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Thanks Grunf
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When Douglas did the load test on the C17 .the wing failed a bit short of the ultimate load limit. I do not remember the excact number but after a few weeks the company decleared that the wing had actually passed and the test was wrong
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and do let us remember the wing was tested to destruction ...... it failed at a stress level 45% greater than anything it will endure under all operating conditions ...... That's why the Airbus Engineers are not jumping off high buildings ... :)
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Thank you for explaining the latest details Grunf. I watched Boeing build the first 767 and haven't been involved in a new aircraft program since.
When watching what happens on one of these tests, the wing doesn't "break off" some part of it fails and the wing starts flexing again, relieveing the load. All you hear is a loud bang, then get out the calculator and find whats busted and where. I wouldn't know the changes Airbus will need to make to fix this, but it could be as simple as adding a few pounds by making a component a fraction of a millimeter thicker, or changing a simple design detail like moving a hole or changing a radius. It is even possible that the strength may be improved by removing material if the failure was caused by an unsuspected stress concentration, as was necessary on the old F27 landing gear. I guess i'm showing my age...... |
Hello.
Sunfish, hats down to you. I always respect the "old" guard. Where would we be without you guys ? As for the possible changes I don't think it would go that far as adding or removing something. Updating FEM would do the math. I would be surprised to see the weight penalization. But still all this is hypothetical since we do not know what was the failure and under which condition. I do not think that Airbus will release that info. Cheers |
Just to add - if the guys flying the 380 South of Toulouse yesterday are reading this (perhaps someone can pass it on): The big beast sure looked impressive, especially with the bank wound up to what looked like at least 60 degrees...I had to stop the car to admire the "show", keep up the good work.
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a380 wing problem
hello grunf,
quote from grunf:" last fail safe model was 747". grunf, in that case, well... , give me a 747 forever! |
Blackmail,
well yes structural analysis advanced since the late 60s (747) so nowadays in application is damage tolerant structure. How good it is? Well, the aircraft are lighter, definitely, since you do not need to put all the redundant load path (additional structure that will carry the load if something fails). On the other hand it is so hard to do a good prediction with damage tolerant structure on when and where it is going to fail so it can be prevented. If you don't have a good MRO it can go bad, really bad. Anyway, inspection intervals for structure are set in a conservative manner in order to cover for all eventualities. Still...:hmm: This methodology was always prone to different interpretations. Let's hope that 380 will get out of flight test without big problems... |
Strange enough nobody ever said what the basis for that test is, and therefore what the 150% represent.
Considering that Airbus has a freighter variant on the line ans some people have been talking about a stretch one could actually consider that the tested load limit was based on a highter weight than the 'standard' aircraft has. |
Hunter58,
you might be right. However since freighter version usually has different floor structure plus a completely different loading it is possible they will certify it later as a derivative. One way would be to follow what you have said (for stretch it is the same thing). They do talk a lot about that which is unusual. Again, they do not share what the real reasons are so at the end, I believe, they will certify the aircraft without problems. |
Originally Posted by Grunf
TightSlot:
It does go more or less along the lines I have already posted. You do get a result based on the initial set of assumptions. If you do not have the good result you change the assumptions. One thing you can change is your model (FEM = finite element model - the "wire" model that in specialized software (FE) simulates the structure exposed to conditions it will see in flight). The closer are your simulated results to tests the better it is. If you are close on the lower side that gives you few options: 1) Change the model so it can better represent the real conditions 2) Declare a weight penalty (ouch!) and pay the price 3) Reinforce the failed part of the structure and redo the test (tooooooo expensive) Also a certain level of "convincing" and political approach is always present. All this rules can be interpreted in different way. What I quoted before from CFAR 25 is just a small portion of all that has to be satisfied in order for an aircraft to get its CofA (certificate of airworthiness). it is up to a local CAA (regulatory body) to make that decision. Therefore, it is not a straight yes or no answer.
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ElectroVlasicu,
It is OK what you suggest but it is costly. Namely item 2 (replacing the part that broke) is acceptable only if it is the SAME part (same geometry and specifications i.e. materials). One more comment, of a general nature: There is no wing braking before the whole certification testing. If and when the test is finalized and all certification data completed and accepted by a local CAA, then it is possible to go through with wing braking. This is not a part of required ultimate loads test. So, Vlasicu, your suggestion is already somewhere in between the options 1, 2 and 3. Cheers |
As a stress engineer who has performed lots of FEA, most of which has been in support of certification and some of which has been in support of failed tested items, I can confirm that ElectroVlasic's option 4 is basically how it works.
If you take the example of the wing, if you can prove the load path and boundary conditions of the area surrounding the failed component (for example by test data from strain gauges and the like) then you can generate a finite element model of another component of whatever design, and providing the interfaces are the same apply the known loads and boundary conditions and bob's your uncle. The certifying authorities need to be convinced that your models are correct, and if they are then no further physical testing is required. |
Originally Posted by ElectroVlasic
I thought option 4) was
I also have lots of experience in certification and failed test items and would offer the more likely path as:
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