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?


From a deleted question: by @ffejrekaburb

From an email from the author of the SeattleTimes article, Dominic Gates:

The description of MCAS provided by Boeing for regulators (FAA and foreign) during certification, is this:

MCAS “was added to address potential nose-up pitching moment at high angles of attack at high airspeeds outside the normal flight envelope.”

Elsewhere in the documents, it’s made clear that MCAS was expected to kick in when a MAX approached a “wind-up turn,” which is essentially a banked downward spiral. Of course a commercial jet would never in normal flight do such a maneuver. But in flight tests for certification, the test pilots are required to show that the plane can approach that and not lose lift on one wing and flip over.

Can you post that answer for me? Thanks, Dominic Gates

An article from the SeattleTimes published in March makes mention of a high speed stall. The SeatlleTimes article linked to in OP mentions that in a wind-up turn:

Pilots who pull back forcefully on the column — sometimes called the stick — might suddenly feel a slackening of resistance.

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 (engine) pods...

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 second 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 SeattleTimes 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
  • $\begingroup$ Thanks for the additional sources. I don't think that the "lack of smooth feel" comes from a servo limitation or the Mach trim. It's more likely that it refers to stick force lightening (uncertifiable prior to buffet onset) or reversal (uncertifiable up to deterrent buffet). Mach trim wouldn't have helped since this is maneuvering stability rather than speed stability. $\endgroup$ – JZYL Jul 22 '19 at 1:47

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