Boeing 737MAX8

These are the switches. If these switches are flipped to “cutout” the automatic and electric trim stops working and so does the MCAS in the Boeing 737MAX8. They form part of the “runaway stabilizer-memory items” that all Boeing 737 pilots are taught during their type-rating.

(Taken from: mentour_pilot,
Petter, Public figure
Catalonia, Spain)


A runaway stabiliser isn’t the same as MCAS operation though.

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Re: mentour_pilot, an experienced 737 pilot. Ask him if you have doubts. Can contact him via Instagram.

Doubts about what?!

The validity of his comments.

His comments are valid.

The application of one checklist to another situation isn’t.

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Yeah sorry, that’s what I meant. Sorry for not being specific.

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Your position. I am not a pilot, he is. Ask him.

Dispute with the professional pilot. He can illustrate you better than me.

Oh I was only here to help @Stu understand.

I would respectfully disagree to the Pilot on instagram. Don’t worry 😉🤗 (If that were the case)

Are you a pilot? I am not. I just posted the opinion of a real 737 pilot.

Most definitely not. 🤗🤗 that was just a unreal scenario. (I’m sure that is creditable.)

No problem. Have a nice day!

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But not one on here? 😉

in terms of what the pilots would see happening what is the difference between a “classic” runaway stabiliser and an MCAS induced runaway stabiliser?

The MCAS applies nose down pitch through the stabiliser utilising the same actuators as the STAB system.

Remove the power to the stab automatic systems and you ‘remove’ the ability of the MCAS system to pitch the aircraft.

It’s very similar to STAB RUNAWAY in the 777 but our cutout switches are to the left of the throttle quadrant.


Nothing except you can get STAB runaway at any speed and you ‘should’ only get the MCAS at high AOA and close to stall low airspeed.


thanks. so i am not seeing much difference then between diagnosing and applying the runaway stabiliser items for a “classic” runaway stabiliser vs an MCAS induced runaway stabiliser (unless I am missing something?).

i appreciate the pilots of the Lion Air crash the pilots may have not known the underlying cause of the runaway stabiliser was the MCAS system, but that doesn’t seem to change what is observed and what actions need to be taken - the discussion of what induced the problem can be left to a later time.

The big cause for concern is that MCAS applying a nose down pitch programmed for low energy flight into a jet flying at 400kts is going to produce something like the rate of pitch down and descent we are seeing in these accidents.

MCAS in the correct flight profile is great, at higher speeds it could well be a killer.

We will see in the accident reports.

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Yes although MCAS is only supposed to operate for 10 seconds at a time. Be careful with terminology here because this wasn’t an MCAS induced runaway stabiliser this was MCAS doing what it was supposed to do, albeit based on duff gen. It’s supposed to trim (forward) and so seeing it do so isn’t going to be surprising. Also there are 9 other conditions/indications caused by a faulty alpha signal to consider AS WELL AS the the automatic trimming. Imagine seeing, potentially, all of those indications and it’s going to be quite easy to ignore or not notice subtle auto trimming when you might expect it to happen anyway.

A runaway stab trim would trim constantly, hence the name, in either direction and would be a lot more obvious.

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