A U-2 spy plane passing overhead caused the air traffic control for Los Angeles and a wide surrounding area to crash, leading to hundreds of services being grounded, authorities have revealed. Controllers had to resort to emergency back-up procedures, calling each other manually to keep track of planes already flying in the busy airspace while the system was rebooted and fixed.

The Cold War-era U2 plane flies at high altitude, around 60,000 feet (18,000 metres) under "visual flight rules". The Federal Aviation Administration said the computer perceived the plane as a low-altitude operation and began frantically trying to reroute it down to 10,000 feet while keeping other planes out of its way.

The number of adjustments this required to the routes of other planes overwhelmed the software, the FAA said. "[It] used a large amount of available memory and interrupted the computer's other flight-processing functions," the FAA said in a statement. The computer involved was part of a system known as Eram.

A Pentagon spokesman confirmed that the U-2 had flown over southern California last Wednesday as part of a routine training mission but said he could not confirm that it was responsible for the computer glitch. "The U-2 filed all the proper flight plan paperwork and was conducting its operation in accordance with those filings," said army Colonel Steve Warren. It was not unusual for the plane to be flying over the area and the incident had not prompted the military to change the way it conducted such operations, Warren said.

When the system failed air traffic controllers in southern California had to call their counterparts at neighbouring centres to update them on each plane's flight plan, said Nate Pair, the president for Los Angeles of the National Air Traffic Controllers Association.

The FAA said it had changed the way flight plans were logged in such cases to make sure it would not happen again. "The FAA resolved the issue within an hour and then immediately adjusted the system to now require specific altitude information for each flight plan," said Lynn Lunsford, an FAA spokesman, adding that the computer had also been given more memory for processing flights. The fault was not caused by any signal from the plane's sophisticated equipment, the FAA said.



Bob Hope airport in Burbank, California; John Wayne airport in Santa Ana, California; and McCarran International in Las Vegas were among other facilities affected by the order to keep planes grounded. So were flights in other parts of the country that were bound for a wide swath of airspace in the south-western US covered by the system involved.

Reuters and the Associated Press contributed to this report