Pratt & Whitney officials say they have identified a promising remedy for rubbing in the F135 engine that caused a catastrophic fire in June and destroyed a Lockheed Martin F-35 Lightning II, limiting test flights of the developmental fleet.

Contrary to statements by Lt Gen Christopher Bogdan, chief of the US F-35 Joint Program Office, engine manufacturer P&W says seal plating within the third-stage fan overheated during a “relatively aggressive” training manoeuvre.

The engine flexed within the fuselage, causing a “hard incursion” of foam rubber that lines the internally bladed rotor, raising the material’s temperature to twice its intended threshold. The phenomenon causes micro-cracking in the titanium arm ringing the fan, says Bennett Croswell, president of Pratt & Whitney military engines.The heat caused the arm to come loose from both the second- and third-stage internally bladed rotor section of the turbine engine, and shattered and punctured a rear-aft fuel cell, causing the fire. Bogdan has repeatedly blamed the rotor fan for causing the failure.

All fighter engines experience similar rubbing, but the rate and depth to which that particular engine dug into the rubber stator lining was unexpected, Bogdan says. Both he and Croswell acknowledge that initial modelling of the engine’s function underestimated the extent to which the engine would need to be “burned in” before performing advanced manoeuvres.

Potential fixes include changes to the materials used to manufacture the fan blade and the rubber lining that caused the rubbing, and “pre-trenching” the rubber lining to allow the fans to glide freely without making contact. The service and P&W are also considering a set of low-stress flight profiles that would gradually burn-in engines already installed on operational aircraft.

Rub-rig testing to characterise a “good rub” versus a “bad rub” is ongoing. The engine failure was not the result of poor quality materials or manufacturing, Bogdan adds.

“This has to do with the material properties and it has to do with the model that Pratt & Whitney developed a long time ago that would have estimated how much rubbing took place, and quite frankly it was underestimated,” Bogdan says.

Until a fix is identified, the engines are not being mated with Lockheed airframes. All new aircraft, beginning with the programme's eighth lot of low-rate initial production, will receive engines that include the permanent fix, Bogdan adds. Because the engines are modular, new fan sections can be retrofitted to the 150 existing aircraft with the potentially defective engines, he says.

By the end of September, Croswell says the root cause of the engine failure will have been identified and the malfunction will be recreated in a test lab.

Bogdan plans to downselect from six possible fixes to the best one by the end of October. A prototype of a pre-trenched engine should be flight tested by 24 October, he adds. Croswell intends to begin retrofitting existing engines in November, with emphasis on the 21 system development and demonstration aircraft. Those aircraft at least should be flying at full envelope by the end of the year, Croswell adds.

Bogdan said just weeks ago that the US Marine Corps could miss its planned 1 July 2015 initial operating capability if the test fleet was not up and running at full bore by the end of September.

Source: Flight International