Transponder for Flight 93 is Turned Off 1

The FAA’s Cleveland Air Route Traffic Control Center loses United 93’s transponder signal, but continues to track the flight using primary radar and visual sightings from other aircraft.2

1 “Day Of Timeline” Pentagon Memorial Fund n.d. Web. March 17, 2011
2 9/11 Commission Report” National Commission on Terrorist Attacks Upon the United States July 22, 2004. Web. March 17, 2011 page 29