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Mach Airspeed Trim

Thu Mar 31, 2011 1:54 pm

What exactly is Mach Airspeed Trim and what does it mean when you get an error on the PFD alerting you to Mach Airspeed Trim/ Rudder Ratio? I ask because this occurred in the Birgenair Boeing 757 crash off of Puerta Plata, and the video I watched never explained what it was. Thanks in advance.
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RE: Mach Airspeed Trim

Thu Mar 31, 2011 2:32 pm

Mach Trim is a system that resists the nose down tendency - aka Mach tuck - of the aircraft as one Mach number increases.
This is due to the so-called *center of pressure*, i.e; the point (very theoretical point) where the lift is applied to the wing, moving aft.
Generally, usee on non-fbw aircraft on manual flight.
The 737 used to have a very noticeable onset of the Mach trim at around .70M.
On FBW-equipped airplanes, it's part of the control laws so it is quite transparent.

The rudder ratio is something quite different : It is about reducing the amount of needed rudder moment when the speed increases. So one could say, full rudder deflection only happens at low speeds.
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RE: Mach Airspeed Trim

Thu Mar 31, 2011 10:08 pm

The point about the warnings mentioned is that the some systems on the aircraft sensed it was flying too fast when in fact it was flying too slow. Rudder Ratio warning indicates a speed error in the rudder ratio system (rudder ratio in low speed mode with high speed sensed). Similarly the Mach Trim warning.
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RE: Mach Airspeed Trim

Thu Mar 31, 2011 11:09 pm

Thank you jetlagged and Pihero
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