Proving hazardous relative motion <1E-9
Proving hazardous relative motion <1E-9
(OP)
Hi All,
This concerns relative motion between an aircraft system and the local structure, during a crash, which could cause a hazard.
This requires an explicit demonstration by a combination of test, analysis and experience that the probability of the hazard occurring is < 1E-9. I will therefore need to attribute a probability to the chance that there is hazardous relative motion.
Any ideas how a test could provide that level of confidence? A simple drop test using critical loading conditions? Or apply a factor on the loading to take account of scatter, providing a confidence level?
Alternatively, is there a suitable analysis method?
Thank you
This concerns relative motion between an aircraft system and the local structure, during a crash, which could cause a hazard.
This requires an explicit demonstration by a combination of test, analysis and experience that the probability of the hazard occurring is < 1E-9. I will therefore need to attribute a probability to the chance that there is hazardous relative motion.
Any ideas how a test could provide that level of confidence? A simple drop test using critical loading conditions? Or apply a factor on the loading to take account of scatter, providing a confidence level?
Alternatively, is there a suitable analysis method?
Thank you





RE: Proving hazardous relative motion <1E-9
What could cause the relative motion? Would this be your structure tearing loose during an otherwise survivable crash?
Probabilities sound like a challenge. I am aware of all sorts of yield and ultimate stresses, but I do not have Gaussian curves of them, showing me σ and all that. I would also need Gaussian crash decelerations, in all possible directions.
If you not trying desperately to save every last gram of mass, why not just pick a survivable deceleration and a fairly generous safety factor.
--
JHG
RE: Proving hazardous relative motion <1E-9
"...during a crash, which could cause a hazard."
I've seen information that states that there's a maximum g-load assumption that's considered survivable. Beyond that point, there's no concern about hazards because it won't bother anyone anyway.
There should be a formal requirement written down which states what the maximum g-loads in various conditions and directions. Then you can work from the one most-applicable g-loading number.
Maybe. Wait for others to confirm. Not my area.
RE: Proving hazardous relative motion <1E-9
http://www.assakkaf.com/Papers/Journals/Uncertaint...
http://www.wseas.us/e-library/conferences/2012/Kos...
Your requirement seems ludicrous, on the face of it, given that whatever crash shock value you use is probabilistic uncertain already. Likewise, the crash duration is rather arbitrary as well. Crashing into a mountain is different than crashing in a forest, or crashing into the ocean.
In any case, your 10^-9 probability requires at least 6-σ above mean
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers
RE: Proving hazardous relative motion <1E-9
Exactly what specification, regulation, etc are You talking about?
FAR? MIL-STD? JAR?
Regards, Wil Taylor
o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]
RE: Proving hazardous relative motion <1E-9
I will take survivable crash load factors from the FAR.
The difficult bit is to argue a probability of failure based on (maybe) crash load distribution and material allowables. Clearly the published ultimate crash load factors already contain a safety factor (1.5?) but I'd need more info on how this was derived to build a case that I achieve 1E-9 overall. I dimly remember that 1.5 is supposed to broadly relate to 1E-3 or 3sigma, so that could be a start.
@IRStuff - I couldn't find any more on the crash shock specification.
Regarding the next step, we plan to use a drop test to show that the relative movement of structure and systems in a particular area - following application of crash loading - does not exceed a given level.
Any ideas on an appropriate test factor to use? ie, to help argue (to 1E-9) that this level of movement is not exceeded?
RE: Proving hazardous relative motion <1E-9
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers
RE: Proving hazardous relative motion <1E-9
RE: Proving hazardous relative motion <1E-9
I had to review a 'telephone book thick' structural analysis report, and they didn't incorporate probabilities into each calculation. They simply chooses the worst case at every step of the calculation. This stacks up additional safety factors along the way. At the end, they show that the structural failure in question can't happen, at all, in the case studied.
If the failure ever actually did happen, then it would be due to some other external factor. Not because of some statistical variation.
RE: Proving hazardous relative motion <1E-9
You may be making your problem more difficult than it needs to be. You are being too vague for us to help you.
If you refer to "FAR" requirements you may be talking about reliability requirements such as FAR 25.1309 which have NOTHING to do with structural requirements.
If you mean dynamic loading such as seat structures or maybe landing gear impact, you are dealing with energy absorption, not reliability or statistics.
Before embarking on any new design, it is crucial to understand the requirements to be met, otherwise a lot of time and effort will be wasted proving things that don't matter.
Since there is nothing secret about the Federal Aviation Regulations (all published online) or the guidance documents which support the testing and analysis needed to show compliance with these regulations (also available online), why not be more specific and tell us what you're doing, and what regulation you need to satisfy?
STF
RE: Proving hazardous relative motion <1E-9
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers
RE: Proving hazardous relative motion <1E-9
The only thing I can remember about moving bits and stationary bits, like flight controls passing through structure, where there is a nominal separation and you need to consider the effect of structural deformation (though i'd've thought that was for limit loads).
another day in paradise, or is paradise one day closer ?
RE: Proving hazardous relative motion <1E-9
Start with Wiki: "AC 25.1309–1 is an FAA Advisory Circular (AC) (Subject: System Design and Analysis) that describes acceptable means for showing compliance with the airworthiness requirements of § 25.1309 of the Federal Aviation Regulations..."
The actual ref (PDF): FAA AC 25.1309-1.PDF. Middle of Page 5.
As far as I know, one doesn't drag the probabilities along into structural calculations. It's Newtonian physics, not quantum mechanics.
RE: Proving hazardous relative motion <1E-9
another day in paradise, or is paradise one day closer ?
RE: Proving hazardous relative motion <1E-9
This would therefore mean that the equipment in question must simply be designed with normal safety margins against a normal crash shock scenario, so that it's unlikely to break loose during a crash that might be otherwise survivable. The rationale is to ensure that a survivor of the crash itself doesn't get killed by equipment that's broken loose.
This is why we're told to have our stuff either in overhead compartments or under our seats in a plane. But, the constraints on our carry-ons are fairly loose, so they simply want them to be unlikely to kill someone that might have survived the crash.
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers
RE: Proving hazardous relative motion <1E-9
I think it's fun to guess at the OP's intentions, too, but until the OP clarifies his/her problem we'll still just be guessing.
STF
RE: Proving hazardous relative motion <1E-9
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Proving hazardous relative motion <1E-9
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers
RE: Proving hazardous relative motion <1E-9
What is Engineering anyway: FAQ1088-1484: In layman terms, what is "engineering"?
RE: Proving hazardous relative motion <1E-9
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers