Re: Trim Activated by the Autopilot

Date:         31 Dec 99 02:09:34 
From: (PS2727)
Organization: AOL
References:   1
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I think the answer to your question lies in the task given the designers
of autoflight systems. If one parameter is to allow for pilot input
while autopilots are engaged this would not be a problem. Unfortunately
pilots sometimes don't fully understand the implications of "helping"
the autopilot in its job. Most airplanes I have flown have cautions
about overriding autopilot inputs and to disconnect it if its not doing
what you want.

The elevator trim movement by the autopilot looks at only one thing: it
seeks to relieve pressure on the actuator by trimming the stabilizer so
that the elevator is faired to the stab. If the actuator is holding
constant down elevator to maintain level flight it will trim the stab to
relieve the force and unload the actuator. Now if the pilot applies up
pressure the autopilot will add down trim in an attempt to help unload
the actuator which is now fighting the pilot input. If it continues, the
trim continues to run until it hits the limit. This is the scenario in
a few of the accidents and the solution is to train the pilots
accordingly. Unfortunately in some cases the pilots did not even know
the autopilot(s) were on. This seems amazing until you see how these
things work. There is little warning sometimes and requires constant
attention to keep up with the mode of operation.

Whether its a design flaw is a matter of opinion but I think we have
taken a giant step backward in safety by complicating the task rather
than by simplifying it.