Sunfish gives an excellent intro to FMEA for non-experts. lomapaseo then goes deeper into it
Originally Posted by lomapaseo
I doubt that Boeing assumed a level of "catastrophe" for this error in their FMEA and under what FAR would they perform this type of analysis for such an error....... ?
Good question. Let me say a little more about FMEA.
An FMEA requires you to identify
* everything that can go wrong (all ways in which things can fail, or "failure modes")
* what the possible causal consequences of such a failure can be (and one failure mode might have many different possible consequences)
but it doesn't stop there. You then
* classify the possible consequences into four or five categories. These categories include one typically called "catastrophic" (the worst: say, leading to loss of the airplane) and one called "minor" (the least bad).
and then
* perform a risk analysis: how often are the failure modes likely to appear; given that a failure mode has manifested, how likely is it that a certain effect (from the many that may follow this failure) is exhibited?
This last is not strictly part of an FMEA, but inevitably follows.
There are then requirements that say how often a particular effect is allowed to appear. For "catastrophic" that would be "not expected within the service life of the fleet". An applicant for an airworthiness certificate must show that the failures in their respective categories satisfy these probability requirements. See LLoyd and Tye, Systematic Safety, CAA Publications 1982, for the most concise explanation of categories, classification and risk classification.
So lomapaseo's suggestion requires some refinement. The bolt separating itself from the assembly would likely be a failure mode in the FMEA. That itself would not be assigned a severity category, I don't think, because it is a failure mode, not an effect. The question is then what effects it might engender. And at least one of them turns out (we now know) to be catastrophic. The question would be whether this effect was foreseen in the FMEA.
PBL