When the system failed Wednesday, a backup helped safely guide flights already in the air, but hundreds of planes across the nation headed for Southern California were ordered not to take off as an air traffic control facility about 40 miles north of Los Angeles effectively rebooted.
The problem had nothing to do with spy-related signals sent by the Cold War-era plane.
The plane flies at around 60,000 feet under "visual flight rules." According to the FAA, a computer perceived a conflict between the altitude and the use of visual flight rules, and began trying to route the plane to 10,000 feet. The number of adjustments that would need to be made to routes of other planes throughout the area overwhelmed the software.
"The extensive number of routings that would have been required to deconflict the aircraft with lower-altitude flights used a large amount of available memory and interrupted the computer's other flight-processing functions," the FAA said in a statement.
The Pentagon confirmed Monday that an Air Force U-2 spy plane was conducting training operations in the area. It is not unusual for a U-2 to operate in the region, and the necessary flight plan had been submitted for the high-flying plane, Col. Steve Warren said.
The connection between the U-2 and the outage was first reported by NBC News.
Since the incident, the FAA has been analyzing what went wrong with its En Route Automation Modernization system. The computer system, known as ERAM, allows air traffic controllers at several dozen "en route centers" around the country to identify and direct planes at high altitudes.
The Los Angeles en route center controls high altitude air traffic over southern and central California, southern Nevada, southwestern Utah and western Arizona — except airspace designated for military use.
In its statement, the FAA said it has adjusted ERAM to require altitude details for flight plans.
"The FAA is confident these steps will prevent a reoccurrence of this specific problem and other potential similar issues going forward," the agency said.
When the system failed, air traffic controllers in Southern California had to call their counterparts at neighboring centers to update them on each plane's flight plan, according to Nate Pair, the president for Los Angeles Center of the National Air Traffic Controllers Association. While that was more onerous than normal operations — when computers automatically pass along updates — the system still worked, Pair said.