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Comment by inferiorhuman | Hacker News Reader

Technically MCAS activation isn't runaway stabilizer trim, and I'm sure that tripped up the Lion Air pilots. Check out the quick reference for a runaway stabilizer[1]. Note the steps:

* Control airplane pitch attitude manually with control column and main electric trim as needed

* If the runaway stops, stop.

Well, electric trim input on the yoke will stop MCAS temporarily. Of course if these guys had any experience on an NG they're already used to the computers trimming the plane in a counterintuitive manner via the speed trim system (STS). So not only is MCAS not a runaway trim situation, but pilots flying the NG will get used to the computer trimming the stabilizer "at random".

1: http://www.737ng.co.uk/737-800%20Quick%20Reference%20Handboo...

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This sort of MCAS failure presents itself to the crew as a trim runaway, and the instructions in the document continue:

  4 If the runaway continues:

      STAB TRIM CUTOUT switches (both)   . . CUTOUT

      If the runaway continues:

        Stabilizer trim wheel    . . . . .   Grasp and hold

  - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

  5  Stabilizer    . . . . . . . . . . . .   Trim manually

(That bit about grasping and holding the trim wheel was a surprise to me.)

This is what the Lion Air crew of the flight before the one that crashed did, in response to the same sort of MCAS failure, and completed the flight without further trouble. While Boeing made a serious error in hiding the differences between this variant and its predecessors, it seems that the prior trim runaway procedure does work for this sort of MCAS failure (unless evidence to the contrary comes out of the Ethiopian Airways investigation.)

Boeing is "right" in the sense that, if you follow the normal 737 flight manual, you'll be fine, even with MCAS enable or even MCAS misbehaving due to the sensor readings being bad or whatever.

I think the problem is that the symptoms are different from other 737s, and if you don't KNOW about the MCAS stuff, that's very surprising. So as humans, even though logically the same procedure applies, instinctively you don't think to do it. And given the very short time frames involved, and the margin of error, it's not too surprising that these events have occurred.

Perhaps the need-to-retrain criteria needs to include the following amendment:

If the craft exhibits new symptoms, even if the response to those symptoms are covered correctly be the old operations manual, pilots will need to be retrained to account for the new symptoms.

This sort of MCAS failure presents itself to the crew as a trim runaway

No, it doesn't and that's the problem. A runaway won't stop when you hit the trim buttons.

> A runaway won't stop when you hit the trim buttons.

Depends on what causes the runaway. If the autopilot goes berserk, the trim buttons will override it (if I recall correctly, it's been a long time).

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