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Old 19th Nov 2018, 22:13
  #1402 (permalink)  
Join Date: Nov 2018
Location: Brisbane
Posts: 25
Originally Posted by infrequentflyer789 View Post
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.
MCAS apparently only operates with AP disengaged. The fact that the EAD refers to the runaway trim procedure seems to indicate that the FDR indicated runaway trim. As I've mentioned elsewhere, if MCAS is implicated it means the pilots were flying by hand and counteracting MCAS auto-trim activity reasonably successfully up to a point. What happened to change that situation and trigger the horrific descent is very much unknown.
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