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Brenoch
22nd Aug 2007, 21:43
Haven't found an answer to this in any books available and I haven't been able to figure out a good way to test it in the sim so here goes;
During a triple channel approach does the autopilot trim the stabilizer at full rate or at half?
Looking at schematics it would suggest that the right autopilot would trim the stabilizer through the right trim motor and left/centre would use the trim motor powered by the centre hydraulic system. (Valid for 757, 767 is obviously L and C hydraulic but the idea is still the same)
I do realise these schematics are overly simplified in the pilots manual but it looks much the same in the AMM.
Cheers

None
23rd Aug 2007, 01:35
There are three modes of stabilizer trim for the 757/767;
Electric - via dual pitch trim switches on control wheel
Alternate - via alternate switches on control stand
Automatic - via autopilot. In this case, only one stabilizer trim contol module is used, and trims at one-half the electric or alternate trim rates.

Spanner Turner
23rd Aug 2007, 13:47
All Autopilot Trim is half-rate for 757/767

That is generally true but it "can" operate at full-rate when a multi-channel autoland is commenced!

Below from the Boeing 767 engineers training notes.

Autoflight Trim
FCC auto stabilizer trim signals are sent to one SAM. The control
and arm outputs that result use the same path as described under
manual electric trim.
If the pilot or FCC is not making a trim input, mach trim circuits in
the SAM will control trim operation using ADC inputs.
Trim rate in these modes is half of maximum as only one STCM is
controlling one hydraulic motor/brake combination.
Autoland mode with multi-channel engage of the autopilot can
operate at dual motor rate if required.

The condition that it "CAN" operate at full-rate is described below.


Multiple Channel FCC
During multiple channel operation, two or three FCC’s are engaged.
The FCC (first engaged) provides trim commands to its SAM,
immediately, when the elevator is deflected out of its neutral
position. The stabilizer moves at half rate speed.
If a failure occurs on the 767-300 that results in autoland 3 (three
FCC’s engaged) defaulting to autoland 2 (two FCC’s engaged), the
FCC not controlling trim engages the SAM not previously
performing the auto trim function. The two FCC’s command the two
SAM’s to trim simultaneously resulting in the stabilizer moving at
full rate (0.5°/sec). This full speed trim permits quick retrimming of
the airplane for go-around during a missed approach.

Brenoch
23rd Aug 2007, 21:27
Thank you ever so much Spanner Turner,
That's what I suspected and always thought. I was deep in the wiring diagrams yesterday trying to figure it out but I could not grasp when the A/P would infact trim at full rate.
The reply previous to yours was obviously out of a pilot's manual and they never tell you the truth.
I owe you a beer (or beverage of you choice, champagne not included)...
Cheers :D

Spanner Turner
24th Aug 2007, 09:53
I owe you a beer (or beverage of you choice, champagne not included)...



Brenoch,
No problems mate. If you're ever Down Under I'll gladly accept the kind offer of a cold brown beverage. (I'm yet to meet a Gingerbeer who would select champagne as a drink of choice)


The reply previous to yours was obviously out of a pilot's manual and they never tell you the truth.



Can't have you "Drivers" having ready access to all our secrets can we!
Next thing you'll want to know the location of "The Golden Rivet"
Our engineers training notes in hardcopy form take up about 3-4 ft of shelf space, so a little more in depth than your stuff ever could be. (and that's just the mechanical stuff - add the same again for electrical/instrument/radio)

A caveat to my answer above - it applies to the 767. I've never worked or been trained on the 757, so there may well be differences. If you only knew the differences under the skin between a 767-200 and a 767-300 you wouldn't believe it, so there's every chance that the 757 could be a whole different animal.

:ok:

None
24th Aug 2007, 17:49
I've learned something new.

Thanks.

Jumbo Driver
24th Aug 2007, 20:51
Next thing you'll want to know the location of "The Golden Rivet"

As I remember, it could be found down in the F7 bay of the 1101/1151 ...


JD
:)

Piper19
26th Aug 2007, 15:16
Looks like the 757 and 767 differ in multichannel operation. From the 757 engineer trng notes:
One motor operating: autotrim, mach and speed trim, all other modes after hyd failure.
Two motor operating: manual and alternate elec trim.

"Autotrim commands are generated from the FCC first in CMD and processed through its associated SAM. Only 1 autotrim channel (1 SAM and 1 FCC) is enabled at a time, even when operating multichannel.
On a multichannel landing: trim fault in one FCC sequences the trim path to another FCC and SAM, and the "NO LAND 3" message is displayed only after touchdown. A loss of both trim paths activates a "NO AUTOLAND" alerrt." This is the mean difference with a single channel landing.

Brenoch
28th Aug 2007, 01:28
Spanner Turner;
I have about 3-4 ft of shelf space spare since the missus left.. :}
Did a bit of training down there for Air Tonga donkers years ago but I look forward to my return to downunder, if not for a good game of whatever as long as I get to have a few VB's... ;)

gimmesumvalium
31st Aug 2007, 06:05
Spanner Turner,
Thanks for the response re. Auto-trim.
Could I prevail upon you to provide the Auto-brake parameters for the B767-300 similar to your response regarding the B747-400 in another thread.

Also, I would like to offer my services as Marketing Manager re. your technical information...

I can come up with a business plan for this...
GSV

Spanner Turner
1st Sep 2007, 10:37
Spanner Turner,
Thanks for the response re. Auto-trim.
Could I prevail upon you to provide the Auto-brake parameters for the B767-300 similar to your response regarding the B747-400 in another thread.



Look here for the figures you seek!!

http://www.pprune.org/forums/showthread.php?t=290322