"It's because if you lose Left and Center hydraulic systems, you lose Stab Trim (although u can get it from the right thru the PTU). "
Is this inability to use the Right A/P applicable to the 767 also, Nial? (As I understand it, the PTU can't be used in association with Autotrim).
My 767 engineering training notes don't specifically tell me what would happen to the Right A/P with the L&C Hydraulics depressurized, but, if I understand what I am reading, loss of Stab Trim (with the Right A/P engaged) would simply generate an A/P Caution message (with no yellow bars through the EADI APFD pitch mode annunication). The elevator would simply be made to work harder than normal.
Although A/P engagement requirements differ on the ground from in the air, yesterday, I was able to engage the R A/P on the ground without it tripping off. I used V/S and HDG HOLD as the active pitch and roll modes. When I commanded a climb, the elevator moved to (presumably) the required position, no flags/messages appeared and the A/P remained engaged (I can't even recall seeing an A/P Caution msg).
Thanks.
Rgds.
Q.