Managing the skies: Air traffic control

The most common image of air traffic control is of an airport control tower with men and women directing aircraft around that airport. However, the entirety of the air traffic control system is far more complex. In the U.S., the system is divided into three major segments, with each segment overseen by the FAA Air Traffic Control System Command Center (ATCSCC).

There are 21 ARTCC’s (Air Route Traffic Control Centers – referred to as Centers) spanning the airspace. These Centers are responsible for monitoring the flight’s progress enroute, and maintaining adequate aircraft separation, usually at altitudes above 10,000 feet. These centers are also responsible for lining up merging aircraft in sequence as they begin their descent toward an airport.

Within each of those centers, usually near major airports, there are smaller portions of airspace controlled by TRACON (Terminal Radar Approach Control – referred to as Approach). A single TRACON may have several airports within its airspace, and usually controls the airspace within a 40-mile radius of those airports. The Approach controllers’ responsibilities include transitioning aircraft from the enroute phase to the approach phase into a destination airport located within the TRACON’s airspace, or transitioning after takeoff to the departure phase until the flight is handed off to the Center (ARTCC) for its enroute phase.

The third segment of the airspace system is the Air Traffic Control Tower (or Tower). The Tower is responsible for airport operations including flight data, clearance delivery, ground control and local control (usually within five miles of an airport). Tower controllers need a clear view to the runways and other locations on the airport, and radar is typically only used for tracking aircraft on the ground, or very close to the airport. Takeoff and landing clearances are delivered after a visual check of the runway status confirms there are no other aircraft that could create an incursion.

Although the airspace system contains charted airways similar to interstate highways, pilots are permitted to file a flight plan with any route they choose, even if those routes are off an airway. The movement of aircraft through the various segments of airspace can be compared to a “zone” defense that might be used by a football team. As an aircraft moves through a given airspace division, it is monitored by controllers in that division, and once the aircraft leaves that area and enters another, the responsibility is transferred to a different set of controllers responsible for their zone. However, many factors within each piece of the airspace can have an impact on the route and timeliness of a flight. Controllers must be able to anticipate problems and react when they occur to help prevent delays, diversions or even cancellations.

Weather is the most common reason for a delay. There are three areas where weather affects flight schedules: at the origin airport, enroute, and at the destination airport. A passing thunderstorm or snow squall or the need to de-ice the aircraft can cause major impacts at an airport. These issues are usually temporary, but they have lingering effects on flight departure and arrival times. When an aircraft does not depart on-time, later flights have to wait due to the backlog. The spacing interval on a runway between takeoffs and landings is fixed, based on safety considerations, and cannot be compressed. The net effect is that all flights scheduled to arrive from the airport with the weather problems will arrive late, and that could continue hours after the weather has passed.

Weather can also have a major impact on the enroute phase of flight. Storm systems can occupy a large amount of airspace, and force Center controllers to reroute traffic around the weather. Controllers are challenged with routing a large number of aircraft through a smaller amount of airspace, and are sometimes forced to issue delays, or airborne holding to aircraft, while they wait for the airspace to clear of traffic.

The wind direction and the runway direction of an airport can also lead to problems. Many major airports have more than one runway, however, when strong winds occur, it could force the airport to use the runway that is most favorable for the wind conditions, since it would be too dangerous to takeoff and land with a strong crosswind. When airports are forced to use a single runway, traffic jams can occur. Arrivals and departures are not as frequent, and therefore airborne holding is increased, and ground parking becomes limited. These situations trickle down to other departure airports and can cause those flights to be delayed as well. Since there is no room to park the arriving aircraft, planes are held on the ground to avoid congestion in the airspace, which can be very costly.

The Command Center maintains control over any planned delays, or route changes as environmental factors, or large traffic volumes affect the airspace. Traffic Management Personnel predict, on national and local scales, traffic surges, gaps, and volume based on current and anticipated airborne aircraft. These personnel evaluate the projected flow of traffic into airports and sectors, then implement the least restrictive action necessary to ensure that traffic demand does not exceed system capacity. Various programs or plans have been created to give controllers the ability to mitigate delays as much as possible, therefore saving passengers time and saving operators money. A common misconception to private jet fliers is that air traffic delays can be avoided on private jets when airlines are subject to delays or cancellations. Any airport that is facing a delay program affects all aircraft, regardless of whether it is private or commercial. The advantage to private jet operators is that smaller airports can sometimes be used to avoid traffic delays – however, the weather usually has a bigger impact on small airports than larger commercial airports.

The Future of Air Traffic Control

Air travel has increased dramatically since the U.S. federal government deregulated the airline industry in the 1970s. However, new airport and runway construction has not kept pace with the increase in air traffic. This has put excessive pressure on the air traffic control system to handle thousands of flights per day, with numbers increasing each year. The FAA is developing systems such as increased GPS capabilities to help air traffic controllers track and communicate with aircraft. Six other programs are reshaping operations in the National Airspace System:

Automatic Dependent Surveillance–Broadcast (ADS-B); Data Communications (Data Comm); En Route Automation Modernization (ERAM); Terminal Automation Modernization and Replacement (TAMR); NAS Voice System (NVS); and System Wide Information Management (SWIM).

NASA is also working on new technologies and concepts in air traffic management that will not only provide some relief from holiday travel headaches, but increase the efficiency, safety, and environmental friendliness of air transportation. Researchers are testing new tools at FutureFlight Central, a comprehensive, 360-degree simulation of an air traffic control tower at NASA Ames Research Center in California’s Silicon Valley. During a test at FutureFlight, data is recorded on all elements of the simulated airspace — including voice transmissions between pilots, and summary statistics of aircraft activity, such as taxi times, runway waits and departure rates — allowing NASA researchers to replay an entire simulation run and examine how their tools performed in the hands of real users. Their analysis of these different steps in the process allow them to recommend ways to optimize the routing of planes, the timing of their movements, and the communication among different parties responsible for making a hectic airport move like clockwork.