"On April 30 2014, an FAA air-traffic system that processes flight-plan information experienced problems while processing a flight plan filed for a U-2 aircraft that operates at very high altitudes under visual flight rules," FAA spokesman Lynn Lunsford said.

She added the computer system had "misinterpreted" the U-2 as a more typical low-altitude flight and become overwhelmed in trying to make sure its flight path did not conflict with other air traffic in the area.

"The FAA resolved the issue within an hour, and then immediately adjusted the system to now require specific altitude information for each flight plan," she added.

The agency said it had now added more flight-processing memory to the computer system.

The SR71 is a gas guzzler. It leaks fuel when cold and only tightens up at supersonic speeds.

It is a wonderful plane but it is just not economical.