(9:40 a.m.) September 11, 2001: Flight 93 Transponder Signal Turned Off; Flight Still Closely Tracked

The transponder signal from Flight 93 ceases. [CNN, 9/17/2001; MSNBC, 9/3/2002; MSNBC, 9/11/2002; 9/11 Commission, 6/17/2004] However, the plane can be—and is—tracked using primary radar by Cleveland flight controllers and at United headquarters. Altitude can no longer be determined, except by visual sightings from other aircraft. The plane’s speed begins to vary wildly, fluctuating between 600 and 400 mph before eventually settling around 400 mph. [Longman, 2002, pp. 77, 214; 9/11 Commission, 6/17/2004]