757/767 Mach Trim
 

I can't find any reference to Mach Trim in the Boeing manuals or QRH. Presumably it has one and is totally automatic. That suggests that it can also go wrong. Would anybody be able to enlighten me to how it all works.
Thanks in anticipation - SFD:ok:

757 mach trim
 

In my manual, you will find it under Flt Controls (Sect 9 ?). There is a small mention under "Automatic Trim"

A search finds the following discussion:

Mach Trim

Thank you all. I was searching the flight deck on a 767 and couldn't find any reference to it. I'll check out the 757 on Sunday. I'll also learn how to use the search function to demonstrate my initiative.

Regards
SFD

mach trim is simply a speed stability function so the airplane cannot run away with itself - ie worst case is mach tuck. If you trim out your airplane and add thrust - you can see the trim change automatically . , I think its active all the time ..

Mach trim is a sub function of auto stab trim. All it does is tries to keep the elevators fared by moving the stab to nullify the input. It will do this if the elevators have been displaced from the fared condition by a constant amount for a pre determined amount of time - fifteen seconds in the case of the 747, ten in the case of the 767 ( I think ). The time delay is intended to prevent the auto stab trim gizmos chasing the autopilot, which could lead to porpoising.

The mechanics and signalling are different between the all of the Boeings, but the concept is fundamentally the same.

BTW, a Boeing SRP ( Service Related Problem ) regarding a stab trim seizure on a 767 has been classified as non safety, as in their opinion the aircraft can still be flown to a safe landing with the stab fully displaced one way or the other.

Wouldn't like to put that to the test myself.

757 certainly has Mach trim, I recall this from details emerging from the investigation into the AeroPeru accident, where the Pitot tubes and static vents had been covered during washing? and not uncovered. One of the many error messages received was that of mach trim, albeit erroneously. I have no idea rethe functionality, ie auto fx or manual though, would be interested to know if anyone could enlighten.

Regards

This is straight from 767 airframe training books for ground engineers.


General Description
The mach trim mode controls the stabilizer trim during flight when
no autopilot is selected (no FCC engaged) and no other stabilizer
trim command exists. The controlling SAM commands the stabilizer
trim as a function of change in mach number to enhance the
longitudinal stability of the airplane. A speed increase results in an
airplane nose up trim while a speed decrease results in an airplane
nose down trim.
NOTE:
When not engaged, the mach trim control law is referenced
to stabilizer position. After engagement, the control law
commands the stabilizer trim system to reposition the
stabilizer in response to variations in Mc (computed mach).
Operation
Both SAM’s contain the mach trim circuits and receive mach data
from the two ADC’s.
After initialization the controlling SAM automatically engages the
mach trim mode and commands the stabilizer to move at half rate of
speed when:
• The airplane is in the air (20 sec delay).
• No manual electric trim switches input is being made.
• No alternate electric trim switches input is being made.
• No FCC is engaged (no autotrim)
• Flaps and slats are retracted.
• The airplane speed changes.
• No SAM fault is present.
The mach trim schedule provides for a greater stabilizer trim
correction as the mach number increases.
Movement of the elevator control column in an opposite direction to
the airplane longitudinal trim causes the elevator control column
cutoff switches to stop the mach trim mode.
Maintenance Practices
A failure of the controlling SAM to perform in mach trim mode
results in the other SAM taking over control. Depending upon the
cause of the failure, faultballs are set on the failed SAM. Failure of
both SAM ’s to perform the mach trim function is not annunciated.
Mc (mach) data which is identified as failed by the ARINC 429
status matrix is discarded and the previous valid value is used. If
four or more invalid values of a parameter are received within eight
successive samples, the failure is set as an ADC fault.
27.41 − HORIZONTAL STABILIZER − STABILIZER TRIM OPERATION B767 − ELECTRICAL / INSTRUMENT BOOK 11 − Page 58
M00086A8.PST
MACH TRIM MODE

Thanks everybody. That is Top Banana. I was surprised at how many different interpretations about 75/767 Mach trim exist within the pilot work force where I am employed. Even the 3rd party Tech Refresher instructor had a different opinion (subsequently proven totally wrong).

While there is a 757 / 767 topic up I hope this question is not off topic too much, I am curious is anyone can confirm how the duel type ratting actually works for pilots on these types. If you do a 767-300 type ratting, do you also get tested on the 757? Would there be any questions / relating to a 767-200 ?

Thanks
cf
:ok:

connection fee - you get both types on your licence when you do the course for either, but you are not qualified to fly the other. To do that, you need to do a differences course and a sim check. Thereafter, you do alternative Base Checks (as they used to be called) or LPC/OPC's, this time round you'd do 757, in 6 months' time your sim check would be 767, and a year from now 757 again.

Also, how does it work in regard to different engine configurations. I see on one web site there is some half a dozen or so options of engines available. Does different engines mean there are different speeds and emergency or normal procedures to learn? Do limiting temps/press vary then?

Thanks.

757/767 rating & continuing qual
 

The training program I went through used both the 757 and 767 sims. For continuity, there was a grouping of three or four 757 sims, then the next grouping would move to the 767.

Ground school centered around the 757, then talked about differences for the 767.

The IOE included both types. When I go to Continuing Qual training, I never know which type they will give me until the training is posted on my schedule. I have had the 757 for training only once in the last 5 years.

On the line, the 767 is the equipment I fly 99% of the time. I see the 757 about once every 6 months, and find it not to be any problem.

For the fleet, I see 4 different varieties of engines. Neither presents a problem, but I prefer the FADEC types.

The company has a section of the VOL I dedicated to differences between the different aircraft. It's a great review summary.