Re: Trim Activated by the Autopilot

Date:         31 Dec 99 02:09:36 
From: (JWizardC)
Organization: AOL
References:   1
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While I cannot necessarily speak from any real authority on this
subject, I think I can shed a little light on it.

The reason that autopilots have large trim capability is because they
have little control authority. Generally, an AFCS will be able to
command movement of 10-25% of total elevator and aileron/spoiler travel.
The theory is that if the autopilot were to go totally bananna-bonkers-
bozo, it would not be able to send the aircraft into a zoom climb or
barrel roll.

Autoflight systems generally use two types of pitch trim. First is 'mach
trim', which compensates for airplanes' tendancy to nose down or 'mach
tuck' as airspeed increases and the Center of Lift moves aft on the
wing. The second is just a normal, low-speed trim, usually just a
different actuator than the one(s) the pilots use. In any case, any time
the autopilot trims the aircraft away from 'neutral' (e.g. hands-off)
trim, there is some form of indicator to warn the pilots that it is
happening. The manuals I have read all say the same thing: 'hold the
controls tightly and disengage the autopilot and autopilot trim. Be
prepared for large control forces to be required to maintain flight path