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Old 19th Nov 2018, 12:43
  #1397 (permalink)  
infrequentflyer789
 
Join Date: Jan 2008
Location: uk
Posts: 808
Originally Posted by Concours77 View Post


That’s an interesting catch. If MCAS senses (erroneous) AoA, and pilot catches the “runaway” within the first ten seconds, and starts to wind trim back, when MCAS “stops”, he will crank in unnecessary NU trim, then as MCAS begins a second cycle, well, you get the point, if the pilot is unaware of the function and intermittent cycle, he will chase the trim. FAIL TRIM induced Porpoise! Without specific knowledge of the operation’s cyclic timing, well, don’t try to tame the beast you don’t know.
abnormal.
I'm not sure if I am totally barking up the wrong tree here, but there may be another possibility to consider as well:

What happens if MCAS and STS are both active? We have no information as to how they interact in that case, whether one system inhibits the other, whether outputs are summed or or-ed, whether the reactivation timers are identical and in-sync or not.

As I understand it, if speed (data) is increased STS will trim nose up, if AOA (data) is high MCAS will trim nose down, what if both? If the activation times are different the trim will be up then down, which might lead to a report of "sts trimming wrong way" in absence of knowledge of MCAS, no? So the pilot might be fighting an aircraft that is porpoising all on it's own. Note that this may not look like a classic "runaway trim" either, it isn't at this point running away in either direction.

Now if speed is reduced, will STS and MCAS (assuming indicated AOA remains high) both trim nose down, depending on the cyclic timing, possibly suddenly and at the same time (additive commands? - we don't know)?


But like I said, I may be totally barking up the wrong tree and have misunderstood STS/MCAS.
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