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 8:56.1 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...more