Thanx again Nick,
I had long forgotten about 6 sigma for vibration fatigue and gust loads - i had better read up to refresh my memory! I remember now a conversation with a mathemetician friend of mine who explained that non-linearities in actual airframe structural deformation meant that 6 sigma was almost impossible to achieve in practice. However, this is not the forum or thread for that discussion.
One thought about rotorcraft powertrain: most engine failures can be attributed to either wear (eg cambelt, valve and seal) or material degredation (eg turbine creep, conrod failure, or head deformation). One interesting point is that brushless DC electric motors (like the ones in the french TGV train) and batteries do not suffer catastrophic failures, but lose performance with time as the electronics degrade. To achieve the speculated FAA 1E-9 failure requirement, an electric emergency drivetrain might be suitable.
This may well be better suited to a fly by wire approach to design, where triplex or even quadruplex systems achieve the failure requirements. While i am not a fan
of unecessary high disk loading in a design, it is interesting to speculate on what solutions might be offered for this problem.
Mart