The Pentagon allowed developmental F-35 Joint Strike Fighters, grounded because of a power issue, to return to flight operations yesterday, but kept a ban on ground operations for two production aircraft.

An Air Force Safety Investigation Board, which convened Aug. 4, is still reviewing the failure of an Integrated Power Package (IPP) on a conventional-takeoff-and-landing (CTOL) variant of the F-35 on Aug. 2 at Edwards AFB, Calif. Investigators have blamed a faulty valve, though a permanent fix has not been finalized.

“The government and contractor engineering teams determined the program could resume developmental test flight operations while the investigation continues,” Joe DellaVedova, spokesman for the Pentagons’ F-35 program office, said yesterday in a statement. “This assessment was made after reviewing data from ground and flight tests which showed, with revised test monitoring procedures governing the IPP, the aircraft can be flown safely.”

A root-cause investigation into the failure showed a valve on the IPP–which generates power to start the engine and cool the aircraft–did not function properly, DellaVedova said. Honeywell [HON] makes the power package.

“Monitoring of valve position is a mitigating action to allow monitored operations,” he said. “A permanent resolution is in work.”

The developmental test aircraft that returned to flight operations are assigned to Edwards and Patuxent River Naval Air Station, Md. They were cleared to resume ground operations on Aug. 10.

The program office issued a “precautionary suspension” of ground and test flight operations for the 20-aircraft F-35 fleet on Aug. 3, the day after CTOL aircraft’s IPP failed during a standard ground-maintenance engine run. The pilot and ground crew were not injured that day.

The Pentagon, however, has not allowed the resumption of ground operations for two production F-35s at Eglin AFB, Fla., the program office said yesterday. Those aircraft do not have the same monitoring systems the developmental versions do.

“The completion of the root cause investigation and any corrective actions are required to return to unmonitored operations,” DellaVedova said.

The F-35 program office is still assessing how the grounding has impacted the aircraft’s test-flight plans and production operations, he said, adding: “The program, however, has built margin into the test schedule to accommodate incidents that occur in the development effort.”

The grounding has generated speculation about the multi-service, multi-nation F-35 program’s continued support and funding.

Airframe builder Lockheed Martin’s [LMT] Low Rate Initial Production (LRIP) 1-3 contracts are over-budget by hundreds of millions of dollars, and the heads of the Senate Armed Services Committee have questioned the program’s future.

Air Force Maj. Gen. C.D. Moore, deputy director of the Joint Strike Fighter program office, said on July 27 he believed most of the risk with the F-35 program will be gone by late 2013, after which he predicted the effort will be in “calm water.”

Speaking at an Air Force Association breakfast in Arlington, Va., Moore said the program faced three “key challenges,” related to software development and integration, full-system qualification testing, and affordable production and sustainment.

The Pentagon restructured the F-35 effort earlier this year, expending the development phase, reducing the near-term buy of production aircraft, and delaying the Marine Corps’ variant. The program is expected to go through a high-level Defense Acquisition Board (DAB) review this fall, when its baseline cost figure will be updated.