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 http://www.pentagonmemorial.org/learn/911-timeline
2 9/11 Commission Report” National Commission on Terrorist Attacks Upon the United States July 22, 2004. Web. March 17, 2011 page 29 http://www.9-11commission.gov/report/911Report.pdf