Engineers with the Pentagon’s F-35 program are implementing a series of short- and long-term fixes to the aircraft’s engine problems which resulted in an engine fire and temporary grounding of the plane this past summer.
A root-cause analysis determined that the fire and engine failure were caused by excessive rubbing between polyimide material on the rotor of the engine and the engine rotor’s titanium plates, Lt. Gen. Christopher Bogdan explained.
Polyimide is a rubber like material that the fan-blade can touch in order to create a seal preventing air from back-flowing into the back area of the engine's compressor, JSF officials explained. There is supposed to be natural rubbing in the engine this fashion, however the F-35 engine failed due to a “hard rub” of these materials, resulting in excessive heat or what’s called a “thermal heating chain of events,” Bogdan explained.
This problem ultimately led to the cracking of the rotors and engine failure, according to the root cause analysis.
“We designed this airplane to rub in certain spots. The third rotor fan section of the engine is supposed to rub against a permanent portion of the engine. We underestimated in the design of the engine how much could rubbing potentially occur. We saw a hard rub in excess of anything we would have expected to see, resulting in heat that we never expected to get,” Bogdan said.
The excessive rubbing brought the temperature of the engine up to 1,900 degrees, a temperature much higher than the 1,000-degrees that was expected. The high heat caused micro-fracturing in the titanium piece of the rotor which, over several weeks of flying, lead to engine failure.
“High cycle fatigue caused the rotor to liberate from the airplane. The fire was not caused by the engine but by the pieces of the engine that flew out through the upper fuselage fuel tank,” he added.
Short Term Fixes
One of the short term fixes now underway with several existing test-airplanes includes flying the airplanes on two one-hour sorties doing very controlled maneuvers to “burn in” the engine such that it rubs an appropriate amount, Bogdan explained.
“In two sorties you can burn in the engine in a controlled way such that where this rubbing occurs has now been ‘burned in’ so to speak. Anything else you do with the airplane inside the envelop will not cause any more rubbing than it has already seen because it has already trenched a canal,” Bogdan said.
This involves flying the airplane in a very defined profile with certain specific altitude, G-force and roll-rate conditions in order to achieve the properly trenched, or ”burned in” canal in the engine.
This “burn in” process has already been validated on four F-35 test airplanes and is now being examined by air worthiness authorities in the Navy and Air Force for additional use on fielded jets, Bogdan added.
Another short-term fix now underway on two Air Force F-35As includes a process called “pre-trenching.” Bogdan described this as a process wherein portions of the engine’s stator, a non-rotating section that hooks to the frame of the engine, is specially pre-trenched or configured to avoid excessive rubbing.
“We expect to see some rubbing and some trenching. What we are going to do now is we are going to go in and pre-trench a canal in that material so that when we put the fan blade in there – no matter what we do on the airplane G-wise, altitude wise – it won’t rub anymore,” Bogdan explained.
During the test of this pre-trenching fix, two Air Force F-35s were able to fly a full envelope of potential missions without any excessive heating. Over the next two to three months, all 19 test airplanes will be configured with this pre-trenching fix, Bogdan added.
This approach is also being examined by air worthiness authorities for use on fielded jets, Bogdan explained.
Overall, both short-term fix approaches are needed because the pre-trenching method requires fabrication of a new stator, which take time to produce.
“Right now we’re only producing about one whole set of stators per week. It would take us a while to produce enough for all the airplanes. With the burn-in procedure we can start getting to the same results by flying airplanes instead of getting into the pre-trenching. That why it is important to get both solutions out there,” Bogdan said.
An important element of the pre-trenching process would need to include effort to make sure that the trench is not so big that there is airflow back in the compressor section which reduces the stall margin, Bogdan explained.
Long Term Fixes
Alongside these short-term fixes, the F-35 program and engine-producer Pratt and Whitney are also immersed in the exploration of a series of potential long-term fixes. Bogdan said F-35 program engineers are now analyzing five different long-term options presented by Pratt and Whitney.
The options include changing the polyimide material to a different substance that can better handle greater temperatures or treating the part of the titanium blade that hits the polyimide, Bogdan added.
“You could actually treat the tip of that titanium such that if you do get heating it can withstand 1,900 degrees,” he said.
Yet another option would be to put pre-trenched polyimide in the engine, Bogdan said.
The F-35 program plans to decide upon a long-term fix, which may involve a combination of options, by the end of this calendar year, Bogdan added.
“We have yet to decide on what combination of these different sets of options. We will put an engineering change in, qualify it, and sometime toward the end of 2015 we will start producing engines with the fix and go back and retrofit any of the produced engines that are not in engines yet,” he said.
The cost of fixes to fielded engines, either burn-in or stator procedures, will be handled by Pratt and Whitney. The cost of re-engineering or producing new solutions from production engines will also be handled by Pratt and Whitney, Bogdan explained.
However, the non-recurring engineering costs of production cut-ins or re-engineering of the engine will be paid by the government, per the initial F-35 development deal signed 14 years ago, Bogdan said.