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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 '19 at 16:32
  • $\begingroup$ @user3528438 How is that different than a 737NG? $\endgroup$ Apr 28 '19 at 16:44
  • $\begingroup$ The Ethiopia and Indonesia crashes did not happen slowly! $\endgroup$ Apr 28 '19 at 16:55
  • $\begingroup$ Closely related: aviation.stackexchange.com/questions/61910/… $\endgroup$ Apr 28 '19 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 '19 at 0:18
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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 '19 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 '19 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 '19 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 '19 at 11:39
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    $\begingroup$ How does stick force reversal work with a fully irreversible actuation system? $\endgroup$
    – Koyovis
    Jul 22 '19 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 '20 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 '20 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 '20 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 '20 at 8:10
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    $\begingroup$ @Jpe61 Agreed, did so. $\endgroup$
    – Koyovis
    Feb 13 '20 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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One last attempt to point out the elephant in the room and please don't delete this just because it is my answer. By all means, feel free to criticize it.

What everybody seems to not want to see, is that trim issues are supposed to be non critical by definition. Regardless of what controls the trim surfaces. That goes for all aircraft,... except the 737.

What that means is that these crashes expose a biased certification culture between Boeing and the FAA that dates back 60 years. This played a crucial role in the success of all 737 models. It gave the 737 an advantage over its competitors. MCAS puts that advantage at the expense of aviation safety. It is cutting an already cut corner. Mad Max on top of Mad Max. That is what makes it so dangerous. That is the reason for the crashes and all the dead people. Sweet talking that is not very productive.

The 737 is by concept not overly stable. MCAS attempts to overcome an unacceptable part of that. It does so by controlling the pitch using the trim surfaces. The 737 configuration does not allow for automated trim surface control. It effectively puts two pilots behind the yoke. Should MCAS fail, one of those pilots trims down, while the other pitches up. That, no 737 can not deal with. (due to what happened in the 60's)

EDIT: To answer the question: "What is MCAS trying to fix on B737 MAX?" you can look in two directions. Either you conclude that it must be something unique to the 737 Max, so you look at those things. That is to an enormous extend jumping to conclusions, because that is by no means certain. The other thing to look at is what MCAS does. For some strange reason, I'm the only one looking in that direction. Worse than that, I get people aggravated by writing about it. Not because its a wrong answer. If you change the question to: "What does MCAS do on B737 MAX?" which is not all that much of a change, it clearly points the way I'm looking. Meanwhile a question asking for a reason gets nothing but excuses. Its as if I'm insulting someones religion. Totally weird. MCAS was designed to work on dedicated trim surfaces only, which the 737 simply doesn't have. But hey, who cares? The craziest thing about MCAS is, that if Boeing would have simply not installed MCAS, that most likely would never have resulted in any crash. Ask any capable and experienced Max pilot. Sure the pitch would surprise him, but no way would it make him crash. At the angles at which this happens, he is concentrated on flying the airplane. Its not a whole lot different from weird heavy turbulence and he can easily handle that. MCAS serves absolutely zero aerodynamic purpose. No aircraft does, ever did or ever will need MCAS. MCAS is faking it for the money. The reason it failed is for the money. Yes, it sells lives. The thing it is trying to fix on the 737Max is the financial insolvability of the project. Nothing more, nothing less. If you don't want to know that answer, then what's the purpose of asking the question?

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    $\begingroup$ Here's my opinion on why this answer is not great. Lets remember that the question asks "What is MCAS trying to fix" (ie, what is it's purpose). Your answer starts by talking about FAA requirements. Next it says how MCAS complies with this. Then it describes what it is, not what it's purpose is. So far you're 3 paragraphs in and have gone nowhere near the question. Then you go on to describe its deficiencies. Finally you talk about Boeing's motivation in creating it. Nowhere do you actually answer the question satisfactorily - maybe 1 or 2 sentences which might go some way to doing so. $\endgroup$
    – Jamiec
    May 4 at 15:48
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    $\begingroup$ I admit I write lousy answers. I am not trying to please anyone or get credits, which makes my answers less consumable I guess, but they are not wrong or based on just my opinion. $\endgroup$
    – Berend
    May 4 at 16:36
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    $\begingroup$ It's not about pleasing people or getting credit, it's about providing an answer to the question asked. That's why this is a Question and Answer site, not a discussion forum. A "lousy answer" is one that doesn't address the question at all, even if it's just opinion based. $\endgroup$
    – FreeMan
    May 4 at 16:43
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    $\begingroup$ @Freeman Does this answer address the question? I really tried to make it do so. $\endgroup$
    – Berend
    May 4 at 16:47
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    $\begingroup$ Can you tell me how this does not address the question? I mean, the question asks for the issue that made MCAS necessary, right? $\endgroup$
    – Berend
    May 4 at 16:59

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