To: Jet Noise
Under normal conditions,unrelated failures are not considered, with certain exceptions. If you were performing a Safety Hazards Analysis on the hydraulic system you could factor in a failure of a supporting system such as the electrical system. Since the electrical system is very important to the entire aircraft its’ failure would be factored as a single input to an AND gate within the hydraulic system diagram and the frequency of all failures that went into that particular AND gate would be calculated. In any case the electrical system and the hydraulic system would be the subject of their own Hazards Analysis and in most cases total system loss could not occur more frequently than 1 10-9 for the entire fleet of that particular aircraft.
The fallacy about this type of analysis is that the systems are treated separately and not at the aircraft level. To do so would prove that at the aircraft level the probability of a single point failure causing loss of the aircraft would occur at a frequency well below 1 10-9 flight hours.
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The Cat