F135 Engine Failure Investigation Makes Progress

AIN DEFENSE PERSPECTIVE » SEPTEMBER 19, 2014

by  CHRIS POCOCK

A Pratt & Whitney F135, the engine that powers the F-35 stealth fighter. (Photo: P&W) The inset shows the location of the plate seal, between the second and third fan stages, that rubbed excessively to cause an engine failure.

September 19, 2014, 8:20 AM

Root-cause analysis of the F135 engine failurethat grounded the Lockheed Martin F-35 fighter fleet in July will be completed by the end of the month. Meanwhile, the problem is already sufficiently understood for Pratt &Whitney to have devised an interim fix. F-35s are flying again, but with borescope inspections mandated every three flying hours. The flight envelope restrictions that were previously imposed now vary, with four key development aircraft cleared for greater maneuverability.

“Pratt & Whitney has put its ‘A-team’ on this problem and has taken accountability,” Lt. Gen. Chris Bogdan, F-35 program executive officer, told the Air Force Association conference this week. The company has agreed to pay for modifications to the fan section of the 150 engines already delivered, he added.

Bennett Croswell, P&W president for military engines, said that “there’s more movement of the engine” within the F-35 airframe “than we thought when we designed it.” Both Bogdan and Croswell emphasized that some movement is normal. The problem had not been identified earlier in the development program, because at the time the aircraft had not been cleared for “more aggressive maneuvers,” Croswell said. The problem could not possibly have been detected during engine ground testing, he added.

The problem is excessive rubbing of a polyamid plate seal between the second and third stages of the F135’s fan. The rub strip is flat when a new engine is built, Croswell explained, but during acceptance tests it is designed to be abraded into a groove, or trench. This method ensures that air does not leak forward, a migration that wouldreduce engine efficiency. But in the engine incident that happened while an F-35 was taxiing at Eglin AFB on June 23, the plate seal heated to 1,900 degrees C, causing microcracks to form and propagate in the arm, which eventually failed and penetrated a fuel tank, causing a fire. During inspections, three more engines (out of 165 delivered) were found to have “hard rubbing” that could potentially lead to a similar failure.

“We are validating the root cause on a ‘rub rig’ at our West Palm Beach facility,” Croswell continued. “We’ll test different densities of polyamid, and the orientations that occur in its formation.” P&W and subcontracting supplier Cobham are “pre-trenching” some stators before engine assembly, to determine whether efficiency is significantly affected. Another approach could be to specify a uniform set of flight maneuvers to “burn in” the trench, Croswell said.