Flight 77’s transponder, a transmitter that broadcasts the course, speed, and altitude of the airplane, is turned off at 8:56 am. The hijacker pilot refuses to answer any radio messages, adding to the uncertainty of making a decision to dispatch military aircraft to intercept the plane. For air traffic controllers, the lack of a transponder signal means they cannot find the Boeing 757 until it crosses the path of a ground-based radar.1

1 Goldberg, A. Papadopoulous, S., et al “Pentagon 9/11” Historical Office, Office of the Secretary of Defense n.d PDF. March 16, 2011 page 12 http://osdhistory.defense.gov/docs/Pentagon%209-11.pdf