The temporary grounding of the F-35 Joint Strike Fighter has been lifted after investigators determined the turbine blade crack that prompted the stand down was an isolated event.

“Based on the findings from our initial on-wing inspection to our subsequent inspections in our lab, we determined that root cause is sufficiently understood for the F-35 to safely resume flight,” says Pratt & Whitney, the manufacturer of the fighter’s F135 engine.

The fleet was grounded on Feb. 21 following the discovery by borescope of a 0.6-in. crack in a third-stage low pressure turbine (LPT) blade on U.S. Air Force test aircraft AF-2 at Edwards AFB, Calif.

Investigators were initially concerned the failure could have been a repeat of a high-cycle fatigue problem that was uncovered with the LPT during engine testing in 2007 and 2008. However, Pratt says “there is no evidence of high-cycle fatigue or low-cycle fatigue, and there is no evidence of fatigue progression on the turbine blade. We believe that a key factor for the blade crack stemmed from the unique operating environment in flight test.”

The manufacturer adds the engine “had operated more than four times longer in the high-temperature environment of the flight envelope than the typical F-35 duty cycle. The exposure in this high-temperature part of the envelope led to a separation of the grain boundary on this single blade.” This type of separation failure is also known as a creep rupture. This occurs when deformation begins under constant load and high temperatures, and over time reaches a point where the blade material fails.

The F-35 Joint Program Office adds in a statement that the specific engine involved “had been operated for an extended time in the high-temperature environment in its mission to expand the F-35 flight envelope. Prolonged exposure to high levels of heat and other operational stressors on this specific engine were determined to be the cause of the crack. No additional cracks were found during inspections of the remaining inventory.”

No further specifics have been given about why the issue should strike aircraft AF-2 in particular. However it is known AF-2, one of the flight sciences test fleet at Edwards, was scheduled to be testing a new skin design intended to counter scorching around the horizontal tail. These surfaces suffered scorching and delamination during sustained high-speed/high-altitude flight, resulting in a restricted test envelope. New surface coatings had been tried, but did not cure the problem, and AF-2 was set to test an all-new skin design early in the year. Such testing would involve an unusual amount of sustained high-power engine operation of the type referenced by the investigators.

Pratt says the “mitigating actions will use the advanced engine prognostics and health monitoring systems already in place to ensure a safe return to flight.”