The radar coverage of the United States airspace is nearly complete. In particular the northeastern area, where all four hijackings took place on 9/11, has no “gaps” whatsoever in radar coverage. Nonetheless there was radar loss on 9/11 with respect to the third hijacked plane, American Airlines Flight 77, which was reported to have hit the Pentagon.
American 77 took off at 8:20 a.m. EST and was hijacked more than half an hour later. It began to change its course at 8:54 and, while slowly turning to the left, its transponder was switched off at 8:56. Until then it had been displayed on the radar scopes of air traffic control via the Higby radar site. This was a “beacon-only” site, meaning a site that could only display transponder signals. When American 77 ́s transponder was turned off, the plane was no longer visible to Higby radar.
Nonetheless the area was covered by additional radar sites.3 Several sites that were not “beacon-only” tracked American 77 after its transponder had been turned off. However the plane was lost to controllers because of the way computers processed the radar data — and because of an unexplained wide-ranging radar failure.