FAA faces dilemma over 737 Max wiring flaw that Boeing missed

Re:FAA faces dilemma over 737 Max wiring flaw that Boeing mi... by cmurf on Sunday February 16, 2020 @05:12PM (#22344062)

potential for an electrical short to move the jet’s horizontal tail uncommanded

I think this is a reference to the stabilizer, because the elevators are controlled by cable connected to the yoke and are hydraulically powered. Since stabilizer (trim) can overpower elevator force, uncommanded changes in this control surface could be really bad, depending on how a short manifests into control surface movements.

I can't assess the relative probabilities: a short happening vs the fix inducing some other problem. But I do wonder whether there's another way to mitigate it.

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Re:FAA faces dilemma over 737 Max wiring flaw that Boeing mi... by Gwypaas on Sunday February 16, 2020 @05:44PM (#22344200)

Today it's mitigated by the pilot. Runaway Stabilizer is a memory-item. [1]

This is the same memory item Boeing thought would mitigate MCAS, since that is essentially a runaway stabilizer trim, although in retrospect behaving in a very different manner making it much harder to diagnose.

[1]: http://www.b737.org.uk/runawaystab.htm

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Re:FAA faces dilemma over 737 Max wiring flaw that Boeing mi... by kds3 on Sunday February 16, 2020 @05:59PM (#22344269)

Runaway Stabilizer / MCAS cases can be mitigated by the pilots (they can use cutoff switches and do manual trim), but not this kind of electrical short:

> Furthermore, the electrical power in that wire could circumvent the cutoff switches in the cockpit that, in the event of such a stabilizer runaway, are used to kill electrical power to the tail. Theoretically, the pilots could be unable to shut it off.

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Re:FAA faces dilemma over 737 Max wiring flaw that Boeing mi... by mlyle on Sunday February 16, 2020 @06:15PM (#22344323)

It's not a great mitigation, but the last thing on runaway stabilizer's memory list is to "grasp and firmly hold" the stab trim wheel.

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Re:FAA faces dilemma over 737 Max wiring flaw that Boeing mi... by SAI_Peregrinus on Sunday February 16, 2020 @06:44PM (#22344457)

The electric motors can apply more power than a human can resist. So that's not exactly helpful.

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Re:FAA faces dilemma over 737 Max wiring flaw that Boeing mi... by mlyle on Monday February 17, 2020 @12:45PM (#22349214)

The system is engineered so that an operator holding the trim wheel wins, via a clutch mechanism.

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Re:FAA faces dilemma over 737 Max wiring flaw that Boeing mi... by userbinator on Sunday February 16, 2020 @07:43PM (#22344697)

I believe there is a clutch mechanism. There are videos on YouTube of pilots doing this (in a simulator.)

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