B767 B757 Mach Trimmer

[BANANASBANANAS]

The 757 has a mach trimmer but the 767 does not. Can anyone explain why the 767 either does not need one or, what it has instead?

Thanks.

[woderick]

What makes you think that the 767 does not have Mach Trim ?
It does !

[Capt Fathom]

If the 767 has a mach trimmer, it is well hidden.

[woderick]

From the 767 AMM - cut and paste so the Boeing logo missing but .......


MAINTENANCE MANUAL
MACH TRIM SYSTEM - DESCRIPTION AND OPERATION ____________________________________________
1. General (Fig. 1) _______
A. The Mach trim mode of the stabilizer trim system provides incremental
horizontal stabilizer position in response to changes of Mach. The mode
is only engaged when airborne (after a 20 second delay) and the manual
and autotrim functions are not engaged.
B. This chapter contains Mach trim stability operation. Refer to
22-22-00/001 for a general description of the Automatic Stabilizer Trim
System. Refer to 27-41-00/001 for the component details and system
operation of the Horizontal Stabilizer Trim Control System.

[Permenant Standby]

Just checked my flight crew manuals...it appears that the 767 doesnt have one, and does not have any indications of it failing. However they dont tell us lots of things about it these days...the manuals get smaller by the minute!

Regs

PS

[atyourcervix73]

As far as Im aware both 757/767 have mach trim systems..(Im current on both)...the 757/767 also exibit neutral mach trim throughout the normal flight envelope (straight out of my CBT that was kindly provided many moons ago) I havent been able to find any reference to fail mode announciations, but I would assume EICAS warnings or cautions.

[BANANASBANANAS]

Thanks for all the replies chaps. Time to eat some humble pie. Woderick is absowutely wight!

Thanks to an incredibly helpful and knowledgeable engineer in DXB who dispatched BI092 this morning I now have 9 photocopied pages from the maintenance manual and am a little bit wiser.

All replies much appreciated.

Bananas

[Capt Fathom]

Interesting! In a past life, I flew 767's for over 15 years and have never heard of a mach trimmer. Even more bazaar is that it never failed! Which leads me to believe it is not installed on the ones I have flown!
There is no reference in the Ops Manual, Limitations, QRH, FCTM or DDG.
Is it a option or are they installed on all 767's ?

[Wodrick]

It's not a Mach Trimmer as such i.e. a discrete system, it's the Mach Trim MODE of the pitch trim system.
I guess you have been lucky with no failures, although I don't recall a failure on our fleet.
The EICAS is "MACH/SPD TRIM"

[NSEU]

Is it a option or are they installed on all 767's ?

All 767's. The Mach trimming system is found in the SAM's.

And I'm not surprised it doesn't fail. The system is solid state and I'm sure it's rarely used. The mach trim mode is only active with the flaps and slats up (when the A/P is not engaged and when you're not using manual trim).

Rgds.
NSEU.

[mondriver]

Well...not sure if it's the same on the 767...(and I would guess that it was), but the 757 "Mach/Spd Trim" works using IAS when flaps/slats are extended, and Mach No. when they are retracted.

In both cases, as NSEU stated above, the Mach/Spd trimmer is only "trimming" (if it needs to) when you are manually flying (autopilot not engaged) and you are not electric trimming...!!

I'm off to sit in a dark room...

[sideshowbob]

I have had a mach/spd trim EICAS message on a 757, so they do fail ocassionally.

[Blacksheep]

Two mach trim questions in one day, how unusual.

There are lots of hidden control functions in your aircraft these days. Don't even ask about neutral shift sensing and compensation...

[oceancrosser]

The thing is that the B757 has a Mach/Speed trim system, the B767, at least according to any books I have as an instructor on the twin sisters doesn´t. At least it definitely does not have a MACH/SPD TRIM EICAS Warning, which comes on every now and then on the B757.
A quick fix is to disconnect the autopilot and toggle the Stablizere trim thumb switches or the ALTN STAB TRIM SWITCH/HANDLE and it usually cures it instantly.
No further need to bother maintenance.

Cheers.

[contact-landing]

okay , Mach/SPD trim ; Mach is used by the system when flaps and slats are up, SPD when they are not up.
But why does it need SPD info here with flaps and slats extended ? A "Mach/SPD trimmer is a device used to avoid tuck-under at HIGH speeds I thought, no need for it at low speeds???

[Old Smokey]

I think a bit of talking in circles here, a Mach Trim system is usually a 'Stand Alone' system, typically achieving it's goal through Elevator movement.

woderick's input contained the vital clue as to how later generation aircraft compensate for Mach Tuck - The Mach trim mode of the stabilizer trim system provides incremental horizontal stabilizer position in response to changes of Mach.

Not a Mach Trim system per se, but a MODE provided by incremental stabiliser movement as directed by the AFCS.

I cannot speak for the B757 (sadly), but the Mach Trim MODE (not system) is very much in evidence on the B767 and B777.

Regards,

Old Smokey