September 11, 2001: Flight 93???s Transponder Briefly Comes Back On Flight 93???s transponder, which was switched off after Flight 93 was hijacked, is turned back on just before the plane crashes, thereby revealing the plane???s altitude to air traffic controllers at the FAA???s Cleveland Center. [Federal Bureau of Investigation, 9/11/2001; MSNBC, 9/11/2002] A transponder is a device that sends a