A330/A340 Stab Trim
Alright, this is possibly going off the technical deep end, but that's how I am (and this is my first post here...woo hoo).
Anyway, the Narita MD-11 crash has got me looking back over past accidents and in particular Sioux City and the total-controll-loss situations. Combined with the dicussion of the MD-11's limited pitch authority and stablizer size, that had me reminded of something I've always wondered about.
The Airbus A330 and A340 both have triple hydraulic systems. But both only have two systems power the horizontal stab trim -- and while there is a mechanically signalled backup, I believe it still relies on hydraulics to move the thing.
And since the RAT powered system is the one NOT connected to the stab, in an engines-out situation (or some other catestrophic failure mode) you could easily end up with no stab trim. By comparison, the 767 has a dual-powered stab but the PES PTU to let the third system drive the stab. Others have electric backups, I believe.
This seems lacking in redundancy. Not going anti-Bus here or anything, just thinking aloud!
Does the fuel trim system provide necessary backup? Or does the A330/A340 elevator system have enough authority (compared to stab trim) to compensate for an out-of-trim situation if necessary?
Thanks,
--Nick