Pitch enhancement on 767

[stilton]

Not to belabour an earlier thread, but along similar lines I wonder why Boeing
has this PES on the 767 for stab trim back up control but not on the 75?

Any ideas?

[ICT_SLB]

If memory serves, it's because the Pitch control methods are different between the two aircraft. 767 has a slab all-flying horizontal stab while the 757 has conventional elevators.

[Elroy Jettson]

A moveable horizontal stabiliser and elevators on all the 76s I have flown. Sorry, no experience on 75s.

[ironbutt57]

As far as I can tell, the stab/elevator config is the same on both aircraft, the PES provides SLOW (1/4 speed) stab control in the event of left and center system failure, if center and right systems in the 757 are lost the stab remains where it was when the failure occurred...the major difference between the two aircraft is the 767 has no automatic trim when the autopilots are disconnected, while the 757 in fact has (mach/speed trim system) failure of this system grounds the 757..which would seem to refute the Boeing claim of no mach tuck tendencies...

[dusk2dawn]

stilton, apart from the cockpit layout the 75 and 76 are two very different birds.

[ironbutt57]

Yes, very very different in many ways...767 much nicer to fly, as it is a "clean sheet" design...

[stilton]

Thanks for the response IB57, I certainly agree the 767 is a far nicer aircraft to fly than the 75.

Perhaps with the higher weights on the 76 Boeing decided to provide more redundancy for control. in the event of the loss of the two hydraulic systems powering the stab

It would still appear to me that it would have been worthwhile to have the same redundancy on the 757 that the PES provides.