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New sources such as Seattle Times and Aviation Week provide a pretty good overlay of the MCAS function on B737 MAX. However, I'm unclear as to the issue that MCAS is attempting to fix. From what I can gather, the larger nacelle led to a shortening of the pylon height, leading to altered aerodynamics at the wing/pylon/nacelle junction. At high AOA, this leads to a higher pitch-up moment than on the NG (dispelling other popular theories, it has nothing to do with the thrust line or nacelle CG).

In Seattle Times, it talks about shock formation at the junction, indicating that this is not a low Mach problem. However, transonic regime is usually not limited by stall but by deterrent buffet. Can someone with the knowledge clarify? If it is a stall issue, is it a lack of stall identification (lack of distinct pitch down) or is the aircraft experiencing a pitch-up without MCAS?

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    $\begingroup$ All airplanes stalls. Most airplanes only stalls when the pilot instructs it to do so. The problem of Max is, it could stall itself, slowly. MACS is added to correct itself from doing so. $\endgroup$ Apr 28, 2019 at 16:32
  • $\begingroup$ @user3528438 How is that different than a 737NG? $\endgroup$ Apr 28, 2019 at 16:44
  • $\begingroup$ The Ethiopia and Indonesia crashes did not happen slowly! $\endgroup$ Apr 28, 2019 at 16:55
  • $\begingroup$ Closely related: aviation.stackexchange.com/questions/61910/… $\endgroup$ Apr 28, 2019 at 17:31
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    $\begingroup$ @Harper They didn't crash from a stall, they crashed when the MACS tries to correct a false stall, and it did happen across several minutes. $\endgroup$ Apr 29, 2019 at 0:18

3 Answers 3

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From The inside story of MCAS: How Boeing’s 737 MAX system gained power and lost safeguards by Dominic Gates and Mike Baker:

During flight tests to certify an airplane, pilots must safely fly an extreme maneuver, a banked spiral called a wind-up turn that brings the plane through a stall. While passengers would likely never experience the maneuver on a normal commercial flight, it could occur if pilots for some reason needed to execute a steep banking turn.

Engineers determined that on the MAX, the force the pilots feel in the control column as they execute this maneuver would not smoothly and continuously increase. Pilots who pull back forcefully on the column — sometimes called the stick — might suddenly feel a slackening of resistance. An FAA rule requires that the plane handle with smoothly changing stick forces.

The lack of smooth feel was caused by the jet’s tendency to pitch up, influenced by shock waves that form over the wing at high speeds and the extra lift surface provided by the pods around the MAX's engines, which are bigger and farther forward on the wing than on previous 737s.

(...)

Under the proposal, MCAS would trigger in narrow circumstances. It was designed "to address potentially unacceptable nose-up pitching moment at high angles of attack at high airspeeds," Boeing told the FAA in a proprietary System Safety Assessment reviewed by The Times.

Another article from The Seattle Times published in March makes mention of a high speed stall:

Designed to activate automatically only in the extreme flight situation of a high-speed stall, this extra kick downward of the nose would make the plane feel the same to a pilot as the older-model 737s.

This indeed points to a stick-force-per-g test, as @Jimmy mentions in a comment. Since the B737 has a fully irreversible hydraulically actuated flight control system, with an artificial feel that is proportional to dynamic pressure but not to load factor, the obvious mechanism for "lack of smooth force feel" would be having to release the column to some extent due to extra nose-up moment that the aircraft generates.

It is very plausible that the added lifting surfaces from the engine pods create a larger pitch-up moment than the 737NG has in the same circumstances. I'm not really sure how the supercritical airflow over the MAX wing differs from that of the NG to create extra pitch-up.

All of that is the original limited cure for the stick-force-per-g tests. If this was the only thing that needed to be fixed, all would probably have remained well. But the first linked article also mentions:

The flight-test pilots had found another problem: The same lack of smooth stick forces was also occurring in certain low-speed flight conditions. To cover that issue too, engineers decided to expand the scope and power of MCAS.

As you can see, The Seattle Times is my source of information as well...there is no additional information on what were the "certain low-speed flight conditions".

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  • $\begingroup$ So this sounds like a buffet penetration issue more than a stall issue? $\endgroup$
    – JZYL
    Jul 21, 2019 at 5:20
  • $\begingroup$ The way I understand it, the forward mounting causes an extra pitch-up moment in a particular situation: high AoA, high airspeed, high bank angle. Flow separation would seem a more likely cause for this than buffeting. $\endgroup$
    – Koyovis
    Jul 21, 2019 at 6:13
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    $\begingroup$ The fact that it is a WUT means it's a stick-force-per-G test. Coupled with reference to high speed and shock formation (assuming the same author got it right), it sounds like a maneuvering characteristic (i.e. stick force per G) issue at high Mach, rather than a stall issue. Whether the uncertifiable characteristic occurs prior to buffet onset or during buffet penetration is not clear. I guess that's where the name MCAS comes from! $\endgroup$
    – JZYL
    Jul 21, 2019 at 6:57
  • $\begingroup$ Indeed, that is the way I understood the original design purpose of MCAS as well, although the original article I read in March makes mention of a high speed stall. The SeatlleTimes article you refer to indeed mentions shock waves and the increased lifting surface from the pods in front of the wing. $\endgroup$
    – Koyovis
    Jul 21, 2019 at 11:39
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    $\begingroup$ How does stick force reversal work with a fully irreversible actuation system? $\endgroup$
    – Koyovis
    Jul 22, 2019 at 2:06
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Was there a problem with stalling on 737 jets before the 737 Max,

The flight characteristics, including high angle of attack operations, of the previous models of the 737 did not warrant the kind of action the MCAS system provides.

why is MCAS needed on the 737 Max 8

The Max 8 engines are physically bigger—69.4 inches fan diameter versus 61.0—heavier, mounted farther forward and higher than previously. This changed the thrust line and flight characteristics. MCAS is used to restore "normal" 737 flight characteristics.

See http://www.b737.org.uk/737maxdiffs.htm for details.

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  • $\begingroup$ Would you agree with the phrase that was added to your answer without any reference? "In particular, the new engine cowlings added enough forward lifting area to affect the plane's handling at high pitch angles." $\endgroup$
    – Koyovis
    Feb 12, 2020 at 23:19
  • $\begingroup$ @Koyovis To the best of my knowledge, I didn't post this answer even though it's attributed to me. I say to the best of my knowledge because I am subject to some dementia, and I've learned I can't always trust my memory anymore. However, I've never flown any model of the 737 and really know nothing of it. My large aircraft were the 727-100 and 747-100/200. I looked at the link in the answer, and I'm as certain as I can be that I had never seen that web page before. I don't know what's going on. $\endgroup$
    – Terry
    Feb 13, 2020 at 0:06
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    $\begingroup$ @Jpe61 Please respect the original poster of the answer. I would never fundamentally change or add to an answer of another user here, but would post my own answer if I reckoned more details are required.. Now Terry's name and face is underneath some text that he never published.. $\endgroup$
    – Koyovis
    Feb 13, 2020 at 8:05
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    $\begingroup$ @Koyovis, sure thing, rolled back. I do not understand how this improves the answer though. The recently added edit by Guy should be removed too. $\endgroup$
    – Jpe61
    Feb 13, 2020 at 8:10
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    $\begingroup$ @Jpe61 Agreed, did so. $\endgroup$
    – Koyovis
    Feb 13, 2020 at 8:13
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Previous 737 models had acceptable stall characteristics.

The Max introduced new and larger engines. There was not enough room for them between the wing and the ground, so they were moved forwards further in front of the wing to make room for them to sit high enough and clear the ground. This affected the handling of the Max at high pitch angles.

But the intention was to introduce the Max without the need for significant pilot re-training and type re-certification, so the change in handling was unacceptable. MCAS - the Manoeuvring Augmentation Control System - was introduced to manage the aircraft handling at high pitch angles and avoid the need for pilot re-training.

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