The U.S. Air Force today lifted a two-week-old flight ban that had grounded Lockheed Martin Corp. (LMT)’s F-35 Joint Strike Fighter, following a power problem on a plane at Edwards Air Force Base inCalifornia.

An investigation found there was a defective valve in the plane’s Integrated Power Package, a turbo machine that provides power to start the engine, according to an Air Force statement.

While the probe continues, engineers determined that it is safe to resume test flights, said Joe DellaVedova, a spokesman for the Joint Strike Fighter Program Office. The F-35 is the Pentagon’s biggest procurement program at a planned $382 billion to buy 2,457 of the stealth F-35 jets in different versions for the Navy, Air Force and Marine Corps.

Flights will resume for 18 of the 20 fighters that the Air Force has been flying, DellaVedova said. Two planes based at Eglin Air Force Base in Florida will remain grounded because they lack the monitoring systems used in developmental test aircraft that can detect any problems in flight, he said.

Flight operations will resume for the rest of the planes, which are based at Edwards and at the Patuxent River Naval Air Station in Maryland.

Budget Target

The F-35 may be a target for budget cuts as the Pentagon is pressed to help lower the federal deficit. The Defense Department will need to find at least $325 billion in cuts over the next 10 years in the first phase of a $2.4 trillion deficit- reduction agreement approved by Congress. Another round of $500 billion in defense cuts may be imposed if Congress fails to approve enough budget savings in other areas.

Before leaving office in June, former Defense Secretary Robert Gates said the Pentagon may need to reconsider how many planes should be purchased.

The F-35 has five more years of developmental testing to complete before it is used in operations, DellaVedova said.

Concern about the plane stemmed from an Aug. 2 incident at Edwards, when the Integrated Power Package, made by Honeywell International Inc. (HON) of Morris Township, New Jersey, failed during an engine run, forcing an engine shutdown. No injuries to the pilot or ground crew occurred.

A valve in Honeywell’s power package did not function properly, according to the Air Force statement. Flights can resume on aircraft while the valve is monitored, it said. A permanent fix for the problem is being developed.

The Air Force has also grounded Lockheed’s F-22 Raptor, the military’s most advanced fighter, because of reported problems with the plane’s system for supplying oxygen to the pilot.

The flight ban on the F-22, in effect since May, remains until an investigation is completed in a few months, said Air Force spokesman Lieutenant Colonel John Haynes.