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Old 6th Dec 2018, 09:47
  #2021 (permalink)  
Jetthrust
 
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Originally Posted by MickG0105
Boeing fails to make it clear that a trigger for MCAS to continue trimming in more nose down trim is a countermand from the crew. Frankly, I think that is astounding. The system will ignore the persistence of the trigger condition after it makes its initial nose down trim adjustment but it will make a further nose down trim adjustment if the crew countermands it! Moreover, it will do that each and every time the crew countermands it. That is a very clear cut case of an automated system that is designed to override a specific crew command. To borrow an image from Gums, when the crew looks to wind off some of the automatically commanded nose down trim, HAL says, 'I'm afraid I can't allow that, Dave.
That was my view also. But, after reading FCeng84’s comments to this statement, I would explain it this way. Let’s assume the system logic was the opposite. That is, if after MACS commands a pitch change, the pilot uses the electric trim switch or manual trim wheel, MACS says “ok, the pilot has deliberately changed the pitch, I will disable myself”. For how long should MACS remain disabled? What happens if 20 min later, the AoA gets into dangerous territory? Clearly, a time period needs to exist at which MCAS becomes re-enabled. So, this reverse logic scenario is actually exactly how it does work. It’s just that the time period, according to FCeng84 who seems to know this system well, is 5 seconds. (Which given how quickly an aircraft could get out of control, is probably quite a long time?)
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