Helicomparitor,
Accepted - in that case make the probability of the second failure 1:1,000 and the probability of one followed by the second (from independent cause) will still be 1:100,000,000.
Crab,
Clearly you are conditioned by your military experience; those of us who left the military and then flew as civilians are conditioned more by that experience.
Examine your proposition; the engine failure has been contained and the helicopter flown back to an airfield. On arrival at the airfield, the helicopter is kept at altitude until an autorotation is initiated.
Now consider the probability of an engine failure (using Helicomparitor's probability and the minute spent on finals 1: 60 x 100,000,000) against that of successfully completing an autorotation with (up to) 19 passengers on board.
Add to that the knowledge that there are far more accidents associated with training for autorotation than the real thing and that most engine failures in flight leading to autorotation do not result in outcomes without damage and the risk assessment is now in perspective.
Jim