The amount by which the F-35’s engine flexes during flight is greater than what the Defense Department initially anticipated, according to the head of the program.
F-35 Program Executive Officer (PEO) Air Force Lt. Gen. Christopher Bogdan said Wednesday that although DoD plans for jet engines to flex, shift and move during flight, it didn’t expect the F135 to flex as much as it did, which lead to an eventual aircraft fire and temporary grounding of the entire F-35 fleet this summer.
“We thought, initially, (engine fan blades) would only protrude into the rubber so much and so quickly, and it’s going much deeper and much quicker when we’re going through those extreme maneuvers on an airplane,” Bogdan told reporters following his presentation at the ComDef 2014 conference in downtown Washington. “We just have to figure out a way to get the ‘burn-in’ such so we don’t experience it anymore.”
Not only do fighter jet engines expand and contract when they heat up, Bogdan said, but when they are put through extreme maneuvers during flight, like nine Gs, as he called them, engines flex and move around within the airframe. Though some parts of an engine don’t move, causing rubbing between engine parts, Bogdan said DoD plans for this by putting certain metals in contact with other parts.
Bogdan said in the case of the F135, DoD planned for this rubbing to happen with titanium third stage fan blades against a “carbon polyimide” material that he called a kind of rubber. High-temperature polyimide carbon fiber composites are often used in non-loading structural components in aircraft, weapon systems and space vehicles.
A pilot put the F135 engine through a special combination of Gs, roll rate and yaw, all at the same time earlier this summer, causing the engine to overheat and develop “microcracks” as it flexed and rubbed. Three weeks later, Bogdan said these microcracks grew so large that they started a chain reaction that led to a fire on an F-35A and to an eventual fleet grounding. Bogdan said the engine fan reached temperatures of 1,900 degrees Fahrenheit “in a few seconds” when it was only supposed to reach 1,000 degrees Fahrenheit.
F135 manufacturer Pratt & Whitney, Bogdan said, will have a third-stage fan prototype done by the end of September as a solution to the problem. DoD will perform tests on it, Bogdan said, before it can be put on an engine and he expects to put it on a test aircraft and fly it by the middle of October.
Bogdan said he did not see microcracks in other engines, just indications of “hard rubbing” and not at the extreme temperatures that caused the microcracks.
“We saw some rubs that we said ‘that’s a little darker color than we would have thought,’” Bogdan said. “That is going into our analysis of what (is) good rub and (what is) a bad rub.”
The engine on the F-35A that caught fire hadn’t been fully burned-in, Bogdan said, which is like a break-in period. Bogdan said DoD would adjust the burn in procedure and that this could be done in “one or two flights” because DoD knows how to do it safely in a test environment with instrumentation and test pilots.
Bogdan said this new burn-in procedure would not be a short-term fix that allows for flight on test aircraft, but a fix that will provide “full-life” because once the engine is burned into the right depth of the wedging (between the fan blade and polyimide material) and the aircraft reaches the edges of the flight envelope without rubbing, it is good to go.
“It’s not a risky thing to do,” Bogdan said.
This is not a complex engine problem, either, he added.
“Within two months, we’re pretty much…right there on what caused it,” Bogdan said. “I look at other airplanes that went through other problems that took months and years to figure out. This one, we’re only 2.5 months into it, and we pretty much know where we are.”
Pratt & Whitney would pick up the tab to retrofit 156 engines with “some kind of solution,” Bogdan said, and that future problems with the engine would have risk split between DoD and the company (Defense Daily, Sept. 3). The initial F-35A conventional Air Force variant fire took place June 23 and eventually led to DoD grounding its entire fleet. DoD lifted its department-wide grounding July 14 (Defense Daily, July 15).
Bogdan said the special combination of in-flight maneuvers that caused the overheating and eventual cracking took place three weeks beforethe initial fire. Pratt & Whitney is a unit of United Technologies Corp. [UTX]. The F-35 is developed by Lockheed Martin [LMT] with subcontractors BAE Systems and Northrop Grumman [NOC].