Though the Defense Department identified the cause of an engine blade crack on an F-35A that lead to a fleet-wide grounding, the long-term durability of the F135 engine is unknown, according to DoD’s F-35 chief.
F-35 Lightning II Joint Program Office Program Executive Officer Air Force Lt. Gen. Christopher Bogdan yesterday characterized the crack as caused by “thermal creep.” Bogdan said the engine used on Air Force 2, the jet the faulty engine was discovered on, was the “workhorse engine” of the program and was doing the majority of the envelope expansion of the entire A-model conventional takeoff and landing (CTOL) fleet.
“It had been exposed to more thermal stress than we thought,” Bogdan said at Aviation Week’s Defense Technology Affordability and Requirements conference in Arlington, Va. “It had been out to…700 knots, it had been out to 5,000 feet supersonic, it had been to seven or eight Gs.”
Bogdan said the investigation into the long-term durability of the F135 would take another few weeks and that DoD “is not out of the woods yet.”
Though the overall F-35 fleet grounding has been lifted, Bogdan said the three aircraft used in this “thermal creep” testing environment, Air Force 2, Air Force 3 and Air Force 6, remain grounded, even though Air Force 3 and Air Force 6 weren’t used nearly as extensively as Air Force 2.
How much time the engine spent in this “high, hot and pressurized situation” could determine the life of the blades on that turbine, Bogdan said. DoD is now trying to identify, based on testing it did with the faulty engine’s airplane, is what level of thermal stress it would it take to get that point on a normal airplane, according to Bogdan.
“I don’t know the answer,” Bogdan said. “It could be 25 percent of the life of the engine, it could be 50 percent, it could be 75 percent.”
Bogdan said if it turns out to be less than 100 percent of what DoD expected the life of the engine to be, then it might have to increase inspections over the life of the engine, or perhaps do engine maintenance on a more frequent level once those jets are in the field.
A cautionary flight suspension was announced Feb. 21 after a one-fifth inch crack was found on a 3rd stage engine turbine blade during a routine inspection at the F-35 Integrated Test Facility at Edwards AFB, Calif. No additional cracks or signs of similar engine stress were found during inspections of the remaining F135 engine inventory. No engine re-design will be required. The F135 engine is developed by Pratt & Whitney, a division of United Technologies Corp. [UTX] (Defense Daily, March 4).
DoD is also examining a problem with a component of the F-35 that caused smoke in the cockpit during a flight Feb. 14. DoD said the incident involved a power system that controls oxygen and temperature in the cockpit, and components have been shipped to manufacturer Honeywell [HON] for evaluation. The issue is believed to be software related and “very minor,” according to a Pentagon official (Defense Daily, Feb. 26).
The F-35 is developed by Lockheed Martin [LMT].