PJ2 great posts
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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
But was the risk of the loss of 4 generators actually 1 in 1,000,000 ^4 as you imply? This would be true if the systems where truly independent, but as this is a new failure mode it would appear to render that assumption incorrect. In fact, the 1 in 1,000,000 number that you quote is now totally meaningless. The risk in now what chance of 1) water leakage 2) failure of the water shield to the MEC?