Re: Airbus trim system?

Date:         11 Jul 98 03:00:28 
From: (JWizardC)
Organization: AOL
References:   1
Followups:    1
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My understanding, limited though it is, is what you were witnessing is
called 'mach trim'. There were several incidents and accidents on older
707/DC-8 types where the aircraft got far enough out of trim due to high
mach numbers (often due to Clear Air Turbulance) that the elevator
couldn't compensate, and the stabilizer trim (the entire stab moves, not
just an elevator tab) motor didn't have enough 'oomph' to get the plane
back into trim. This was called 'jet upset' and caused more than a few
bent airplanes and spilled drinks in the earilier days of jet transport
flying. Secondarily, as an aircraft's speed increases toword the top
half of the mach meter, the airplane has a tendancy for the nose to duck
under. This is called (cleverly enough) 'mach tuck'.  Automatic Mach
Trim helps to compensate for these two effects. It doesn't try to
anticipate the pilot's wishes, it just tries to keep things from getting
too far beyond reasonable.