Pratt & Whitney is investigating the cause of an F135 fan failure that developed in the first stage of the Joint Strike Fighter engine’s three-stage unit during ground tests in Florida in December.

The fan crack occurred on Dec. 23 during accelerated mission tests (AMT) on ground engine FX648 at Pratt’s West Palm Beach facility, as the engine reached 77% of its required life, says F-35 Program Executive Officer Lt. Gen. Christopher Bogdan. Discussing the problem at Aviation Week’s Defense Technologies and Requirements Conference in Arlington, Va., Bogdan says Pratt may have “underestimated the stress at low-cycle fatigue” of the fan, which he says “blew” during the test.

“Our investigation is ongoing, but we have determined this incident does not pose a flight safety risk and will have no near-term impact to the operational fleet,” says a statement from Pratt. The engine maker says it will continue to monitor operational cycles for each engine in service, and is confident there is no safety issue because of the low-cycle fatigue conditions of the failure.

The specific engine involved was the highest-time F135 in the test fleet, with 2,200 hr. of running time, or approximately nine years of service as a test engine. In terms of hours, Pratt says FX648 had more than four times the hours of any engine used in F-35 flight testing, and more than 10 times the hours of any operational F135 engine.

The failure, which occurred in the front stage of the F135’s three-stage fan with the engine operating in conventional mode, did “pretty good damage” to the cold section of the engine but did not effect the hot section or the lift fan, Bogdan says. The unit, which is common to all variants of the F-35 engine and located aft of variable-geometry inlet guide vanes, combines the roles of a conventional fan with that of a low-pressure compressor. The stages are made up of integrally bladed rotors (IBR), the first of which is constructed from hollow titanium. The second and third stages are made from solid titanium.

Pratt declines to comment specifically on whether potential flutter or other factors are being considered in the investigation, but adds “we are looking at a variety of factors, including stresses on the region and the rigors of the AMT test.”

Bogdan says manufacturing and cost challenges associated with the hollow IBR already had prompted Pratt to start a redesign of the fan, which was under way when the failure occurred. Fixes based on lessons learned from the failure in December will be incorporated into the redesign, which involves producing the first-stage IBRs from solid titanium. The redesign is expected to add about 6 lb. to the total weight of the engine, but should ease manufacturing significantly. The change effectively represents the second redesign for the IBR, which was changed to a hollow unit as part of a weight-reduction effort.

Pratt confirms the planned modification is already under way, saying: “Prior to this incident, Pratt & Whitney initiated a redesign of the IBR to further reduce costs. We will be able to incorporate these changes with minimal impact to the operation of the F-35 fleet. We are confident the solid IBR design will resolve the issue.”

Although the F135 has encountered several development issues since engine tests began in late 2003, there have been few reported incidents concerning the fan. A problem causing damage to the first and second stages during durability tests of an F135 in September 2009 was later traced to aerodynamic disturbances caused by a worn bushing in the inlet.

News of the fan failure emerges a bit more than one year after the temporary grounding of the F-35 Joint Strike Fighter had been lifted, following the discovery of a crack in a third-stage low-pressure turbine (LPT) blade on U.S. Air Force test aircraft at Edwards AFB, Calif.