Air Traffic Control

Overview | ATC Facilities | Radar

Overview

Controllers and pilots use a complex system of radars and radios to separate aircraft, and satellite technology and electronic messaging are slowly being introduced. Flights are typically along �airways� in the sky marked by navigation beacons, but increasingly aircraft are given more �direct� routes.

Commercial flights use Instrument Flight Rules (IFR), regardless of the weather, so radar controllers can keep aircraft separated. The pilots must be able to fly only by instruments.

Flights under Visual Flight Rules (VFR), typically by smaller private aircraft at lower altitudes, are not separated by controllers, but may be seen on radar. VFR flights must have radios to talk with controllers and a device to make them more visible on radar screens to fly into major airports or at higher altitudes. Pilots and controllers communicate by radio using VHF frequencies between 118 and 136 megahertz, just above the FM broadcast band. While all airline aircraft are "controlled" every step of the way, general aviation aircraft often fly in uncontrolled airspace, or Class G Airspace. In general, these areas are below the airways used by airline aircraft, and outside the airspace the airlines use on takeoff and approach to landing. Pilots of general aviation aircraft flying VFR 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, or in other controlled air space.

The ATC system, staffed by about 35,000 of the FAA�s roughly 50,000 employees, has four key elements:

• the tower, with control responsibilities within a few miles of the airport;
• a Terminal Radar Approach Control (TRACON) for flights arriving or departing within about 40 to 50 miles of the airport;
• "Air Route Traffic Control Centers" (ARTCC) covering high altitudes over an area that can stretch over several states and the lower-altitude airspace between terminal areas;
• The "Air Route Traffic System Control Center" (ARTSCC), an oversight facility at Herndon, Va., known as "central flow" or "flow control," where strategic planning takes place with airlines. This facility monitors but does not control traffic. Here nationwide solutions are ordered when, for instance, poor weather affects one or more centers.

Only about 550 of the 5,300 public airports in the U.S. have towers. Over 350 of the busiest towers run by the FAA. Less busy airports have control towers run by contractors. There are 22 centers, including one in Anchorage and a combined center/approach control on Guam. There are 185 TRACONs, some serving several airports. Technicians maintain about 45,000 pieces of equipment to support the ATC system.

Before a flight airline dispatchers send a computerized flight plan to the FAA. It goes to the by a radio e-mail known as a "data link," to save on radio transmission time. Crews verify the flight plan and any changes with a "clearance delivery" controller. When the flight is ready, the crew talks to a tower controller in charge of taxiways, asking "ground control" for permission to taxi to the runway. Ground controllers also supervise the movement of vehicles on the airport. Once at the runway, the crew talks to the tower "local controller" for permission to move onto the runway and takeoff. For safety, pilots repeat instructions to confirm that directions are understood.

After takeoff, the "local controller" tells the plane to "contact departure," the TRACON, on a particular radio frequency. Here flight radar monitoring begins.

The departure controller watches the plane as it climbs toward its cruising altitude and then "hands off" the flight to center controllers who follow it through sectors of airspace until it nears its destination. The flight appears on the radar screen as a "data block" listing the airline, flight number (or the N registration number of the plane) and aircraft type. Also listed is the speed, altitude and if the plane is climbing or descending, data sent by an on-board "transponder" programmed with a specific code for the flight. It sends its information each time radar bounces off the plane.

When the plane nears its destination the process is reversed. The center hands the flight off to an approach controller in the TRACON who sequences it for landing. The flight then contacts the local controller in the tower for a landing clearance, and once on the ground checks in with the ground controller for taxi instructions.

The heading a plane flies determines what altitudes or flight level (FL) may be used. A flight level is shorthand for a multiple of 100 feet. IFR flights below FL 290 (29,000 feet) are separated by 1,000 feet vertically, and by 2,000 feet above FL 290. Above FL 180 flights are separated horizontally by five miles. Closer to airports, where aircraft are slowing, the rule is three miles. Aircraft must be separated by either the vertical distance dictated by the flight level, or by the horizontal distance.

Over oceans, with no radar coverage, separation is by flying time, typically five to 30 minutes. Lateral separation of routes is between 30 and 120 nautical miles (1.15 statute miles). Aircraft with highly accurate altimeters can be separated by just 1,000 feet vertically about FL 290.

In areas without radar, such as northern Alaska, the precursor to a satellite navigation/separation system is being tested. Known as Automatic Dependent Surveillance-Broadcast (ADS-B), it sends signals not just to controllers hundreds of miles away by satellite but also to surrounding aircraft. This allows controllers to separate aircraft without radar and shows pilots where other aircraft are in relation to their position.

For detailed information on the air traffic control workforce, please visit the National Air Traffic Controllers Association Website.