Trim Activated by the Autopilot

Date:         17 Dec 99 01:57:37 
From:         "John M. Hunt" <johnmhunt@ipa.net>
Organization: Internet Partners of America
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In glancing over some of the old Flight Safety Foundation accident
reports, as well as Macarthur Job's three books, I have noted a number
of disasters which were at least partly caused by autopilot actuated
extreme trim on the elevator.  All involved stalls near the ground,
the control difficulty arising from an attempt by the pilots to
physically override the autopilot by pronounced forward stick force,
with the autopilot responding by introducing extreme nose up trim.
All were on modern aircraft, not geriatric 727s or such. To compound
matters, sometimes meddlesome autothrottle activity joined in the
battle of wits.  In most instances the pilots were exquisitely blind
to any concern over trim position.

I am simply unable to understand, in today's sophisticated world of
computer controlled everything, why anyone would design a system which
would permit such a flagrant conflict of control authority, and
particularly, one which would necessarily leave the aircraft in an
outrageously untrimmed condition.

I can see modest justification of an autopilot permitting a very small
amount of tweaking of flight path by gentle assistance from the
pilots, presumably to permit refinement in precision of capture of the
glide slope or ILS during the early phases of instrument approach.
Even then it seems unlikely that human skill could compete with or
refine the actions of a modern autopilot (needless to say, I am not
talking about WW II era junk).

What exactly is the reason for deliberately designing a system that
doesn't, at any one time, have a single master in control of aircraft
desired flight path and related control actions?  It seems a bit
implausible that any competent designer would even remotely flirt with
such a design.

Where am I wrong?
John M. Hunt
johnmhunt@ipa.net