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With regard to the recent crashes involving the Boeing 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 for an excessively high angle of attack (pitch attitude), what is the technique or procedure to 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 to disable/disengage the MCAS from pitching the nose down.

UPDATE: This video is the most concise answer to my question that I have found. It shows a crew in a full motion simulator like the Boeing 737 Max 8 that crashed in Ethiopia, then it shows the proper technique to disengage the MCAS and manually trim up in the same scenario.

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closed as off-topic by Jamiec Jun 3 at 7:32

This question appears to be off-topic. The users who voted to close gave this specific reason:

  • "This question does not appear to be about aviation, within the scope defined in the help center." – Jamiec
If this question can be reworded to fit the rules in the help center, please edit the question.

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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
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    $\begingroup$ Note: the new video is not a Max 8 simulator, it's an NG. Also worth mentioning that with MCAS disabled (your question), it takes time to manually correct the trim. $\endgroup$ – ymb1 Apr 4 at 14:54
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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 concise description of the system I found here. The observation was a follow up to an observation that the MCAS isn't an anti stall feature like a stick shaker, but rather ...

  • Just about everything you have read in the media is wrong. MCAS is not stall protection. MCAS does not counter the additional thrust of the more powerful engines. It is only there because the larger engine nacelles of the B737 MAX cause an aerodynamic pitch up moment at high angles of attack that did not meet FAA longitudinal stability and stick force certification standards. The easiest fix was to automatically apply a little nose down trim at high angles of attack.

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, though the Max trim wheels being of a smaller diameter raises a training and effectiveness problem1 that is probably deserving of its own question and answer.
    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. As of mid 2019, the AoA signal (be it from signal error or a faulty AoA probe) has been identified as a part of the problem in this unexpected activation during takeoff and departure.


1 Note from @user40476, slightly edited:
If you are a bit late to switch off both CUT OFF switches, the deflected angle of the THS will create strong forces on the trim screw, more particularly if simultaneously you hold the yoke to maintain an acceptable pitch.
Critical Flight Safety Issue Follows
In these conditions, it becomes very hard to move the trim wheels manually. This is a major problem; the trim wheel diameter in the 737-MAX is much smaller then the trim wheel of the previous models: 737-100, 737-200 etc. This smaller diameter reducing the efficiency of the pilot force on the wheel handle. (Less leverage for the same amount of pilot force applied to a trim wheel).

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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
  • $\begingroup$ 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
  • $\begingroup$ Not my graphic, @Devil07, TheAirCurrent's -- :) While I think you are right, I just need to recall which argument I was reading to link your thought to that. Give me a few. There is some interesting discussion on that here, based on the Lion Air accident. See if it's helpful to you. Beware, a few of the posters there are sharp, and a few are trolls. $\endgroup$ – KorvinStarmast Mar 15 at 20:35
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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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    $\begingroup$ @user40476 Thanks for concisely stating that. I hope you concur with my slight editorial adjustment as I folded in your note. Your point certainly squares with what I've been reading about, and while related to this question, I think for stack exchange purposes, that specific issue is its own question (and for Boeing, its own fix/problem to remedy) among others that these MCAS malfunctions have brought to light. $\endgroup$ – KorvinStarmast May 29 at 12:24
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enter image description here

You can disable the stabiliser 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
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From what I have read, the other answers explain the cutout procedure correctly.

Edit: The ongoing investigation appears to reveal that the aerodynamic forces on the stabilizer may become so high that it becomes hard or impossible to manually correct a nose-down trim. (This is not a B373 MAX issue but concerns earlier models as well.) The forces increase, unfortunately, with upward elevator position and high air speed, which is the situation in the final nose-dives. The remedy is described in the page user40476 linked to:

If the stabilizer put the plane in an extreme nose down position the pilot was advised to first pull the column to decrease the speed. He then had to push the column forward to lower the aerodynamic forces that blocked the jackscrew. Then the manual trim wheel could be turned a bit while the plane continued to dive and again increased its speed. The procedure had to be repeated several times: pull column to decrease speed; push column to decrease the aerodynamic force on the stabilizer and its jackscrew; trim manually; repeat. The technic was known as the rollercoaster maneuver.

Other sites (https://theaircurrent.com/aviation-safety/vestigal-design-issue-clouds-737-max-crash-investigations/, http://www.b737.org.uk/mcas.htm#mantrim) agree that the pilots may simply have been unable to operate the manual trim wheels because of the high aerodynamic forces.

Therefore the simulation in the video you link to where the co-pilot appears to turn the wheel with relative ease is probably incorrect in this respect.

[End edit]

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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    $\begingroup$ But what if it's a system's error and not pilot's? Shouldn't egineers have considered this case? Knowing that this MCAS considers input from just 1 AoA, that pilots could be not be aware of its existence, and that in situations can be hard to track the problem and apply the correct solution, even if someway easy (stab trim off)... $\endgroup$ – ShinTakezou Mar 26 at 11:29
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    $\begingroup$ @ShinTakezou I agree with all your valid objections ;-). I just quoted possible rationales for the decisions as they were made. $\endgroup$ – Peter - Reinstate Monica Mar 26 at 12:49
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    $\begingroup$ I would say what you may read on the following website would also contribute to understand the difficulties that result from switching off the cutout switches: moonofalabama.org/2019/05/… $\endgroup$ – user40476 May 29 at 14:28
  • $\begingroup$ @user40476 Thank you for the pointer. It becomes clearer as the investigation advances how tragic these accidents were. The pilots may well have tried to do the right thing but the plane didn't let them, due to a flawed design. $\endgroup$ – Peter - Reinstate Monica May 30 at 6:51

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