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Comment by chupa-chups | Hacker News Reader

- The MCAS system has more trim authority than the usual (assisted) system

- It was originally intended to have an authority of +/- 0.6 degrees of trim

- Later it was discovered that this it not enough due to the aerodynamic effects of the engines becoming apparent also at lower speeds, which led boeing to increase authority to +/- 2.5 degrees per iteration with unlimited iterations until the maximum range

If you don't believe me, see pprune.org (real pilots discussing the issue) and https://www.satcom.guru/2018/11/stabilizer-trim.html

This is not a software fault.

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Unless I'm wrong disabling the MCAS system leaves the elevator at the current trim as set by the jack screw. That's really not the behavior you want. You want the control to return to neutral. Which probably isn't the way the trim mechanism is designed.

This whole thing smells like a bunch of dodgy decisions. Some of them recent and others historical. For instance not being able to manually adjust trim while the elevator is under load. That's dodgy, they got away with it because runaway trim is really rare.

Not a pilot yet myself, but I know exactly enough about flight physics and control to suspect that while you are likely correct about the MCAS jack-screw positioning being undesired, the solution is probably not to auto-return it to neutral.

If we assume MCAS is only disabled in an emergency, we don't want the result of cutting out an automated system to be further automatic manipulation of the control surfaces, right? That's one more weird behavior for the pilots to have to keep in their situational awareness while doing the most important step (i.e. flying the plane).

> we don't want the result of cutting out an automated system to be further automatic manipulation of the control surfaces, right?

You have two separate things a pilot may need to do. One is disabling the MCAS control action. In that case you do want it to return to normal trim. Second is disabling the Automatic/Electrical Trim. In that case you want it to stop and allow the pilot to set the trim manually.

There's no way to "disable MCAS". There's just "disable the stabilizer trim motor". At which point the motor can't help you restore safe trim.

Yes that's more or less it from what I gather. The trim cutout is designed to stop runaway from due to failure of the motor control circuit. Some ommon failure modes leaves the output stuck on. The only fix is to kill the power.

However if you kill the power to the trim motor and then turn it back on MCAS will blindly add more trim on top of the trim it previously added. It doesn't remember how much trim it's already applied.

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