The 0.6-in. crack found on a blade in the third-stage low-pressure turbine of an F-35A last month was the result of “thermal creep,” says U.S. Air Force Lt. Gen. Christopher Bogdan, F-35 program executive officer.

Though the problem is not thought to be a design flaw in the F135 engine, the Pratt & Whitney propulsion system is “not out of the woods” yet as officials study what implications there could be for durability of the system once it is fielded, Bogdan told an audience March 5 at Aviation Week’s Defense Technology & Affordability Requirements conference outside Washington.

The crack was found Feb. 19 during a routine borescope inspection on the ground, but it led to a fleetwide grounding, which was lifted March 1.

The crack was found on a blade in AF-2’s engine. Bogdan says this was the “workhorse engine on the program,” which has been used for envelope expansion work and, thus, exposed to extremes in operation, including flights at 1.5 Mach; in low-level, supersonic conditions and at 7-8g.

This aircraft was “doing the majority of the envelope expansion for the A-model fleet,” he says.

Two other aircraft in the conventional-takeoff-and-landing fleet, AF-3 and AF-6, remain grounded because they have been flown in similar — though not quite as rigorous — conditions.

Engine experts will require at least two more weeks to explore what implications there are for engine durability, Bogdan says.

“What level of thermal stress would it take to get to that point on a normal airplane” not exposed to such extremes, he asks. “The issue is if it turns out that it is less than 100% of what we expect the life of the engine to be, then we have [a] turbine blade having life-limiting parts on it and we’ll have to deal with that.”

Should this become a problem, customers could be required to conduct additional inspections and/or additional maintenance over what was originally planned.