The nation's air traffic control system is part man and part machine, an elabroate safety net that protects passengers as their planes are passed along an elaborate chain of controllers.
The $2-billion-a-year system employs 43,000 people, most of whom never actually see the aircraft. But radar, radio and computers allow flights to be tracked and directed from the moment they back away from loading gates until they shut off engines at their destination.
On a normal Tuesday, the system accommodates about 14,000 scheduled domestic flights. Yesterday, as a controllers' strike went into its second day, that figure was cut by perhaps a quarter as supervisors and nonstriking controllers formed skeleton crews. But operating procedures remained essentially the same.
For the public, the best-known controller is the stereotypical tower chief who clears planes for takeoff or, in more dramatic moments, "talks down" pilots who have run into trouble aloft.
This position -- "local controller" -- is an important and exadting one, but only one link in an intricate system: Here is how it works:
When a plane is loaded, its crew calls a scheduler in the tower to file a flight plan.A second controller directs the plane as it moves into position on the runway. Takeoff begins on clearance from the local controller, who juggles the plane with others that are landing or in line for takeoff.
As the plane lifts off the runway, responsibility for it is passed to the airport's radar room, where the plane will appear on a scope after it reaches several hundred feet altitude. Here another controller keeps watch to ensure it stays on its assigned path and keeps proper distance from other planes.
About 30 miles out, the airport radar room gives the pilot a radio frequency and tells him to contact a particular controller at the regional "en route" center into whose zone his plane is passing.These centers -- there are 20 across the country -- are the least publicized link in the system, though they employ about a quarter of its people.
For airspace around Washington, the center is at Leesburg. Four giant radar antennas postioned around the facility's 145,000-square-mile zone sweep the air every 10 seconds. Their signals flow into computers that break the zone into smaller sectors for display on a controller's screen.
Leesburg's turf is normally divided into 32 sectors. The controller watches a radar scope and assumes responsibility for each plane that enters his sector -- as many as 30 at a time, flying at different altitudes and directions.
Radio instructions are delivered in terse, standardized phrases. A plan might be told to change course slightly to avoid bad weather, or to climb 1,000 feet to maintain FAA regulations governing separation from another plane.
As a plane passes across one sector's boundaries, the controller instructs it to contact the controller working the adjacent sector, then signs off.
This process continues until the plane reaches its destination airport's airspace, where control is handed off to the airport tower and the takeoff procedure is reversed.
Radar screens display precise, computer-composed symbols marking the position of planes, airports, areas of bad weather, ground obstructions, flight paths and the like.
Development of the transponder, now used on all commercial airliners, allows precise tracking and identification of planes. When radar impulses strike a plane, its transponder "squawks" the plane's identification code, altitude and position.
Using this data, the computer constructs a "data block" of numbers -- flight identification, altitude, assigned altitude and speed -- which it displays beside the airplane's image on the screen.
But the final judgments still are by people -- men and women who in the past year guided more than 5 million scheduled commercial flights from takeoff to landing.