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