Re: Trim Activated by the Autopilot

Date:         10 Feb 2000 05:03:40 
From:         "Chris Dahler" <>
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> Most other models do, except (I believe) the B767. I would welcome any
> information on the B767 AP behaviour in response to manual force on the
> controls.

The autopilot fights you, but it won't disengage no matter how much
force is applied.  However, the trim is inhibited from moving in a
direction opposite to control column force.  Boeing has always designed
aircraft like this: in the older aircraft with the trim wheels, the
wheels were stopped by a mechanical lock.  If the trim was activated and
then the control column was moved in the opposite direction, a very loud
bang was the result as the lock whacked into place, stopping the wheels.
Quite annoying.  The 757/767, of course, has no wheel and thus no noise,
but the lockout is the same philosophy.

On another note concerning this thread about audible warnings: Boeing
does include a warning.  If the trim is not within a predefined takeoff
range (defined on the trim position tape as a green band), the takeoff
warning aural will sound as the power is advanced.  Obviously, this only
works on takeoff.

For other regimes of flight, part of the problem that you and others
aren't taking into account is that it would be rather difficult to
create an aural warning for a wide trim variance.  Depending on the
aircraft's weight and speed and the way the CG is distributed at the
moment, the trim will vary all over the place.  On the 757, I've seen it
from almost at full AND to almost full ANU at various times during
various flights.  It's fine to speculate on having an aural saying
"Trim" or something like that, but where are you going to define the
limits of the warning?  It would be practically impossible.