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(8:56 a.m.-9:05 a.m.) September 11, 2001: Flight 77 Disappears from Indianapolis Center Radar Screens

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According to the 9/11 Commission, “Radar reconstructions performed after 9/11 reveal that FAA radar equipment tracked [Flight 77] from the moment its transponder was turned off at 8:56 [a.m.].” However, for eight minutes and 13 seconds, between 8:56 and 9:05, this primary radar data is not displayed to Indianapolis Center air traffic controllers. “The reasons are technical, arising from the way the software processed radar information, as well as from poor primary radar coverage where American 77 was flying.” [1]

According to the Washington Post, Flight 77 “was hijacked in an area of limited radar coverage.” The Post adds that there are two particular types of radar system. “Secondary” radar “is the type used almost exclusively today in air traffic control. It takes an aircraft’s identification, destination, speed, and altitude from the plane’s transponder and displays it on a controller’s radar screen.” “Primary” radar, on the other hand, “is an older system. It bounces a beam off an aircraft and tells a controller only that a plane is aloft—but does not display its type or altitude. The two systems are usually mounted on the same tower.” Normally, “If a plane simply disappears from radar screens, most controllers can quickly switch on the primary system, which should display a small plus sign at the plane’s location, even if the aircraft’s transponder is not working. But the radar installation near Parkersburg, W. Va., was built with only secondary radar—called ‘beacon-only’ radar. That left the controller monitoring Flight 77 at the Indianapolis Center blind when the hijackers apparently switched off the aircraft’s transponder [see 1], sources said.” [2]

In its final report, the 9/11 Commission will include an elaborate explanation for the loss of primary radar contact with Flight 77, saying it is because “the ‘preferred’ radar in this geographic area had no primary radar system, the ‘supplemental’ radar had poor primary coverage, and the FAA ATC [air traffic control] software did not allow the display of primary radar data from the ‘tertiary’ and ‘quadrary’ radars.” [3] The Commission will note that two managers at the Indianapolis Center[who?] who assist in the search on radar for the missing aircraft do “not instruct other controllers at Indianapolis Center to turn on their primary radar coverage to join in the search for American 77.”. [4]

  1. [9/11 COMMISSION, 6/17/2004]
  2. WASHINGTON POST. 11/3/2001. 
  3. 9/11 Commission Report,26 July 2004,Page 460
  4. [9/11 COMMISSION, 6/17/2004]


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