
ET302 Used the Cut-Out Switches to Stop MCAS - acqq
https://leehamnews.com/2019/04/03/et302-used-the-cut-out-switches-to-stop-mcas/
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erentz
This was quite a revealing bit of the puzzle when it was announced. In short
(lay persons interpretation): due to a separate issue they had with the
airspeed disagree indicator warning they followed the course of action for
that which was to speed up the plane to prevent potential stall. You’re
flying, you don’t know for certain the speed, so go a bit faster.

But this meant when they had the AoA/MCAS issue they cut off the MCAS and were
going too fast to be able to manually trim the aircraft due to the fact MCAS
had driven the stabilizer to full deflection. At that speed and that amount of
deflection and pulling back with full elevator to try keep the nose up the
forces were too great to manually trim.

The solution to that is to ease up on the elevator allow the nose to go down
and manually trim then nose back up. But they were too close to the ground
still to do that safely. So they needed the electric trim enabled, but that
means having the faulty MCAS enabled which they needed off.

Suggested work arounds were to set flaps to 1 while leaving the drab trim
cutoff disabled which would’ve disabled the MCAS and let you still
electrically trim. But even if you thought of this you can’t extend flaps at
400kt.

If all this is true then this accident is definitely not as cut and dry as
everyone thought initially and the solution of “just cut the Stab Trim cutout
switches and trim manually” isn’t a solution in certain failure scenarios,
namely this one.

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the_fonz
MentourPilot is a great channel... I subbed before the Max debacle.
[https://mentourpilot.com](https://mentourpilot.com)

It makes sense that even with two pilots trying to manually spin the trim
wheels from a full nose down configuration would be nearly impossible due to
aerodynamic loading. IIRC relatively unloaded manual trim wheel movement is
difficult even with both pilots extending the trim wheel folding handles and
spinning them with maximum effort. I would like to know if commercial
simulators accurately simulate manual trim wheel difficulty under high
aerodynamic loading such as full nose down trim and high speed.

I think the electric trim cutout switches should be repurposed as "automation
trim cutout switches" so that pilots can continue to use the yoke trim
switches under high aerodynamic forces where manual trim reversion isn't
possible and electric power is still available... and pilots could still
revert to trim wheels during electric power loss conditions. You wouldn't
disable the brake booster in a car because cruise control went haywire.

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erentz
> think the electric trim cutout switches should be repurposed as "automation
> trim cutout switches" so that pilots can continue to use the yoke trim

Those trim cutout switches were actually first put in place long ago to cutout
the electric trim on the yokes for situations when they get stuck or otherwise
runaway.

So really what’s needed now is a different set of MCAS disable switches.

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phire
This shouldn't be marked as dupe.

Despite the similar title to the WSJ article, this contains heaps of new
original information based on actual simulator testing that hints why the
pilots might have re-enabled the cut-out switches.

~~~
acqq
Exactly. This is the original source of an attempt of one real 737 pilot to
reproduce the conditions similar to those that happened during the last
crashes, using the existing (non-MAX) Boeing 373 flight simulator.

The company he works for requested the takedown of the video, but the video
shows a lot that the public shoold know, and the conclusions are still there.

Moderators maybe please take a new look here.

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sschueller
Aren't all these scenarios tested in simulators and in the actual aircraft? If
it's not possible to recover the aircraft why was this not know and why was
this plane still flying after the first crash?

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prewett
Planes are complicated, and you can't test for all n! different things that
can go wrong. I think the case that:

a) an MCAS sensor is giving incorrect data AND

b) the data is leading MCAS to set the trim to the max AND

c) the plane is going so fast that the force of the air over the elevator is
too great to manually override AND

d) the plane is too low to temporarily drop altitude to reduce the force on
the elevator to allow a manual override

I don't think this is an obvious set of things to foresee happening,
especially D.

It's the same way with software. There's just too many things to think of, and
errors that slip through the test cases are inevitable. I've had what I've
considered to be pretty thorough test cases and had a bug that was "obvious".

I'm not excusing Boeing, but it isn't gross incompetence either.

