F135 Damage ‘Significant’ Or Easy Fix?

F135 Damage ‘Significant’ Or Easy Fix?

UPDATED: Congressional Staffer Questions Pratt; OSD Document Says Damage to Engine Was “Significant”

This is a tale of at least two perspectives. Pratt & Whitney says it knows what failed on the F135 test engine and can fix it relatively painlessly. The company has identified a “worn bushing” as the likely cause of the recent test engine failure and can make changes to the engine “with little to no impact on cost or schedule,” a senior company official said.

“During a recent qualification test on an F135 CTOL engine, an incident occurred resulting in damage to the forward section of the engine. Pratt & Whitney conducted a thorough review and inspection of the engine,” the senior program official said.


The investigation found that a worn bushing is the “potential cause, which tells us this issue can be addressed with little or no impact on cost or schedule. Pratt & Whitney will immediately implement a minor modification to the fan section for all F135 initial service release (ISR) flight and production engines. This modification can be incorporated into an assembled engine without teardown,” the senior program official said.

But a congressional aide, told of Pratt’s comments, dismissed them, saying that the company was trying to make things look as good as possible to protect the company while funding for the engine programs is decided.

The aide noted that a Sept. 14 Pentagon fact sheet about the incident says that the engine damage was “significant.”

A Pratt spokesman said “we can argue about significant and whether it is or not.”

However, the congressional aide also noted that this is the third failure the F135 has experienced, adding that caution is warranted given that significant problems with the F100 engine that powers most F-15s and F-16s “didn’t really show themselves until two years after initial operational capability.”

The OSD document says that “an approximately 1 inch by 1.5 inch piece separated from a blade on Rotor 1 of the fan. At the time the separation occurred, High Cycle Fatigue sweep tests were being conducted, but high loads at this location on the fan blade were not expected.”

The OSD document said the engine would “undergo tear down and root cause analysis. This is a lengthy process that includes very detailed metallurgy and crack propagation analysis and will take several weeks.”

The Pratt spokesman said the company “will continue to do further metallurgical examination” but did not need to tear down the entire engine. The fixes to the blades are “imminent” and will be done as each engine comes down the line, the spokesman said.

No decision has yet been made about the failure’s impact on the test program, the OSD document says. The engines being used for current flight tests “uses an older design of this fan airfoil, and is not expected to exhibit a similar problem,” the OSD document adds.

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So how much time in an engine shop does this “staffer” have? Must be 20–30 years (retired E-8 or E-9?) since he seems to know all about jet engine development problems and solutions.

More likely that that the aide works for a representative that’s beholden to the competition.
Either straight up campaign funding or jobs for constituents.

Well, the story doesn’t make a whole lot of sense. One one hand Pratt is talking about a bearing in the fan section of the engine. Yet the damage seems to have come from a piece of a blade that failed in HCF (high cycle fatigue) which could be due to a 1 per rev stress from sometining up stream, or due to HCF resulting from a tip rub. Pratt says that the engines that are flying have a different fan blade (older design) that is not affected by the same failure as this event. So, my take is that a redesign (possible weight reduction) may be involved with the failure. The bearing thing is confusing however.
But this is the kind of thing that flight test and endurance testing should find. Doesn’t mean that Pratt is hiding anything just is unfortunate timing. Much better to find problems at the front end of the program than at the beginning or well into production.

Steve

The Congressional Staffer and the P&W rep seem to be talking past each other. P&W says the potential cause is a worn bushing and the fix is relatively easy, while the Congressional Staffer says the damage to the engine was significant. They both could be right. And yes, the P&W rep is trying to put the company in the best light and I am sure the Congressional Staffer has reason to want the F136 to be funded. Doesn’t mean that they are not speaking the truth.

so they’re talking about a worn bushing, and a blade failing, with additional fuss about older blade airfoils not being affected… they could easily be connected. The worn bushing allowing slop in the rotation which stresses the fan and causes it to weaken along stress lines in the new airfoil design as the fan wobbles back and forth with the slop from the worn bushing.

This scenario make sense to anyone else in explaining the inconsistancies in the various stories?

A fan hat falls apart by itself would be “significant” and require a whole new fan re-design, but a fan that fails because of a worn bearing or bushing is not significant since all it would need is a more durable bushing, which is easier to retrofit than designing a whole new fan assembly.

Not sure if any of you have visited the f135’s website, but it does clearly state that the engine went through 5 hours of testing at “supersonic conditions” and endured the equivalent of 8 years of use. One would think that in 8 years of use that the military would surely take apart the engine to ensure there is no damage. If you don’t consider that point, then buy a new vehicle a never give it an oil change or any other maintenance and see how long it lasts you.

The worn bushing may have been on an variable inlet guide vane or flapper vane stem ahead of stage 1 fan blades. The worn bushing could cause abnormal vane rotation or displacement and induce a 1/rev excitation as noted in earlier post. Early HCF results. Odd thing is the loss of blade tip is likely high order vib mode again resulting in fatigue failure.

“One would think that in 8 years of use that the military would surely take apart the engine to ensure there is no damage.“
There are periodic inspections of engines, from magnetic plug, oil contamination(worn parts) to a visual borescope inspection of the internals of an engine. As well as a visual(eyeballs down the intake) every time an engine is started.
All aviation engines have a time to overhaul limit, that includes military engines.

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