What is MCAS trying to fix on B737 MAX?

Question

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?

Answer 1

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".

Answer 2

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.

Answer 3

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.

Answer 4

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. Its as if I'm insulting someones religion. Totally weird. 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 Ralph J. Sure the pitch would have surprised him, but no way would it have made him crash. At the angles at which this happens, he is flying the airplane. Its not a whole lot different from weird heavy turbulence and he can handle that easy. My opinion, if you don't want to know an answer, don't ask the question.