ATC Zero: even with backup systems and master plans, a catastrophic failure may still have critical air traffic control being done by cell phone.(SYSTEM NOTES)

It was 11:27 a.m. on September 25, 2007, when a loud "ka-thunk" followed by a loud screeching sound emitted from speakers throughout the cavernous Air Route Traffic Control Center (ARTCC) located in

Memphis, Tenn. Two miles away, in a building owned by a major communications company, a vital communications component known as a switch controller catastrophically failed.

This switch handles communications between Memphis controllers and controllers in other FAA facilities, as well as radar data and radio communications with pilots within an area approximately 250 miles around Memphis.

Air Traffic Controllers manning dozens of sectors that together comprise the Memphis Center airspace were startled by the unusual sounds. They glanced at their radar displays. All aircraft were accounted for. At least they were for the moment. They began checking the status of their equipment by calling pilots and other controllers. Some were successful. Most were not.

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Sectors that were affected changed the radio transmitters and receivers on their Voice Switching and Control System (VSCS). When that backup system …