The job of the air traffic controller is to provide for the "safe, orderly, and expeditious flow of aircraft" through our country's national airspace system.
Similar to the handling of a baton in a relay race, one air traffic controller after another takes responsibility for a specific leg of the trip ensuring that the aircraft is safely separated from other air traffic and vehicles.
As a first step, a pilot files a flight plan with the flight service station, or an airline files the plan automatically with the FAA. This plan outlines the route the aircraft will take and alternative plans in the event of an emergency or weather-related problems. Once the flight plan is approved, the pilot is ready to contact the ground controller for taxi instructions.
Takeoff and Departure
The ground controller notifies the pilot when it is safe to push the aircraft out of the gate or enter the controlled movement area at the airport, issues instructions to a runway, and places the aircraft in a departure sequence with other aircraft taxiing about the airport.
The local controller in the tower assumes control of the aircraft and integrates its movements into the flow of traffic arriving and departing the runway. The local controller issues a departure clearance and grants permission to enter the runway and depart. After takeoff, the local controller will assign the aircraft a frequency change to the departure controller, stationed in a radar room which may be at the airport or several miles away. The departure controller assumes responsibility for the plane through its ascent while safely avoiding other aircraft.
En Route
Once the flight departs the airport, controllers in one or more of the 21 regional Air Route Traffic Control Centers, also called en route centers, take over in sequence. Each center controls all aircraft, military and civilian, in its defined portion of airspace - called a sector. The en route controllers direct and separate planes flying in their sector. They coordinate with pilots on weather conditions, and issue instructions on speed, route, and altitude to ensure positive separation from other aircraft. When the aircraft moves into a new sector, the next controller takes over.
Approach and Landing
As the plane approaches your destination, the en route center organizes the traffic into several streams and flows the traffic towards the airport. The center will "hand off" responsibility for the plane to the approach controller located in the same room as the departure controllers and will adjust the plane's speed, altitude and flight path by issuing instructions to the pilot. Once the plane has been cleared for approach, responsibility for the plane is transferred to the local controller. They ensure that there is enough spacing between departures and arrivals, both in the air and on the runways. After landing the local controller gives responsibility for the flight to the ground controller who ensures safe passage from the runway to the gate.
