About Flight 77
Off the Screen
Until six minutes before the second airliner hit the World Trade Center, Flight 77 appeared to air traffic controllers in the FAA's Washington Air Route Traffic Center in Leesburg, Va., to be just another flight they were routinely tracking. When the Boeing 757 reached central West Virginia, it was routinely "handed off" by Leesburg to the next air traffic control center, outside Indianapolis.
Flight 77 continued west, appearing on the radar screens with a data block identifying the airline, flight number, altitude and type of plane - information from the plane's transponders, which are signal transmitting devices. The plane inched across the screen, the radar updating every 12 seconds.
Then Flight 77 began to turn slightly - and abruptly disappeared from the radar screens. Suddenly there was no transponder signal.
Federal officials say the terrorists apparently shut down the transponders in all four hijacked airliners as they took control. The air traffic controllers didn't know it, but Flight 77 was making a U-turn and heading east.
Normally, when an aircraft's transponder cuts off, the plane is still visible as what's called a "primary target" or "skinpaint" - a target the radar is picking up but can't identify. The controllers in Indianapolis kept watching for Flight 77 to appear over Kentucky, Ohio or Indiana - but they weren't looking for it to reappear far to the east, over West Virginia where the plane had come from, sources said.
Back in Leesburg, air traffic controllers knew at about 9:05 a.m. that they had a new eastbound plane on their radar, but they didn't know it was Flight 77. The aircraft had entered their airspace with no radio contact and no transponder identification.
During the confusion, rumors circulated that Flight 77 might have exploded in midair. It wasn't until 9:24 a.m. that the FAA alerted the military that the plane was heading for Washington.
http://emperors-clothes.com/9-11backups/nd923.htm
Flight 175
http://newsmine.org/archive/9-11/air-traffic-controllers-recall-events.txt
read it, its an interview with two of the air traffic controllers.