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Old 14th Nov 2018, 10:04
  #1158 (permalink)  
Join Date: Mar 2010
Location: L.A.
Age: 52
Posts: 556
Originally Posted by Halfnut View Post

The MCAS function becomes active when the airplane Angle of Attack exceeds a threshold based on airspeed and altitude. Stabilizer incremental commands are limited to 2.5 degrees and are provided at a rate of 0.27 degrees per second. The magnitude of the stabilizer input is lower at high Mach number and greater at low Mach numbers. The function is reset once angle of attack falls below the Angle of Attack threshold or if manual stabilizer commands are provided by the flight crew. If the original elevated AOA condition persists, the MCAS function commands another incremental stabilizer nose down command according to current aircraft Mach number at actuation.

That seems a lot of trim to me. Try flying at 5,000 and 250 kt, and then mis-trimming forward by 2.5 degrees. You will be struggling.

Strikes me that Boeing designed this stall-trim system assuming that the aircraft would be slow and approaching the stall, when 2.5 degrees of trim would be acceptable. But at 250 kt in a normal climb, 2.5 degrees of trim becomes a liability. And if it gives you another 2.5 degrees of trim, you are uncontrollable.

Sounds like Boeing made this system on the cheap. The reason aircraft have stick pushers, is because the push is instantaneous, on the stick, and its withdrawal is instantaneous too - so you can pull out of the dive. But if you ‘push’ via a trim input, you cannot easily undo that trim - and so you cannot easily recover from the dive. And as a basic flying principle, we always fly with the stick, and trim out the stick pressure. You never fly with the trimmer, which is what this system is trying to do.

Furthermore, an erroneous stall-trim input at 250 kt (due faulty AoA indication) will give huge stick forces that the system did not intend - and perhaps the designer and test pilots never considered. A faulty stick push can be easily overcome with stick pressure - it was designed that way, and haveing already got the T-shirt on that one, the aircraft is easily flyable. But if a system gives you 2.5 of trim, and then another 2.5 of trim, you are going down, because the stab-trim is much more powerful than the elevator. Did anyone ever test this...?


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