F-35B Bulkhead Cracks Found on Stress Test

[sandozer]

Looks like bulkheads will require beefing up after this report on the F-35B.

Lockheed F-35 for Marines Delayed as Test Exposes Cracks - Bloomberg (http://www.bloomberg.com/news/2014-02-21/lockheed-f-35-for-marines-delayed-as-test-exposes-cracks.html)

[Rhino power]

Ancient news... Cracks discovered on F-35B bulkheads - 10/11/2013 - Flight Global (http://www.flightglobal.com/news/articles/cracks-discovered-on-f-35b-bulkheads-391647/)

-RP

[sandozer]

You have a good heads up on the re-design RP. looks like Reuters just getting up to speed today.

UPDATE 1-F-35 cracks need redesign, Marines still on track for 2015 combat use | Reuters (http://www.reuters.com/article/2014/02/21/lockheed-fighter-idUSL2N0LQ1UP20140221)

[Rhino power]

The bulkhead cracking issues were actually known about as far back as 2011 (possibly further), must've been a slow news day at Reuters and they needed some filler... ;)

-RP

[Surplus]

must've been a slow news day at Reuters and they needed some filler...

Bit like the cracks then :}

[Rhino power]

Bit like the cracks then...

Made I larf did that...:)

-RP

[GreenKnight121]

Rhino - the article says this is a new cracking issue, not the same as that found in 2011: The cracking “is significant enough to warrant changes to the design” of the bulkhead, Jennifer Elzea (http://search.bloomberg.com/search?q=Jennifer%20Elzea&site=wnews&client=wnews&proxystylesheet=wnews&output=xml_no_dtd&ie=UTF-8&oe=UTF-8&filter=p&getfields=wnnis&sort=date:D:S:d1&partialfields=-wnnis:NOAVSYND&lr=-lang_ja), spokeswoman for the Pentagon test office, said in an e-mail. “This is a new defect that must now be addressed through a production change and a retrofit plan.” .


However, note the following:
1. "projected lifetime, which in the case of the Marines’ F-35B is 8,000 flying hours."
2. "The ground testing aircraft had accumulated 9,480 hours when testing was stopped"
3. "To provide an extra margin of assurance, the Marine, Air Force and Navy versions of the F-35 are all required to undergo tests for the equivalent of 16,000 flight hours."

So the test aircraft was past the currently-planned service life when the cracks developed.



In many ways, this is comparable to the F/A-18A/B/C/D fatigue issue - which required replacement of the fuselage center-barrel to prevent cracking.

Remember, the original Hornets had been designed for a service life of only 6,000 hours, but were reaching their "fatigue life" (based on stresses recorded by aircraft instrumentation) in an average of 4,800 hours.


In this case, though, the need for changing the center section has been identified long before any aircraft need it.

I am confident that a proper "fix" can be developed and implemented long before any flying aircraft start developing cracks.



Yes, the F-35B has only 202lb more leeway before its weight reaches the contractual "limit".

Contrary to some assertions, this is not the "max growth weight", but the "max start-of-service" weight. There will still be "growth allowance" left, as well as future allowable weight increases as engine power is increased over time.

[longer ron]

So the test aircraft was past the currently-planned service life when the cracks developed.

Possibly !
Or possibly when the cracks were found - which would be a slightly different story !

[Just This Once...]

GK, there is an error margin between actual fatigue and that experienced by the testbed, hence the higher target. The fact that the F/A-18 experienced real-world fatigue issues at an earlier point than the testbed suggested is testament to this additional margin. It would be overly optimistic to expect the F-35 to meet its projected lifetime without the planned rectification.

The presumed logic of increased thrust overcoming the weight gain is a bit shaky with the F-35B due to physical limitations of the lift fan.

[t43562]

More Cracks Found In F-35B's Second-Life Testing (http://www.aviationweek.com/Article.aspx?id=/article-xml/awx_02_21_2014_p0-666195.xml)

This suggests that this is *not* old news recycled.

[Rhino power]

Really not good news at all, if they are indeed 'new' bulkhead cracks...

Where's our resident ray of F-35 sunshine Spaz when we need him, to assure all is actually well... ;)

-RP

[NITRO104]

2. "The ground testing aircraft had accumulated 9,480 hours when testing was stopped"
This is barely over half the contracted life, or about 25years of service.
The problem is real and nothing to downplay.

[SpazSinbad]

For 'Nitro104':

Cracking Found in Marine Joint Strike Fighter Won’t Delay Program 24 Feb 2014 Dave Majumdar
"...
According to the joint statement: “9,480 durability test hours equate to more than 17 years of operational flying.”

The F-35 is required to have an airframe life of 8,000 flight hours—which must be verified via durability testing though two complete structural lifetimes...."
Cracking Found in Marine Joint Strike Fighter Won't Delay Program | USNI News (http://news.usni.org/2014/02/24/cracking-found-marine-joint-strike-fighter-wont-delay-program)

[NITRO104]

Thx.
I assumed 40 years of service life, but it appears JPO calculates with 30 years life span, which is particularly problematic at current cost rates and would bring the total fleet procurement in often cited 'next 40/50 years', to almost 200% of the current cost.

[PhilipG]

The cracks in the bulkhead (S) that have so far been found are in the lighter weight aluminium alloy that was developed to give a weight saving over the Titanium used in the A and C models.
I have not seen anyone taking about any new alloy to replace the one that has failed in a number of structures. I assume that if all the light weight aluminium was replaced by the original Titanium, that the plane would be overweight.
A bit of a quandary...

[glad rag]

Na they'll just scrap everything else to pay for a [relatively] minute fleet of wonder aircraft....

...still on the good side, we splurged a dose of Harriers to the yanks for peanuts, perhaps it is time for some payback with some A-10's coming our way, perhaps even for the AAC instead of those ridiculously expensive helicopter upgrades/replacements :ok:

[Engines]

PhilipG,

Sorry, no - the bulkhead that has cracked has always been aluminium. The titanium bits that were replaced were the keel beams, which were that way to meet the C model requirements for catapult launch and recovery. Those were changed out to new aluminium beams on the A and B.

Structures fail for all sorts of reasons, including selection of material, but for most combat aircraft, high strength aluminium is really the best choice. The main reason structures fail is due to failings in design, mainly failing to correctly model and predict load paths and load magnitude. Fatigue also plays a massive part, and here things get really complicated, depending on how you apply fatigue spectra. Clever people all around that stuff.

I don't for one moment want to suggest that 'all is well'. It's not and fixes will have to be found. My own feeling is that a rapidly executed weight reduction programme led to some structural design features that should have been better. But I could very well be wrong.

That said, the sky is not falling in here. It's perhaps best to remember that the F-35 team set themselves a very, very high bar for achieving a complex yet light weight structure. Nobody is perfect. The designers of the Tornado wing box found that out. So did the C-5A wing team. So did the F-22 aft fuselage team. So did the Buccaneer team. So did the Hornet team. So did the Comet team.

It's easy to criticise. It's a bit harder to do.

Best Regards as ever to all those doing the doing.

Engines

[PhilipG]

Engines,
Sorry if I got it wrong but the press article did say that the F35B bulkhead was one of those that is in Titanium on the A and C.
As a matter of interest do you know how much weight the aluminium saved? Also do you know if the metallurgical issues with the alloy is in the process if being solved, I say metallurgical as I assume the component is the same shape as the Titanium ones?
Philip

[longer ron]

It's easy to criticise.

very true :)


It's a bit harder to do.

Also very true...especially when you try to do it on a really bad design :)

[GreenKnight121]

However, exaggeration is REALLY easy to do, isn't it?

[longer ron]

Actually I was being Really polite about the thing ; )

[walter kennedy]

In the old Rhodesian air force, they measured the progress of the cracks in that big structure that held everything together in the elderly Canberras!

[Flight_Idle]

Walter, cracks can be strange things.


As a very junior airman, I was watching a guy doing ultrasonic testing on the dry bays of a large transport aircraft. The guy explained about the 'Standing wave' etc, then moved his probe over a 'Join in the metal' & carried on. I felt a bit embarrassed to point out that the 'Gap' was in fact a crack which led into the wing, covered by paint.


It got laughed off & I never heard any more about it, a crack it was though, strange **** happens.


There are times when the mark one eyeball actually works. Aluminium alloys may work well most of the time, but they can throw up suprises.