# Thread: "Six nines" reliability vs. safety factor

1. ## "Six nines" reliability vs. safety factor

Aircraft are supposed to have a 99.9999 % chance that they will work over some specified time interval.

If a picture weighs 25# and you want the industry wide safety factor of four, the fastener should pull out at 100 # or more. Personal safety calls for factors of at least 10.

Is there some way to correlate these two?

2. ## Re: "Six nines" reliability vs. safety factor

what is the relationship between aircraft and picture?

3. ## Re: "Six nines" reliability vs. safety factor

Not with the data given.

If you assume that the only relevent failure mode picture hanging is "fastener could not hold 25#", and you tell me that standard deviation of the maximum load, then I can corelate them. If we build picture fasteners to hold, on average, 100# but the SD around that spec is 33#, then they will not hold a 25# picture ~1% of the time, i.e. they will only be 99% reliable. If, on the other hand, the SD around that spec is just 11#, then only ~10^(-12) of them will fail, i.e. they will be 99.9999999999% reliable.

Of course, I doubt that the assumption that this is the only relevent failure mode is a good one in practice.

4. ## Re: "Six nines" reliability vs. safety factor

Mr. "I am", I think you have answered my question.
Thanks.
BTW, the SD of the pull out strength of fasteners is probably +/- a few pounds, depending on the wall material.

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