as during interactions with other controllers, both in the tower and in other facilities. Although each facility has its own area of traffic jurisdiction, the tower and radar controllers are constantly working closely together to ensure traffic safety.

The control tower is aware of every aircraft movement that occurs on the airport surface. Even if an aircraft is to be towed from one position to another, the tower must approve the operation. From the time the aircraft is pushed back from the loading gate until it lifts off the runway during its takeoff, it is monitored and directed by the control tower.

The ground controller issues taxi instructions to the aircraft when it is ready to taxi from the terminal loading gate to the departure runway. The aircraft’s progress is closely monitored by the ground controller, and the aircraft is informed of any other aircraft and other ground traffic in its immediate vicinity as it moves along. Basically, aircraft on the ground are controlled in the same manner as vehicle traffic on the streets. They are given specific taxi instructions telling them which taxiways to use to get to the end of the runway for departure. Taxiing aircraft are told when to stop and when to go at different taxiway intersections and when to give way to other aircraft as the tower establishes the aircrafts' positions in the departure sequence.

The most critical safety areas on the airport surface are the active runways. Mixing high-speed arriving and departing aircraft using the same surface requires a controller’s full and undivided attention. Factoring in the numerous runway crossings, both by taxiing arriving or departing aircraft and by ground vehicles, the runway becomes an area with an extremely high potential for disaster. Literally hundreds of thousands of lives encounter this potentially dangerous environment every day across the nation.

To ensure the safety of the aircraft during runway operations, close interaction and coordination are required between the two air traffic controllers responsible for its success or failure. The ground controller is responsible for the movement of all aircraft on the airport that are not on the runway. The local controller is responsible for all airborne traffic around the airport and any aircraft that are on the runway. Therefore, if the ground controller wants to taxi an arriving or departing aircraft across the active runway, he must first obtain permission from the local controller. The local controller may instruct the ground controller to hold the aircraft short of the active runway or she may give permission for the aircraft to taxi across the runway.

Regardless of which instruction is given to the ground controller, the instruction is repeated back to the local controller verbatim. This is a built-in safety measure that ensures there is no miscommunication or misinterpretation of the instructions by either controller during this sensitive operation with such a high potential for disaster if mistakes are made. This is one operation that advanced technology has not replaced. There are still certain times in air traffic control when nothing can replace old-fashioned eye-to-eye contact and face-to-face coordination to ensure safety of aircraft involved.

Sequencing and landing the arriving aircraft, as well as clearing departing aircraft for takeoff, is the responsibility of the local controller in the tower. This controller must also adhere to strict safety rules and guidelines when balancing arriving and departing aircraft. Factors such as the aircraft’s performance capabilities and potential for wake turbulence must be constantly taken into consideration.

A great amount of skill and judgment is required to perform the duties of an air traffic controller. These attributes, combined with the safety mechanisms the FAA has developed and implemented, make the complex world of air traffic work in a seemingly simple manner.

See Also:

  • Air Traffic Controller Training
  • Air Traffic Flow Management
  • Air Travel
  • Collision Avoidance Systems, Airplane
  • Federal Aviation Administration
  • National Airspace System
Further Readings
  • Eisfeldt, H.; Heil, M.; Broach, D.. Staffing the ATM System: The Selection of Air Traffic Controllers. Burlington, VT: Ashgate, 2002.
  • Federal Aviation Administration. FAA JO 7110.65U, Air Traffic Control. Washington, DC: Government Printing Office, 2012.
  • Gesell, L. Air Traffic Control: An Invitation to a Career. Chandler, AZ: Coast Aire Publications, 1990.
  • Kirwan, B.; Rodgers, M.; Schafer, D.. Human Factors Impacts in Air Traffic Management. Burlington, VT: Ashgate, 2005.
John R. Griffin
Embry-Riddle Aeronautical University
© 2014 SAGE Publications, Inc

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