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With regard to the Ethiopian crash of the 737 Max 8 aircraft, where according to preliminary reports, it appears to be related to the MCAS which is a safety system that pushes the nose down to correct an excessively high angle of attack (pitch attitude), how difficult is it for the pilots to simply disable the MCAS and manually fly the plane?

To clarify my question, what I'm asking is can the pilot disengage the MCAS and if so, how do they do that? Is it just flipping a switch or popping a breaker, or is it a complicated set of procedures?

UPDATE: The answer to this question explains how "difficult" (or easy) it is to disable/disengage the MCAS from pitching the nose down.

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    $\begingroup$ This question could be just as answerable without bringing up the two recent 737 Max crashes at all. You might consider just editing that part out; that would go a long way toward alleviating concerns that you want speculation on an accident, rather than operational details on an airplane. $\endgroup$ – a CVn Mar 15 at 19:08
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    $\begingroup$ @aCVn I think context is important. Many people use this site, not all are aviation nerds, by providing context to the question it establishes why the question and answers, are relevant and important. At least, thats my opinion. :) $\endgroup$ – Devil07 Mar 15 at 20:20
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    $\begingroup$ Related: Can computer imposed inputs be overridden on the Boeing 737-MAX? $\endgroup$ – ymb1 Mar 15 at 20:59
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    $\begingroup$ While this is an interesting question, if we're considering it for practical purposes, we also ought to ask what the purpose of MCAS is and what the consequences of disabling it would be. $\endgroup$ – David K Mar 17 at 16:22
  • $\begingroup$ The purpose of MCAS is to avoid stalls. It does that by shoving the nose down. The consequences of disabling it that it won't try and auto-recover from a stall. $\endgroup$ – ggb667 Mar 18 at 12:49
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MCAS doesn't have its own on/off switch

It is a fly-by-wire feature designed to account for a particular flight regime that would not (or was not expected to) be encountered very often in normal operations, and is intended to account for some of the aerodynamic effects of the LEAP-1B (CFM International) engine installation for this model. Its activation requires a number of preconditions, but we'll get to that in a moment.

A few things that should disable it (with caveats)

  1. Lower the flaps. It is intended to work only if the flaps are up.

  2. Turn the Stab Trim switches to OFF. This disables the horizontal stabilizer's trim completely, and reverts to manual trim (there are two guarded stabilizer trim switches in the aisle stand, see Windshear's answer). This means that the pilots must move/rotate the trim wheels in order to apply pitch trim during flight.
    enter image description here

    The manual pitch trim appears to be what a few crews did prior to the LionAir crash in October 2018. It is unclear how many of the crews knew that it was MCAS, versus any other trim or pitch anomaly. The previous LionAir crews on the accident aircraft ended up flying to their destination manually. (Original source is the Preliminary Report from that accident).

    ... pilots encountered problems involving the AoA as well as the pitot tube used to measure airspeed. In a flight in the same plane the day before, to Jakarta, the pilot experienced many of the same symptoms as the pilots on flight JT610: the stick shaker activated during rotation, an indicated airspeed warning alert appeared, and the aircraft began automatically pushing the aircraft nose down.

    The pilot, after determining that his flight display system was malfunctioning, ran a runaway stabilizer non-normal checklist which led to the MCAS being disconnected when the stabilizer trim switches were turned off. The copilot flew the rest of the flight using manual controls and without autopilot.

    That Jakarta flight was using an angle of attack sensor that had been replaced after the previous Lion Air flight to Denpasar experienced problems. {snip} However, it is not clear whether the pilot communicated that he ran a runaway stabilizer non-normal checklist during the flight, which might have alerted the airline’s engineering staff that there was still a problem. (source)

  3. Enable autopilot. It is supposed to only work if the autopilot is off. But...CAVEAT

    That last part is a little bit complicated: the autopilot may not stay on if it - the autopilot system - keeps getting spurious signals from the AoA sensors, or if the pitot static system is providing bad data to the FCCs.

How do you know that you need to disable MCAS?

A related question is "when do you know that it's time to turn off Stab Trim switches?"
The problem for a given crew is: how do you know what sub-system is giving you trouble? The AD released in December of 2018 updated the Runaway Stabilizer procedure. Since there are other malfunctions possible in the trim system for pitch, such as a trim runaway, the same (or similar) symptom may manifest during malfunctions with different causes. (See the example above). If additional symptoms are present, such as a stick shaker activating from an erroneous AoA indication or some other fault, which problem do you solve first?
Which problem are you having?
Are you having both problems, or even something else?

That concern, the relationship between different potential failure modes, gets to the heart of

  1. What is in the pilots manuals?

  2. What training the crews do, or do not, get on this sub-system?

    The MCAS is only supposed to activate under certain conditions (in theory): high angle-of-attack, flaps-up, flight with autopilot off. This includes high angle of bank flight in such conditions.
    enter image description here

    CAVEAT

Take a look at that last bullet in the lower right of the figure. It's only partially correct. When the pilot overrides with yoke trim, MCAS will try again five seconds later (after the pilot has stopped) unless the triggering condition, or signal, goes away.

"In the event of erroneous AOA data, the pitch trim system can trim the stabilizer nose down in increments lasting up to 10 seconds," Boeing explained to operators in a bulletin issued last November, following the JT610 accident. "The nose down stabilizer trim movement can be stopped and reversed with the use of the electric stabilizer trim switches but may restart 5 sec. after the electric stabilizer trim switches are released. Repetitive cycles of uncommanded nose down stabilizer continue to occur unless the stabilizer trim system is deactivated." (source)

It is this system behavior that is believed to have ultimately overcome the best efforts of the pilots in the LionAir crash. They were airborne for about six minutes before they were no longer able to overcome the horizontal stab position with their trim controls. (Caveat: current as of early 2019. When the final report comes out this may become a lot clearer, and may need revision).

There is an easy-to-understand video of the relationship between the jack screw and the horizontal stabilizer function here. Whenever the jack screw is moving the horizontal stab, a cockpit cue that this is occurring is the rotation of the manual trim wheels (seen at about 3:40 in the video).

Why is this activating during takeoff/departure?

That MCAS appears to activate(sometimes) during routine flight regimes is believed to be catching some crews by surprise - and not just the two accident crews. There is a recent article in Atlantic that excerpts some NASA reports by aircrews in the US about some cases of unexpected/odd pitch behavior in this model aircraft.

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    $\begingroup$ is it fair to say that MCAS controls pitch through the automated Stab Trim system? So when MCAS is activated from a high AOA sensor reading, pilots will see Trim wheels spinning forward to reduce pitch? $\endgroup$ – Devil07 Mar 15 at 20:26
  • $\begingroup$ @Devil07 Which line are you getting that from? For that matter, what you just asked is its own question. suggest you ask it. I am addressing the diasabling of that feature. Whatever disables the trim beyond manual control is what disables the MCAS function. Apparently, trim over ride from the yoke does not. I've seen an explanation of the details ... give me a chance to find a link. $\endgroup$ – KorvinStarmast Mar 15 at 20:28
  • $\begingroup$ @Devil07 FWIW, I have been watching the arguments about the stab trim system and this systems interaction for a few months, since Lion Air, and am leary of jumping into that briar patch. $\endgroup$ – KorvinStarmast Mar 15 at 20:33
  • $\begingroup$ you're answer is great, I'm just trying to understand how it all works together. If MCAS causes aircraft to pitch nose down when it senses excessive AOA, then doesn't it do it through use of trim? If so, when MCAS is pitching nose down, won't the trim wheels physically turn by themselves? Your graphic says "MCAS moves horizontal stabilizer trim". $\endgroup$ – Devil07 Mar 15 at 20:33
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    $\begingroup$ To answer your question, yes, as explained in this video $\endgroup$ – KorvinStarmast Mar 16 at 13:51
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enter image description here

You can disable the stabaliser trim which will prevent MCAS from making inputs to the aircraft.

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    $\begingroup$ This is what I was thinking. If MCAS is pitching nose forward through use of stab trim, then the trim wheel will be visibly spinning forward (or will it?), if it is, then disable it and take manual control of trim. $\endgroup$ – Devil07 Mar 15 at 20:13
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    $\begingroup$ Yes, the trim wheels will spin in case MCAS sends input. By disabling the stab trim, inputs to it by any system will be disabled. $\endgroup$ – Windshear Mar 15 at 20:43
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    $\begingroup$ that what I thought. Good answer and great picture! $\endgroup$ – Devil07 Mar 17 at 5:50
  • $\begingroup$ (stabaliserstabiliser) $\endgroup$ – Peter Mortensen Mar 17 at 10:46
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From what I have read, the other answers explain the cutout procedure correctly. It is also important to keep in mind Korvin's remark that trim anomalies can occur for a variety of reasons. What I have not seen mentioned often or clearly enough is a crucial difference that distinguishes MCAS from regular trim control, a difference which was apparently not communicated clearly enough to the pilots. As an article by Leeham news points out:

[MCAS is] not stopped by the Pilot pulling the Yoke, which for normal trim from the autopilot or runaway manual trim triggers trim hold sensors.1

This is a difference to previous B737 models which was not mentioned in the manual (as there was no reference to the MCAS at all) and thus was unknown to pilots before the FAA advisory was published.2 From the NASA incident reports published in the Atlantic it becomes clear that the FAA advisory after the LionAir crash was discussed among the pilots. Apparently there were a few incidents where performing the proper cutout procedure saved the airplanes, among them the preceding LionAir flight.


1 The reason for this design decision being that providing the pilot a way to easily override

would negate why MCAS was implemented, the Pilot pulling so hard on the Yoke that the aircraft is flying close to stall.

In other words, MCAS is a mechanism installed to correct an already present pilot error. Logically, this corrective intervention by the flight computer can only be cancelled through a dedicated operating sequence. It's also worth mentioning that even extreme yoke action, working on the elevators only, is not able to fully compensate extreme stabilizer positions, the control surfaces MCAS uses (apart from, I suppose, creating unpleasant flight characteristics).

2 An article in the Seattle Times on March 17, 2019 presents that argument as well.

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