Three of the four aircraft hijacked as part of the 9/11 attacks had their transponders turned off: > Controllers track airliners such as the four aircraft hija

Three of the four aircraft hijacked as part of the 9/11 attacks had their transponders turned off: > Controllers track airliners such as the four aircraft hijacked on 9/11 primarily by watching the data from a signal emitted by each aircraft's transponder equipment. Those four planes, like all aircraft traveling above 10,000 feet, were required to emit a unique transponder signal while in flight. > On 9/11, the terrorists turned off the transponders on three of the four hijacked aircraft. With its transponder off, it is possible, though more difficult, to track an aircraft by its primary radar returns. But unlike transponder data, primary radar returns do not show the aircraft's identity and altitude. Controllers at centers rely so heavily on transponder signals that they usually do not display primary radar returns on their radar scopes. But they can change the configuration of their scopes so they can see primary radar returns. They did this on 9/11 when the transponder signals for three of the aircraft disappeared. [1] http://www.9-11commission.gov/report/911Report_Ch1.htm http://www.9-11commission.gov/report/911Report_Ch1.htm