ATPL(A) Systems Q:

[Bort Simpson]

I was wondering if anyone out there in cyberland could assist me with one unresolved question I have regarding the B767 stabiliser/elevator/pitch trim.

I was asked a question in my exam about how the pitch trim is operated in a 76 when the autopilot is operating it. I have looked through both sets of systems books I have, and through an extract version of the 76 manual and none really provides what method the system uses to actuate the trim (when operated by the AP). They do however, cover the pilot operating side of things (i.e. electric trim switchs, and the stab trim levers that mechanically activates the system). What part do hydraulics play? Is the stab trim adjusted by electiral servo motors?

The reason I ask is that I missed out on the pass mark last time by a smidgeon, so I am trying to be more thorough this time around. Any insight would be appreciated.

Thanks in advance,
Bortman

[gaunty]

This might be a start, it's B737 but I suspect the B767 maybe the same ??

http://www.b737.org.uk/flightcontrols.htm

Have a google around for it.:ok:

[Bort Simpson]

Thanks Gaunty,
I had a look at the link. With the info there and what I have previously read, it appears that the autopilot application to the Stab trim uses electrically powered servo motors. It doesn't really say how the stabiliser is then moved, but I guess (stab in the dark!) that these servomotors would not be there to power a hydraulic actuation. (hydraulic pressure is already available in the aircraft and probably doesn't need a new local source). Rather, they would more likely be used to power a mechanical actuation. Such as a nut and bolt type system where the bolt screws through the actuation arm, to in turn move the stabiliser.

So it seems like, the autopliot control of the stab trim (to my thinking anyway) is an electronic/mechanical actuation. Assuming that the 73 and 76 systems are similar in this instance.

If anyone else has anything else to add, please feel free to.

Bort.

[whaet]

hey mate
try this one out:

http://www.smartcockpit.com/plane/boeing/B767/

will probs be on there somewhere...

whaet

[Bort Simpson]

thanks for the link Whaet.
The stab trim is clearly indicated there to use hydraulics and there is no mention that the autopilot control of the stabiliser trim doesn't use the hydraulics as well. Looking at the use of the PES (pitch enhancement system) in this system (fm the R hydraulic sys), it adds to my thinking that the electric and autopilot control of the stab trim is indeed electir/hydraulic and the stab trim levers are mechanical/hydraulic.

maybe????.:confused:
I seem to remember when I got this question when I attempted the exam back in Dec that I put that the AP control of the stab/trim was electric/hydraulic. My memory may be playing tricks here.

Thanks for the input thus far guys.
B.

[mustafagander]

B767 stab trim is hydraulic powered and electrically controlled. There are 2 stab trim control modules. L is powered by L hyd system (is the R hyd PTU still operational?) and R is powered by C hyd system.

Electric trim (control wheel switches) uses both control modules (as does manual trim) and automatic (A/P driven) trim uses either L or R module - only 1 at a time.

BTW I think the PES is the PTU using L system return fluid.

Early a/c used cables to open the appropriate valves to operate stab trim while later a/c have electric actuation. The "handles" are manual and the "switches" are electric.

[Bort Simpson]

thanks for your help an info there mustafagander.

what you've written is consistent (but much clearer) with what's in the study books and manuals I have read.

PES, PTU. Yeah sounds like this is the same thing but different name. The diagrams I've looked at have the PES using the L system fluid too.

Bortman:)