According to this article, a horizontal stabilizer runaway on the 737 MAX, unlike with all other 737s, cannot be countered with yoke inputs:

Older 737s had another way of addressing certain problems with the stabilizers: Pulling back on the yoke, or control column, one of which sits immediately in front of both the captain and the first officer, would cut off electronic control of the stabilizers, allowing the pilots to control them manually.

That feature was disabled on the Max when M.C.A.S. was activated — another change that pilots were unlikely to have been aware of. After the crash, Boeing told airlines that when M.C.A.S. is activated, as it appeared to have been on the Lion Air flight, pulling back on the control column will not stop so-called stabilizer runaway.

Why would the addition of MCAS require removing the yoke trim override mechanism? In the event of a trim runaway, the pilots' first reaction is going to be to pull (or push, depending on the direction of the runaway) the yoke, and requiring them to perform a separate, different action in the heat of the moment to disable the autotrim mechanism is an unrealistic expectation; allowing yoke inputs to cut out the autotrim relieves the pilots of the burden of having to recognise the problem as a trim runaway in order to solve it, and allows the problem to be solved in the same manner as other unwanted pitch excursions (by pushing and/or pulling on the yoke(s)).


2 Answers 2


Allow me to emphasize the article quote differently:

That feature was disabled on the Max when M.C.A.S. was activated

For a regular trim runaway without MCAS activation, the yoke can still cut out electronic trim as before. The only change was inhibiting this behavior when MCAS is active. MCAS is designed to improve handling at high angles of attack. One way to get to a high angle of attack is with lot of yoke input, so it would somewhat defeat the purpose to stop MCAS in that situation.

The designers chose to trust MCAS (and its sensor inputs) over the pilot inputs in this situation. It certainly seems now that this trust was misplaced, but this has more to do with the system design than the general philosophy. Fly-by-wire planes have many protections that will change control input depending on sensor inputs. In normal law, Airbus alpha protection will prevent the angle of attack from increasing, even with full aft sidestick input. It compares three sensor inputs to do this, rather than trusting only one, but even that is not infallible.


You have to consider why MCAS was installed. It's there to save the day when the pilots have not kept the angle of attack to appropriate levels, very commonly due to disorientation.

If MCAS could be overridden just from the pilot pulling back on the yoke, it defeats the purpose of it being there in the first place. Hence, when MCAS is active, the autotrim must be cut out via a different method.

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    $\begingroup$ I disagree: MCAS was installed to help pilots lower the nose when they are close to a stall in a high thrust situation, where pushing on the yoke alone might not be enough. The purpose was never to overwrite a pilot's input and it really shouldn't be active when the pilots are pulling against it (and likely it won't after the update, which is about to be released in the next months). $\endgroup$
    – Bianfable
    Apr 4, 2019 at 11:24
  • $\begingroup$ @Bianfable: According to Boeing, post-update, MCAS still won't automatically disengage if the yoke is pulled, but its control authority will be limited to inputs small enough to counter with elevator deflection alone. $\endgroup$
    – Vikki
    Apr 17, 2019 at 22:40
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    $\begingroup$ @Sean That's interesting. I expected they would add this additional cut out feature. Maybe the changes they mentioned are enough to never be an issue any more. Anyway, my original point remains: MCAS should not fight the pilots, it was designed to make the MAX feel like the NG to fly in high thrust and high angle of attack situations. $\endgroup$
    – Bianfable
    Apr 18, 2019 at 6:45
  • $\begingroup$ @Bianfable You are right, but Boeing want it transparent to the pilot, expecting that a single step of THS 2.5 ° would be sufficient, if the yoke was able to cut this, the runaway would have been declared like a fault by the pilots. $\endgroup$
    – user40476
    Jun 3, 2019 at 15:46

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