F35s grounded. [Archive]

NutLoose

23rd Feb 2013, 13:36

Yes there was this one as an example

160784 B Second TFA-18A (later designated FA-18B). McDonnell Douglas, crashed 8 Sep 1980 Mildenhall Royal Air Force, Middle Wallop, Hampshire, UK near Farnborough after engine failure. MD pilots Jack Krings and Gary Post ejected safely.

They had a big TV request on the news and offered locals and local schoolchildren a reward for any parts of the turbines found and managed to recover most of it to find out what went wrong

Bengo

23rd Feb 2013, 13:53

The Gazelle 163 ( I think- the one in the Wessex 3/ 31 series) was prone to shedding compressor blades. That caused the loss of several aircraft and at least one RAN fleet grounding. The cause was usually an installation issue with the root.

N

barit1

23rd Feb 2013, 15:35

Engineering teams are removing the turbine blade from the plane and plan to ship it to Pratt's engine facility in Middletown, Connecticut, for more thorough evaluation and root cause analysis, according to the Pentagon and Pratt.

If this is to be a competent investigation, USAF should send the ENGINE back to P&W for analytical teardown. I say this because it's possible - maybe likely - that the root cause lies elsewhere in the engine, and the cracked blade is simply the symptom of a upstream fault. :8

Pontius Navigator

23rd Feb 2013, 15:47

Do we not get turbine blade failure on other engines/fleets and would the whole fleet be grounded then? Notwithstanding of course the recent introduction of the F35 and the obvious lack of history when compared to say the Tonka fleet.

I think one clue is in the news item. They need to ensure the support of the other nations involved in the development programme. They cannot afford to press on an risk another failure, so better to ground the fleet when such grounding in not yet an operational issue.

Look also at the 787 which has been grounded by the FAA. Then think of the Comet where they pressed on an lost more aircraft.

The RAF has been known to have tech groundings just they never publicised them to this extent. There was on grounding on the Vulcan fleet circa '65-'67 which lasted IIRC 10 days or more. As this did not prevent launch on deterrent missions, and we didn't wish to alarm anyone, it was kept Secret. the same was true of the Valiant force in '64. They remained operational yet grounded at the same time.

Courtney Mil

23rd Feb 2013, 15:56

Does anyone here think we'll ever build the perfect engine that never fails, never catches fire, never gets upset? Is this the first engine that ever had a blade failure? Is this anything to do with the rest of the platform?

Come on. Wait and see before we judge. They have, after all, built a lot of good engines before, so probably know what they're doing. Above all, the engine isn't the biggest concern in the whole system.

FODPlod

23rd Feb 2013, 16:36

Imagine the widespread panic if civilian airliners ever suffered turbine blade failures. Airbus A330-300 C-FBUS: Aviation Investigation Report A01F0020 (http://www.tsb.gc.ca/eng/rapports-reports/aviation/2001/a01f0020/a01f0020.asp)
Analysis

Modern jet engines are extremely reliable, and in-service failures are rare. This reliability is the basis of the extended range twin-engine operations (ETOPS) approval. When two engines on the same aircraft fail for the same mechanical reasons in such a short time, this reliability is brought into question. In this case, both engine failures were the result of stress corrosion cracking of the second-stage turbine blades, a failure mode that Pratt & Whitney has been actively trying to control through the use of sacrificial corrosion protection coatings. The focus of this investigation analysis was to identify the underlying causes of these two engine failures...

Turbine D

23rd Feb 2013, 19:36

Imagine the widespread panic if civilian airliners ever suffered turbine blade failures.
Luckily, civilian airliners have two engines, some even have four. The F-35 has one and if you have to shut it down, you have none. That is the difference.

Rulebreaker

23rd Feb 2013, 19:51

Reports so far indicate the aircraft with the engine issue was from Edwards. Has it been reported which aircraft was involved as that's also the location of the high alpha test aircraft.

As turbine failures are generally considered uncontainable this must be considered a serious event. As was the case with the a380 trent engine a few years ago issues upstream can be a cause, so as was mentioned above sending the entire engine back to Pratt would be prudent.

Wallah

23rd Feb 2013, 21:03

Lucky for us we have the F136 as an option. Oh wait we don't............

FODPlod

23rd Feb 2013, 22:13

Luckily, civilian airliners have two engines, some even have four. The F-35 has one and if you have to shut it down, you have none. That is the difference.

Are you saying the problem is a permanent show-stopper then?

Turbine D

24th Feb 2013, 01:40

Originally posted by FODPlod
Are you saying the problem is a permanent show-stopper then?
I don't think it is a permanent show-stopper, but one where a time-out is required to determine the scope of the problem. It could be unique to one turbine blade where an undetected, but should have been detected, defect was the source, or it could be a more general defect/material problem which could involve multiple blades. Unlike what some articles report, the turbine consists of a one stage high pressure turbine and a two stage low pressure turbine. Even if this is a stage 2 LPT blade, its failure could cause significant damage aft in the A/B as blade sizes and weights have gotten larger with fewer blades per rotor. So I think they did the correct thing until the true cause can be ascertained. It is unusual for a LPT turbine blade with low operating hours to develop detectable cracking.

TD

Pontius

24th Feb 2013, 02:22

Mildenhall Royal Air Force, Middle Wallop, Hampshire, UK near Farnborough

That's one BIG crash site.

FODPlod

24th Feb 2013, 08:59

TD - Thank you for your clarification.

Haraka

24th Feb 2013, 10:38

"Mildenhall Royal Air Force, Middle Wallop, Hampshire, UK near Farnborough"
IIRC that was the climb-out from Farnborough one. The local kids were rewarded per lump retrieved, until it was realised that the enterprising little angels were breaking bits up in to many smaller lumps before handing them in.

b1beefer

24th Feb 2013, 16:38

'Defence Specialist'?!

barit1

24th Feb 2013, 19:02

Rulebreaker:As turbine failures are generally considered uncontainable this must be considered a serious event...

As has been discussed in the A380 QF32 case, it was the turbine disc failure (IPT in that case) that was uncontained, Turbine blade failures should be contained, at least in civil-certified engines. (I know of some exceptions in military engines, but cannot speak directly to the F135 engine requirement)

SpazSinbad

25th Feb 2013, 07:09

For 'barit1' comment on previous page: http://www.pprune.org/military-aircrew/508639-f35s-grounded.html#post7710920

F-35s Grounded As Precaution After Crack Found in Engine Blade (http://www.asdnews.com/news-47807/F-35s_Grounded_As_Precaution_After_Crack_Found_in_Engine_Blade .htm)

"...Officials are shipping the engine and its associated hardware to Pratt & Whitney's engine facility in Middletown, Conn., to conduct more thorough evaluation and root cause analysis...."

SpazSinbad

25th Feb 2013, 17:09

A good 'history' of F135 engine problems here:

F-35 Grounded After New F-135 Engine Problems By Johan Boeder (Source: JSFNieuws.nl; published February 22, 2013)

F-35 Grounded After New F-135 Engine Problems (http://www.defense-aerospace.com/article-view/feature/142927/the-long-history-of-f_35-engine-problems.html)

There is an apparent factual error here AFAIK:

"...Some facts about what happened
It was a F-135 engine with 700 hours, of which 409 flight hours. The aircraft was the F-35A test aircraft AF-2. The half-inch wide crack was found in a turbine blade of the low pressure turbine section...." Perhaps it is a DUTCH language thing but here is what was reported earlier:

Half-inch crack blamed for F-35 fighter jet grounding: sources - Yahoo! News (http://news.yahoo.com/half-inch-crack-blamed-f-35-fighter-jet-031237599--finance.html) : "...The engine blade crack that prompted the U.S. military to ground all 51 F-35 fighter jets was over half an inch long, according to three sources familiar with the matter..."

Jul 22, 2008 briefing by Pratt & Whitney on engine problems and fixes

http://www.defense-aerospace.com/dae/articles/communiques/Blade-Failure-SECNAV-Briefing-2008.pdf (8Mb)

SpazSinbad

26th Feb 2013, 11:04

F-35 grounding sends pilots to simulators 25 Feb 2013
By Brian Everstine and Aaron Mehta - Staff writers

F-35 grounding sends pilots to simulators - Air Force News | News from Afghanistan & Iraq - Air Force Times (http://www.airforcetimes.com/news/2013/02/air-force-f35-grounding-sends-pilots-simulators-022513/)

"...Matthew Bates, a spokesman for engine contractor Pratt & Whitney, said the damaged engine had arrived to a P&W facility on Sunday and that crews are working on inspecting the crack...."

John Farley

26th Feb 2013, 15:20

Does anybody know if the cracked blade was a rotor or stator blade?

barit1

26th Feb 2013, 15:33

The pdf linked in SpazSinbad's post explicitly points to the LPT rotor 3rd stage blade, calling it "mistuned" and vulnerable to high-cycle fatigue (HCF).

BTW, I'm very surprised to see this P&W pdf briefing appear "in the wild". The pages are marked "proprietary".

EDIT: Well gee, If I were only paying attention I would have noticed the P&W briefing is 5 years old. I can only guess that the current 2/2013 grounding is more of the same. :}

Turbine D

26th Feb 2013, 19:24

SpazSinbad,

Thanks for your post, very enlightening indeed relative to history and cause of early stage 3 blade problems.

It would be my guess the problem this time around will have a different cause. First of all, it happened on an Air Force F-35, rather than on a STOVL version intended for the Marines. In the 2007 time period, the 3rd stage turbine blade failure was associated with the behavior of the 3rd stage vane nozzle just ahead of the blade rotor. The cause was related to an unanticipated wake force function when in the STOVL mode. This caused vibratory stress levels in some of the 3rd stage blades that exceeded the single crystal material capability, notably, at #6 internal cooling crossover hole near the blade leading edge. Since crossover holes do produce a higher level of stress concentration, the failure (crack) started there. To resolve this issue, the #6 crossover hole was eliminated as well as the internal coating that had been used. Some changes to the 3rd stage vanes were also made.

Now there is a cracked 3rd stage blade discovered on the Air Force version of the engine which would seemingly have none of the causes related to the STOVL engine incidents. IMO, the cause of this crack will be different. Hopefully, we will learn what is discovered after the engine teardown and metallurgical examination of this new crack.

TD

Lonewolf_50

26th Feb 2013, 19:34

barit1: thank you for your incisive posts.
Courtney:
Does anyone here think we'll ever build the perfect engine that never fails, never catches fire, never gets upset? Is this the first engine that ever had a blade failure? Is this anything to do with the rest of the platform?

Come on. Wait and see before we judge. They have, after all, built a lot of good engines before, so probably know what they're doing. Above all, the engine isn't the biggest concern in the whole system.
All true.

The media and chattering classes and others thrive on drama, manufactured or otherwise. Turdine D, thanks for the distinction between B and A, which to many laymen watching the above mentioned drama would be obscure. I expect the engineers will figure it out.

SpazSinbad

27th Feb 2013, 02:38

Pratt rules out worst-case cause for F-35 blade crack: sources 26 Feb 2013 By Andrea Shalal-Esa

Pratt rules out worst-case cause for F-35 blade crack: sources - Yahoo! Finance (http://finance.yahoo.com/news/pratt-rules-worst-case-cause-023634091.html)

"WASHINGTON (Reuters) - Pratt & Whitney is 99 percent sure the fan blade problem that grounded the Pentagon's 51 new F-35 fighter jets was not caused by high-cycle fatigue, which could force a costly design change, according to two sources familiar with an investigation by the enginemaker.

Company engineers have concluded that a 0.6 inch-long crack found on a turbine blade in the engine of an F-35 jet at Eglin Air Force Base in Florida was almost certainly caused by lesser issues, such as high heat exposure or a manufacturing problem, that would be easier to solve, the sources said.

"They're 99 percent sure that it's not the worst-case scenario of high-cycle fatigue," said one of the sources.

Flights of the single-engine, single-seat F-35 fighter could resume as early as this week if the Pentagon accepts the findings of Pratt, a unit of United Technologies Corp (UTX), after additional tests to be done Wednesday, said one of the sources, who was not authorized to speak publicly....

...It was not immediately clear if the Pentagon would order a one-time inspection of all F135 engines built by Pratt for the new F-35 fighter, or whether the incident would result in a new recurring inspection requirement. Some inspections of the other 50 fighters already in use by the Pentagon were underway.

Pratt began detailed tests of the engine on Sunday evening at its Middletown, Connecticut facility after the blade assembly was removed from the Florida test plane and shipped north....

...The official, who was not authorized to speak publicly, said Pratt was expected to deliver a comprehensive analysis of the test results to Pentagon officials no later than Thursday evening.

Two sources familiar with the investigation said the fan blade tests would include a "destructive" test that would cut into the turbine blade to better understand how the crack developed.

Engineers believe the crack is either a "creep rupture along a grain boundary" that was caused by prolonged exposure to high heat, or that it was caused by an anomaly during the metal casting process, the sources said...."

matkat

27th Feb 2013, 06:37

To post number 14 the RR Trent 900 issue was not caused by turbine blade failure but as a consequence of the oil feed line in the IP/HP bearing house this in turn led to an oil fire which caused the turbine disc failure.

Barit1, sorry just saw your post and of course totally agree.

Turbine D

27th Feb 2013, 14:34

SpazSinbad,

It is always interesting, but adds to the confusion, when newspaper publications refer to fan blades, compressor blades and turbine blades interchangeably when they are not at all.

So, assuming the problem is with the 3rd stage turbine blade or any turbine blade and not a fan blade, finding the crack to be associated with a grain boundary would not be good as all the turbine blades are supposed to be a single crystal, having no grain boundaries. It is possible however, in a turbine blade containing complex internal passages (for air-cooling purposes) to have an isolated grain boundary form due to an undetected internal defect and stress concentrations that are associated with the defect. We will just have to wait and see what P&W finds in their examination. I kinda expect the finding will be isolated to this one blade and not widespread.

TD

SpazSinbad

27th Feb 2013, 18:33

Pentagon says no additional cracks found in F-35 engines Wed, 27th Feb 2013

Pentagon says no additional cracks found in F-35 engines - Finance News - London South East (http://www.lse.co.uk/FinanceNews.asp?code=oa5okxle&headline=Pentagon_says_no_additional_cracks_found_in_F35_eng ines)

"WASHINGTON, Feb 27 (Reuters) - The Pentagon on Wednesday said no additional cracks were found on F-35 fighter engines during inspections done after the Feb. 19 incident that has grounded the entire fighter fleet and halted operation of the engines on the ground...."

LowObservable

28th Feb 2013, 11:23

Good news for the program, as far as it goes.

However, it's too early to pop the bubbly and say "manufacturing only" because one of the things that we do, when it comes to things that are essential to the aircraft's ability to maintain a positive height AGL on demand, is make sure that they are all manufactured roughly the same way. So the root cause may not be design, but it still needs to be found and sorted out.

SpazSinbad

1st Mar 2013, 00:48

F-35 to return to flight after grounding By AUSTIN WRIGHT | 28 Feb 2013

F-35 to return to flight after grounding - Austin Wright - POLITICO.com (http://www.politico.com/story/2013/02/f-35-to-return-to-flight-after-grounding-88277.html)

"Lockheed Martin's F-35 Lightning II is returning to flight after its grounding last week over a crack discovered in an engine blade aboard an Air Force-model jet, POLITICO has learned.

The decision is effective immediately, said Joe DellaVedova, a spokesman for the Defense Department’s F-35 Joint Program Office. Pentagon officials were planning to issue a statement on the decision later Thursday evening...."

SpazSinbad

1st Mar 2013, 03:49

Pentagon says to resume F-35 flights 28 Feb 2013 by Andrea Shalal-Esa

Pentagon says to resume F-35 flights | Reuters (http://www.reuters.com/article/2013/03/01/us-lockheed-fighter-idUSBRE91R1IT20130301?feedType=RSS&feedName=domesticNews)

"(Reuters) - The Pentagon said on Thursday it would resume flights of its F-35 fighter jets, which were grounded a week ago after a crack was discovered in an engine of a test plane.

"F-35 flight operations have been cleared to resume," Pentagon spokeswoman Kyra Hawn said.

No additional cracks were found during inspections of engines in the rest of the F-35 fleet or in any spare engines, Hawn said.

Matthew Bates, a spokesman for Pratt & Whitney, which supplies the engines for the planes, said the Pentagon's F-35 program office had decided to lift a temporary suspension of flight operations after it conducted extensive tests on the affected engine part. The tests showed that a crack in a turbine blade stemmed from the "unique operating environment" in flight tests rather than a design flaw, he said.

Bates said Pratt had been working around the clock with Pentagon officials to determine the cause of the crack in the engine blade.

"The team has determined that root cause is sufficiently understood for the F-35 to safely resume flight," Bates said.

The engine in question had operated at high temperatures more than four times longer than a typical F-35 flight, which led to a separation of the "grain boundary" of this particular blade, he said...."

SpazSinbad

1st Mar 2013, 09:25

Engine Investigation Clears F-35 To Return To Flight 01 March 2013 By Guy Norris

Engine Investigation Clears F-35 To Return To Flight (http://www.aviationweek.com/Article.aspx?id=/article-xml/awx_03_01_2013_p0-554481.xml)

"...The fleet was grounded on Feb. 21 following the discovery by borescope of a 0.6 inch crack in a third stage low pressure turbine (LPT) blade on U.S. Air Force test aircraft AF-2 at Edwards AFB, Calif.

Investigators were initially concerned the failure could have been a repeat of a high cycle fatigue problem which was uncovered with the LPT during engine testing in 2007 and 2008. However Pratt says “there is no evidence of high-cycle fatigue or low-cycle fatigue, and there is no evidence of fatigue progression on the turbine blade. We believe that a key factor for the blade crack stemmed from the unique operating environment in flight test.”

The manufacturer adds the engine “had operated more than four times longer in the high-temperature environment of the flight envelope than the typical F-35 duty cycle. The exposure in this high temperature part of the envelope led to a separation of the grain boundary on this single blade.” This type of separation failure is also known as a creep rupture. This occurs when deformation begins under constant load and high temperatures, and over time reaches a point where the blade material fails...."

Then an explanation about 'horizontal tail scorching testing' for this particular aircraft.

barit1

1st Mar 2013, 18:20

One must question the style of program management that takes engine hardware which hes been intentionally abused in development test, and is then installed in a single-engine tactical aircraft. Isn't this a needless risk to a high-visibility program?

OTOH, maybe the inspection cycle that "caught" the cracked blade was designed specifically to minimize this particular risk, and was perhaps even applied to this particular engine (or a like fleet). If so, I'll award partial credit for this precaution.

SpazSinbad

1st Mar 2013, 19:23

Not to worry...

Pentagon says to resume F-35 flights | Reuters (http://www.reuters.com/article/2013/03/01/us-lockheed-fighter-pratt-idUSBRE92003H20130301?feedType=RSS&feedName=everything&virtualBrandChannel=11563) By Andrea Shalal-Esa / Reuters 01 Mar 2013

"...The Navy order rescinding the flight ban, or so-called "red stripe," said that engine had experienced the most "hot engine time exposure" of all the engines in the developmental program. It said it would now require reports to monitor and limit similar damage after every 25 flight hours...."
______________

F-35 Engine Issue Isolated: Fleet Cleared for Flight (http://www.aviationweek.com/Blogs.aspx?plckController=Blog&plckScript=blogScript&plckElementId=blogDest&plckBlogPage=BlogViewPost&plckPostId=Blog%3a27ec4a53-dcc8-42d0-bd3a-01329aef79a7Post%3a51f692b8-1fb5-43ad-b598-aee458f5559e) 01 Mar 2013 Amy Butler

JSFfan

1st Mar 2013, 20:25

Ok, they have reduced the bore-scope inspection from 50hrs to 25hrs

Turbine D

1st Mar 2013, 22:06

Hi barit1,
One must question the style of program management that takes engine hardware which hes been intentionally abused in development test, and is then installed in a single-engine tactical aircraft. Isn't this a needless risk to a high-visibility program?
I am thinking this engine was in an aircraft being used to test various thermal paints (heat resistant) applied to some areas of tail that were receiving damage from the afterburner which swivels (not fixed in one plane). So they ran the afterburner in flight for excessive periods of time to determine which paint worked best or perhaps using a preselected paint to determine if it fixed the problem. Then at some point discovered the crack in one 3rd stage turbine blade. I am sure their first thought was oh-oh, the older blade cracking problem has returned.

Anyhow, all of this leads to some questions:
1. Why would prolonged afterburner usage have any effect on the LPT of the engine?
2. Wouldn't engine EGT monitoring result in the fuel schedule being automatically reduced if EGT was being exceeded?

A fairly early and low operating hour blade creep situation leading to creep rupture (cracking) on a single crystal turbine blade would seeming indicate one is perched on the edge of the cliff with not much margin. Perhaps this blade had a not so perfect crystal orientation that degraded creep life or the stress levels of the blade are higher than thought to be. It is hard to say, but there has to be more to the story. The reduced hours prior to borescope inspection is a good thing.

TD

barit1

2nd Mar 2013, 02:16

Turbine D:

Good possibilities, good questions. And I agree the burner usage should not affect the LPT.

However, if the A/B fuel or nozzle schedules were not quite right, there could be a LP rotor rollback which might put the blade into an unwanted vibration region. I have observed this rollback on older engines.

If Pratt's control loops do not provide this protection - well, that's just speculation on my part.

SpazSinbad

2nd Mar 2013, 03:28

'Turbine D' perhaps you missed the link (not well highlighted by me on previous page): http://www.pprune.org/military-aircrew/508639-f35s-grounded-2.html#post7721369

F-35 Engine Issue Isolated: Fleet Cleared for Flight (http://www.aviationweek.com/Blogs.aspx?plckController=Blog&plckScript=blogScript&plckElementId=blogDest&plckBlogPage=BlogViewPost&plckPostId=Blog%3a27ec4a53-dcc8-42d0-bd3a-01329aef79a7Post%3a51f692b8-1fb5-43ad-b598-aee458f5559e) 01 Mar 2013 Amy Butler

"...The crack was found in AF-2, which has been used for testing the aircraft at the edge of its operational envelope. "The engine in question is part of the F-35 test aircraft fleet and had been operated for extended time in the high-temperature environment in its mission to expand the F-35 flight envelope," according to a statement from Lt. Cdr. Kyra Hawn, an F-35 spokeswoman. "Prolonged exposure to high levels of heat and other operational stressors on this specific engine were determined to be the cause of the crack."...

...AF-2 was used to test the new horizontal tail skin, and so it was run through many extra afterburner tests. Officials are sure to implement additional inspections and monitoring actions to understand more about the durability of these engines in extreme conditions...."

Turbine D

2nd Mar 2013, 16:08

Hi barit1,

I would agree with you, there are a lot of subtle things that cause A/B issues including the one you mentioned. During the 2009 time period in testing the F-35, they ran into an A/B screech problem where they couldn't get the engine to full power and high vibrations were a problem. The problem generally occurs at lower altitudes and high speeds. It's a problem you can't wring out in a test cell per se, but can be fixed changing fuel schedules & software and modifications to A/B hardware confirmed by flight testing.

SpazSinbad,

I did see and read the article you posted, but it isn't the full story. A fuller story is available here:

F-35 Cleared For Flight | Defense News | defensenews.com (http://www.defensenews.com/apps/pbcs.dll/article?AID=2013302280031)

Keep in mind the bigger picture here, cost overruns, delays in the testing program, the general cost of this aircraft relative to the various aircraft it is replacing and funding issues have generated much pressure on DoD management. So I don't believe we will see anything but very guarded statements tending to be as "positive" as they can be in newsprint articles. The customer (Lt. Gen. Christopher Bogdan) has stated his general unhappiness with both aircraft and engine at this point in time verses what was promised even although the flight status has been cleared to resume, once again.

TD

lomapaseo

2nd Mar 2013, 23:23

A 3rd stage turbine blade is pretty far back from the high heat zone. However it is susceptible to induced fatigue from aero sources. Multiple AB mis-lights are just one of many (wrong nozzle area etc.).

Once identified easily fixed

SpazSinbad

5th Mar 2013, 20:33

More info:

F-35 Fan Blade Crack Raises F-35 Durability Questions (http://www.aviationweek.com/Blogs.aspx?plckBlogId=Blog:27ec4a53-dcc8-42d0-bd3a-01329aef79a7&plckPostId=Blog%3A27ec4a53-dcc8-42d0-bd3a-01329aef79a7Post%3Aade3159d-afb9-41dd-a3bb-2c4a949858fd) Amy Butler 05 Mar 2013

"...The crack was found on a blade in AF-2’s engine. Bogdan says this was the “workhorse engine on the program,” which has been used for envelope expansion work and, thus, exposed to extremes in operation, including flights at 1.5 Mach; in low-level, supersonic conditions and at 7-8G. This aircraft was “doing the majority of the envelope expansion for the A-model fleet,” he says.

Two other aircraft in the conventional-takeoff-and-landing fleet, AF-3 and AF-6 remain grounded because they have been flown in similar – though not quite as rigorous – conditions.

Engine experts will require at least two more weeks to explore what implications there are for engine durability, Bogdan says. “What level of thermal stress would it take to get to that point on a normal airplane,” not exposed to such extremes, he says. “The issue is if it turns out that it is less than 100% of what we expect the life of the engine to be, then we have turbine blade having life limiting parts on it and we’ll have to deal with that.”

Should this become a problem, customers could be required to conduct additional inspections and/or additional maintenance over what was originally planned."

SpazSinbad

6th Mar 2013, 21:04

Engine crack that grounded F-35 traced to thermal creep 06 Mar 2013 Zach Rosenberg

Engine crack that grounded F-35 traced to thermal creep (http://www.flightglobal.com/news/articles/engine-crack-that-grounded-f-35-traced-to-thermal-creep-383136/)

"Bennett Croswell, president of Pratt & Whitney's military engine division, says a problem with F135 engines that grounded the Lockheed Martin F-35 is due to thermal creep, and is unlikely to affect the aircraft further as it returns to flight status.

The issue was a crack in a third-stage turbine blade on a single engine. As a precaution, the US military grounded all F-35 aircraft until a cause was discovered.

"During [an] inspection we found about 1/6-inch (4.2mm) crack on the turbine blade," says Croswell. "We felt we could continue to fly, and we took that recommendation to the (joint programme office), but on consultation with them we both came to the conclusion it was safer to suspend operations."

Thermal creep from high-temperature, high-intensity testing was found to be the cause of the crack. The engine the tenth engine built, powers the second F-35A, was tested extensively at supersonic speeds and low altitudes, generating significant more heat than expected.

"It was operating at levels 4X an operational mission, and 4X of levels we had qualified the engine for," says Croswell. "That was very good news, you don't want something like high-cycle fatigue or low-cycle fatigue." The issue is not expected to impact operational aircraft for months or years, depending on how the aircraft are flown...."