All Lockheed Martin F-35s are grounded while the programme investigates the root cause of a crack discovered on 19 February in a third-stage low-pressure turbine (LPT) blade deep inside the Pratt & Whitney F135 engine.

The discovery of the blade crack comes five years after the first flight of the F-35B short take-off and vertical landing (STOVL) variant was delayed several months due to fatigue-related blade failures in the third-stage LPT.

The latest crack - discovered only days after the F-35B returned to flight following a 25-day grounding caused by a fueldraulics failure - was discovered on 19 February by a borescope inspection on an F-35A at Edwards AFB, California, and confirmed by an eddy current inspection, P&W says.

"It is too early to know the fleet-wide impact of this finding," says the Joint Program Office (JOPO). But all F-35s are grounded "until the investigation is complete and the cause of the blade crack is fully understood".

The engine in the 19 February incident has run a total of around 700h, of which 409h were flight hours, P&W says.

Though the issue was found in a single engine, there was no clear cause behind the crack. The engine's turbine module has been shipped to P&W's Connecticut test facility for closer inspection.

"We will conduct more thorough evaluations to determine the cause of the indication of the crack," the engine maker says.

Lockheed referred all questions about the grounding to the international joint programme office.

Two previous incidents with the third stage low pressure turbine blades have resulted in F-35 groundings in the past, once in 2007 and again in 2008. Those incidents, which both occurred in the F-35B vertical-takeoff variant, were traced to high-cycle fatigue. Unexpected vibration levels were caused by interaction of the the blade with the wakes from vanes upstream of the third-stage turbine.

The troubled aircraft programme, meant to provide a new fighter-bomber for all three major services of the US military, has been grounded several times before.

Source: Flight International