You have a very reasonable question, rb1957. As I understand from the data I have read, Boeing discovered during certification that the MAX aircraft as designed without the MCAS had a problem meeting the flight performance requirements. This was due to the engines being moved forward and raised up in relation to their position on all of the non-MAX 737 aircraft. I believe (more qualified persons, feel free to chime in and correct me) that the altered Center of Gravity with respect to the Center of Lift caused the aircraft to not be as recoverable as required during high angle of attack upsets. So I believe the MCAS system was the way Boeing intended to compensate for this degradation of high angle of attack recovery performance, by forcing the nose down prior to reach this area of degraded performance.

If I am correct, then since I am not privy to the Boeing internal process, I would speculate that Boeing considered the MCAS to be a better solution than attempting to alter the stick pusher function. And more speculation on my part would be that the stick pusher only involves the elevators, and Boeing decided that the stabilizer trim system was required to compensate for this new challenge due to its far greater impact on aircraft pitch under the imposed flight loads. In short, the elevator alone was insufficient to resolve this new challenge.

So the bottom line would be (again, speculation on my part) the "good systems" as you stated that protected the plane prior to the MAX relocation of the engines to accommodate their larger size no longer protected the plane per regulatory requirements, and moving the stabilizer trim with its greater ability to manage pitch attitude versus flight loading was chosen as the fix.

Darn good question though. I wish I had access to more accurate and detailed "insider" information.

The CG has to be where the CG has to be; the engines don't change that.

What did change is the size of the engines leading to an increase in the lift the engines produce at high angle of attack.

MCAS was for a regulatory requirement that stick force be linearly proportional to AoA. With the additional lift as AoA increased this requirement was not met in the MAX series, hence the use of stab trim to linearize the force. Note that enlarging the stab would likely not have linearized the response as the engines provide a non-linear input.

If MCAS is not enabled then there is a small increase in the chance that the pilot will fully stall the aircraft if it is operated at low speed with no flaps. Note that MCAS is not enabled when flaps are deployed.

A much better explanation than my speculation. Thanks 3DDave! Star!

thx 3DDave. so you'd think the MCAS is on the MMEL (required for take-off, but it can be disabled in flight) ?

I think we're on the same page. I don't think Boeing did a sloppy job certifying the Max. I'm sure a search-light was shone on the MCAS and if anything was amiss it would have been found and discussed. I guess they were caught out be the reliability of the sensor ? Their previous fix was (I think) mostly training, now because of the "fuss" they're doing more (that they don't really need to do, but need to be seen doing).

another day in paradise, or is paradise one day closer ?

It isn't required for take-off or landing it is inhibited by flaps. It's for protection primarily in manual control with flaps up; e.g. Cruise. It's software, so it cannot be MEL. The AoA system is MEL, and Lion Air took off with it still defective like it was the day before.

3DDave, you are clearly more up to date than I am on this incident. I would like to verify one point you made.

"The AoA system is MEL, and Lion Air took off with it still defective like it was the day before."

It is obvious from the analysis report that you are entirely correct in the technical sense, in that Lion Air took off with the AoA still defective. However, correct me if I err, but I believe the maintenance team used the required procedures to troubleshoot and clear the logbook entry. So from an airworthiness standpoint, the fault was cleared, although subsequent events prove that the required procedures were insufficient to diagnose and clear the actual fault.

If I am correct, then the accident flight crew took off with the belief that the fault actually had been cleared. Would you agree with this assumption on my part?

I'm sure the accident crew thought that the ground crew had addressed the problem. I suspect they went through the proscribed procedures ("went through the motions") and "no fault found".

another day in paradise, or is paradise one day closer ?

"If I am correct, then the accident flight crew took off with the belief that the fault actually had been cleared. Would you agree with this assumption on my part?"

According to the flight recorder data images presented, only one AoA sensor was indicating the correct value prior to takeoff.

TTFN (ta ta for now)
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I muffed this - MEL is a list of broken equipment that a plane is certified to fly with. Apparently the default is that everything on a plane has to be fully functional, no matter how small, and the MEL is a list of exceptions with specific limitations on how long/how much the plane can be operated in this condition. It makes sense rather than making a list of 10,000 items that represent all the major parts of a plane to be checked at each flight.

AFAIK the maint crew did not correctly diagnose the problem and poked at some other area and considered what they did as good. They did nothing to confirm that the system functioned correctly under flying conditions.

This is in part because the previous crew misrepresented the extent of the problem and failed to note that stab trim ran sporadically on its own until they were forced to shut it off.

AFAIK the flying crew had no information about the fault as experienced and were therefore completely unprepared to deal with the problem when it recurred.

3DDave, IRstuff and rb1957, good points. As implied by 3DDave, I too would like to read the logbook entry myself from the pre-accident crew to see how they wrote up the upset. I have a lot of personal experience with flight crews (mostly military) being too brief and too generic in the written write-ups. That is why in my younger days (a looooong time ago) I found that being assigned to the maintenance team that performed the mandatory de-brief of each B-52H flight crew was tremendously beneficial as I could ask questions and get much more detailed dissertations from the crew to "flesh out" the abbreviated logbook entries.

When they released the preliminary official report concerning the initial discovery of the problem they failed to note that there was a third pilot deadheading in the cockpit who told the pilots about the trim shutoff. Yeah. People died and they still tried to hide an important fact.

Supposedly, per the report, "After parking, the PIC informed the engineer about the aircraft problem and entered IAS and ALT Disagree and FEEL DIFF PRESS problem on the AFML." Nothing about flying with a stick shaker for nearly 2 hours. Nothing about shutting off the stab trim. Nothing about the trim fighting them for 10 minutes.