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In page Safety engineering:

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Usually a failure in safety-certified systems is acceptable if, on average, less than one life per 109 hours of continuous operation is lost to failure.{as per FAA document AC 25.1309-1A} Most Western nuclear reactors, medical equipment, and commercial aircraft are certified to this level.[citation needed] The cost versus loss of lives has been considered appropriate at this level (by FAA for aircraft systems under Federal Aviation Regulations).[3][4][5]