Re: Airbus Trim System

Date:         26 Jul 98 23:57:20 
Organization: Deja News - The Leader in Internet Discussion
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I don't think the original poster was seeing the auto stab trim compensating
for mach tuck. The Mach trim system was installed to prevent the nose
dropping due to movement of centre of pressure as speed increased to some
level above the normal operating region. It was on the B707 in my experience
and as the nose started to drop the mach trim applied a counteracting pitch
trim force that resulted in positive forward pressure (a push forward) being
required by the pilot if the aircraft was not to attempt to climb. This nose
up pitch or course meant speed bleed and climb so the tendency was to return
to equilibrium flight. The thrust would have to be reduced to prevent speed
build up again. With the current generation of airliners with high speed
wings, high cruise speeds and low drag I would be suprised if mach tuck was
still a problem needing a special trimmer. Certainly I have never read in any
Boeing publication that such a system is fitted or needed. The upsets, and
there were quite a few in those early years of jet transport operation were
researched by NASA at moffit field ,in advanced (for its time) simulators and
there findings were reflected in new crew procedures, improved weather radar
and operating techniques. I recall the upsets were mainly caused by entry
into Cb's with the autopilot height hold engaged. The initial updraft caused
the stab trim to trim nose down to attemtpt to retain the cruise altitude.
Then as the aeroplane entered the area of strong downdraft it pitched down
and commenced a rapid rate of descent. The speed built up rapidly with all
the forward trim still on. The electric trim motors attempted to trim the
nose up but the air loads were to great and the motors stalled. The speed
increased and the wings came off very quickly. The technigue was to trim for
turbulence penetration, disengage the altitude hold, and dont worry about
altitude in th cell. As the trim moved the stab it was immediately manually
returned to the pre cell entry position. Speed was controlled either with
thrust or speed brake and we just rode it out. Once we knew what was the
cause of the trouble and the fix I think the upsets almost ceased. New and
better radar helped too. Current generation of airliners have hydraulic
motors, not electric.

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