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B-737 Speed Trim System

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B-737 Speed Trim System

Old 7th Dec 2018, 12:23
  #81 (permalink)  
 
Join Date: Dec 2002
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More thoughts about system’s architecture and input sensors; primarily relating to MCAS, but perhaps some may relate to STS.

Although the aircraft airspeed system has ‘triplex’ integrity, this depends on the pilots selection of the best 2 out of 3 displays. However, I assume that the inputs into the trim computations are only dual (digital), and thus with a ‘speed disagreement’, in might be possible for an erroneous low value to activate trim.
Similarly an erroneous high thrust input, affecting MCAS.

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safetypee is offline  
Old 7th Dec 2018, 13:25
  #82 (permalink)  
 
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My opinoin, we would be safer without this system working agains us when we hand fly. Those of you who engage the auto pilot passing 400 ft will not understand what Iím talking about, but the few of you who hand fly on a regular basis will.
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i fly manual on a daily basis and the 737 flies like a Cessna. A pure joy. No issue with the speed trim sysyem at all.
de facto is offline  
Old 7th Dec 2018, 15:02
  #83 (permalink)  
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Originally Posted by de facto View Post


i fly manual on a daily basis and the 737 flies like a Cessna. A pure joy. No issue with the speed trim sysyem at all.
Yes, because you cancel it by trimming against it. As I have done for nearly 20 years. My point is, if STS was not there, you would not notice. As the aircraft accelerates you would trim down. Same as with a Cessna.
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Old 24th May 2019, 20:37
  #84 (permalink)  
 
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MCAS is not a system at all - it is just a few lines of code in the flight control computer - other than that it does not exist in any form, part or shape. This code was supposed to give one single and small movement of the horizontal stab leading edge upwards when the aircraft enters one corner of the flight envelope and move the neutral point slightly back in order to satisfy the static stability regulation requirement.
Then during certification testing code was changed and instead of one activation new code allowed stabilizer to move more than once.
That effectively created a never ending loop - most common cause od any computer program gets stuck and only way to clear it is to reset the computer. It was a begginer's mistake. Only this time it was the aircrafts crashed not just code.
I do not know how deep the FAA can go into the code but they should have not ever ever approve this change.
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