A formal Air Force investigation has upheld an earlier assessment of the cause of the service's F-35A aircraft engine fire last year, which led to a temporary grounding of the aircraft.
The report, issued by the Air Education and Training Command Accident Investigation Board, said the incident wound up causing an excess of $50 million in damages.
The F-35A, which was damaged last June during a take-off portion of a training mission, was assigned to the 58th Fighter Squadron, 33rd Fighter Wing, Eglin Air Force Base, Florida.
The Air Forces' Accident Investigation Board president found, by clear and convincing evidence, the mishap occurred when "the third-stage forward integral arm of a rotor fractured and liberated during takeoff."
"The Accident Investigation Board found the cause of the mishap was catastrophic engine failure," the report stated. "Pieces of the failed rotor arm cut through the engine's fan case, the engine bay, an internal fuel tank, and hydraulic and fuel lines before exiting through the aircraft's upper fuselage. Damage from the engine failure caused leaking fuel and hydraulic fluid to ignite and burn the rear two thirds of the aircraft."
The findings are wholly consistent with prior analyses of the incident, F-35 program officials said. Engineers with the Pentagon's F-35 program began implementing a series of short– and long-term fixes to the aircraft's engine problems last year.
An initial 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, Air Force Lt. Gen. Christopher Bogdan explained to reporters following a discovery of the incident.
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 engines 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 last years' root cause analysis.
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"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 Fahrenheit, 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, led 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.
F-35 Fixes
F-35 officials explained in 2014 that several of the short term fixes initially underway with several existing test-airplanes have proven effective. These fixes include 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 October 2014.
"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 in October 2014.
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 was validated on four F-35 test airplanes and was examined by air worthiness authorities in the Navy and Air Force last year in order to pave the way for additional use on fielded jets, Bogdan added.
Another short-term fix which has proven effective 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 the kind of excessive rubbing which led to the incident.
"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 in October 2014.
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 was also examined by air worthiness authorities for use on fielded jets, Bogdan explained.
Overall, both short-term fix approaches were needed because the pre-trenching method required 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 included efforts to make sure that the trench was not so big so as to create airflow back in the compressor section which reduces the stall margin, Bogdan explained in October 2014.
-- Kris Osborn can be reached at kris.osborn@military.com
