"Dick, the probability of 1 AC Bus dropping off is 1 in a million. The probability of 2 dropping off is a million squared. According to CASA, the probability of them all droping off is 3 in recorded aviation history. You cannot plan for every contingency, sometimes its just not your day..."
I don't know anything about flying or repairing aeroplanes but I do know about statistics. If I understand correctly what I am reading the generator control units are close together so failure via water is not an independent event for each one. For non-independent events you don't multiply probabilities. From what happened in this case I would suspect the probability of all four failing via water is the same as the probability of one failing. So it is not one in a giga-billions it is one in a million.
Mind you I never understand how people get failure numbers like one in a million. Do they mean that or do they just mean it is a rare event? Or maybe they are basing it on experience of how long these planes have flown without incident? In the latter case they are assuming the aging of the planes has no effect. News today is suggesting there are more cracks in water shields.
Thanks for all your posts -- makes interesting reading for a lurking passenger.
Michael