The various air traffic control facilities encountered by a plane during its flight

Departure, En Route and Descent

Once your plane takes off, your pilot activates a transponder device inside the aircraft. The transponder detects incoming radar signals and broadcasts an amplified, encoded radio signal in the direction of the detected radar wave. The transponder signal provides the controller with your aircraft's flight number, altitude, airspeed and destination. A blip representing the airplane appears on the controller's radar screen with this information beside it. The controller can now follow your plane.

An airplane's transponder transmits flight data to incoming radar signals.

The departure controller is located in the TRACON facility, which may have several airports within its airspace (50-mile/80-km radius). He or she uses radar to monitor the aircraft and must maintain safe distances between ascending aircraft. The departure controller gives instructions to your pilot (heading, speed, rate of ascent) to follow regular ascent corridors through the TRACON airspace.

The departure controller monitors your flight during ascent to the en route portion. When your plane leaves TRACON airspace, the departure controller passes your plane off to the center controller (ARTCC controller). Every time your plane gets passed between controllers, an updated flight progress slip gets printed and distributed to the new controller.

En Route and Descent

Once your plane has left TRACON airspace, it enters a sector of the ARTCC airspace, where it is monitored by at least two air traffic controllers. The radar associate controller receives the flight-plan information anywhere from five to 30 minutes prior to your plane entering that sector. The associate controller works with the radar controller in charge of that sector. The radar controller is in charge of all air-to-ground communication, maintains safe separation of aircraft within the sector and coordinates activities with other sectors and/or centers. The controllers must monitor the airspace at high altitude (above 24,000 ft/7320 m) and low altitude (below 24,000 ft). The center controllers provide your pilot with updated weather and air-traffic information. They also give directions to your pilot regarding such aspects as speed and altitude to maintain a safe separation between aircraft within their sector. They monitor your plane until it leaves their sector. Then they pass it off to another sector's controller.

Another controller, called the radar hand-off controller, assists the radar and associate radar controllers during times of heavy traffic, watching the radar screen and helping to maintain smooth air-traffic flow.

While you are enjoying your meal, snack, in-flight movie or the view outside the window, your plane gets passed from sector to sector and center to center. In each sector, center controllers radio instructions to the pilots. The path of your plane may have to be changed from the original flight plan to move around bad weather or avoid a congested sector. Your pilots may request a change in altitude to avoid or reduce turbulence. This back and forth between pilots and center controllers continues until you are about 150 miles (241 km) from San Francisco (your destination). At this point, the center controller directs all planes flying into San Francisco to move from high altitudes to low altitudes and merges the descending aircraft into a single file line toward the airport. The controller gives instructions to your pilot, such as changes in heading, speed and altitude, to place your plane in line with these other aircraft. Depending on traffic conditions, the controller may have to place your plane into a holding pattern, which is a standard route around each airport, where you wait until the airport can handle your arrival. The controller continues to give directions to your pilot until your plane is within TRACON airspace.