Autopilot Mach Hold Questions

From: (Al McFarland)
Organization: The MITRE Corporation
Date:         18 Aug 96 20:13:39 
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Anyone here familiar with the behavior of the Mach hold channel of the
autopilots on in-service air transports?  I presume the crew set the
target Mach speed and then the Mach hold channel maintains Mach number
within a certain tolerance above or below this target.  Are the operators
able to adjust the tolerance bands?  Is there a dead band within which no
power adjustments are made?  What kind of control law is used in that
channel?  For normal cruise practice, how much excursion in Mach number is
experienced around the target Mach number?  Assuming that the excursions
are periodic with an interval of time above the target and an interval
below, what would typical periods be?  Anyone know of accessible
references that would address these questions?