Originally Posted by
LarryJ
When you're hand-flying an airplane you are trimming frequently. Even more so in a transport jet as you are operating over a much wider range of speeds and configurations than smaller, slower airplanes. As conditions change, the nose either starts dropping below where you want it or it starts rising above it. You react by holding pressure to keep the nose in the desired position then re-trim to remove the pressure. This reaction is a normal part of hand-flying an airplane and should be an automatic response to an out-of-trim condition.
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After three, maybe four, cycles the pilot should notice that everytime he trims the nose back up the airplane is trimming it back down. This is a stabilizer runaway. He can continue to counter the MCAS activations with his electric trim indefinitely or accomplish the stabilizer runaway procedure which will disable the system for the remainder of the flight.
The procedure is; 1. Grasp control wheel firmly. 2. Disconnect Autopilot (it's already off or MCAS wouldn't be operating) and autothrottle. 3. Stab Trim switches to Cutout.
AoA Disagree messages, or in-depth knowledge of MCAS , doesn't really help because you're busy controlling the airplane and don't have time for detailed thought on system interaction. That would come later when you're writing up the problem in the logbook for maintenance to fix. You fly the airplane. The key factor in identifying a stabilizer runaway is the repeated, or increasing, abnormal need to retrim.
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Thanks for your clear description of techniques and issues in this and other posts.
As a layperson, I'm wondering if there was a typo (in red): would MCAS only be operating when autopilot is
on rather than
off? The former makes intuitive sense to me, but if it's the latter — I am curious why would MCAS only operate when autopilot is off? From what I have understood so far, the MCAS was added to give the MAX a manoeuvring feel consistent with the non-MAX, but if it's primarily intended as a manual pilot aid, I don't understand why disconnecting the autopilot is part of the corrective action.
I think this question is complementary to zdog2x's post just above; it seems like an odd pattern that both of the problem flights crashed soon after takeoff.
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Also, NPR's All Things Considered did a pretty nice summary of the market pressures (fuel) and design constraints (frame dimensions vs. engine size) that led to the conception of MCAS.
Link to summary:
https://www.npr.org/2019/03/22/70597...s-737-max-jets
Link to audio:
https://ondemand.npr.org/anon.npr-mp...7_max_jets.mp3
(This isn't UA specific, but I'm posting here for continuity of discussion.)