A matter of margins: Aging radios, radars, and computers in a 23 000-element system help controllers route aircraft during their 45 million hours per year in U.S. airspace

Whenever you fly - on a commercial airplane, a corporate jet, or even your uncle's two-seater prop plane - if the pilot has filed a flight plan with a Federal Aviation Administration division, you become a part of the air traffic control system. If you fly under instrument flight rules, your plane is a blip on a radar screen somewhere. Its takeoff and landing are two operations among some 43 million that the U.S. air traffic control (ATC) system handles each year. Aircraft under ATC guidance spend 45 million hours annually in U.S. airspace.