Re: Trim Activated by the Autopilot

Date:         31 Dec 99 02:09:35 
From:         "Chris Dahler" <>
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> 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.
> battle of wits.  In most instances the pilots were exquisitely blind
> to any concern over trim position.

In a case such as you described here, the pilots would also have to be
exquisitely blind to the fact that the autopilot was still connected
while they were attempting to manipulate the controls.  At least in the
757/767, the autopilot ceases to have any control over the trim when it
is disconnected, and I cannot envision a scenario in which a pilot would
ever intentionally try to hand-fly the airplane with the autopilot still
hooked up.

> 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.

They don't design systems like this.  When one disconnects the
autopilot, the conflict of control authority ceases.  The pilot should
never be attempting to manipulate the controls with the autopilot hooked

> 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.

No one designs aircraft like this.  The pilot remains the "master in
control" of the aircraft, and if the autopilot doesn't do something he
wants, he either reprograms the autopilot or he disconnects it and
hand-flies the aircraft.  Some aircraft (notably Airbus) are designed
with extreme flight envelope limit protection software that will prevent
a pilot from, for example, stalling the aircraft, but one does not
normally operate an aircraft in such a regime, and it could be argued
that if a pilot allowed his flight path to deteriorate to this point to
begin with, he probably needed some help from his friendly autopilot