tched 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 plane???s identifying information, speed, and altitude to controllers??? radar screens. [Washington Post, 9/