According to Boeing's April 17, 2019, update MCAS is a system that enhances “pitch stability”:

“The Maneuvering Characteristics Augmentation System (MCAS) flight control law was designed and certified for the 737 MAX to enhance the pitch stability of the airplane – so that it feels and flies like other 737s.”

Boeing: The 737 MAX MCAS Software Enhancement

They are talking about stability in a different context.

The handling characteristic that MCAS addresses is the pitch feel at very high angles-of-attack. The pitch feel is lighter which could lead a pilot, accustom to the pitch feel of the 737 NG, to over-control and apply too much nose-up elevator input. MCAS provides a nose-down bias, through the input of nose-down stabilizer trim, to produce a pitch feel that is similar to the 737 NG.

The relaxed stability systems, that are used on some airplanes, reduce the static stability around the lateral axis (pitch) by moving the aircraft's CG aft. This reduces the moment-arm between the CG and center-of-lift (CL) which reduces both drag and stability. This is done during all phases of flight, or during high-speed and cruise flight, so as to maximize the benefit from the reduced drag. Such an airplane would be difficult to fly due to the constant need to correct pitch so stability augmentation systems are needed.

The 737 does not use a relaxed stability system, such as what you described in your post titled "Relaxed Static Stability (RSS)".

The MAX's heavier engines, positioned farther forward, would tend to produce a more stable aircraft, all else equal, due to the more forward CG location. The lighter pitch feel at high AoA comes from the larger nacelles producing more lift in those non-normal situations. It is not a condition that the airplane would experience during normal operations. It is not a feature that was designed in to increase efficiency as is a relaxed stability system.

Do They Need to Do a D Check For Each Max?
 

"(CNN)The day after Ethiopia's minister of transportation released a preliminary crash report on Ethiopian Airlines flight 302, four Boeing employees called an Federal Aviation Administration whistleblower hotline that allows employees and the public to report aviation safety issues. .........."

"Among the complaints is a previously unreported issue involving damage to the wiring of the angle of attack sensor by a foreign object, according to the source."

"Boeing has reportedly had previous issues with foreign object debris in its manufacturing process; The New York Times reported metal shavings were found near wiring of Boeing 787 Dreamliner planes, and the Air Force stopped deliveries of the Boeing KC-46 tanker after foreign object debris was found in some of the planes coming off the production line."


https://www.cnn.com/2019/04/26/polit...rts/index.html

Betteridge's law of headlines applies, so no.

How long the 737-MAX9 grounding? When they will put back in service?

Unknown.

Boeing has not submitted the fix to FAA - so (re)certification process has not even started.

AA and WN pushing back MAX on the schedule to August (for now) should be a hint.

So, is a current 737 pilot certified to fly the MAX with MCAS permanently turn off? If yes, just turn off the questionable feature, why the grounding is necessary? If not, then why the solution of turning off MCAS when it is faulted is even acceptable?

You cannot turn off MCAS by itself. You can only turn off electric stab trim, which is used in many other situations completely unrelated to MCAS. I think it would be quite a large inconvenience to do manual trim all the time.

So MCAS can't be "turned off." Except that now, according to the "fixes," it'll only operate once, in extremely rare instances, and then turn itself off. One time, a small nose-down, and then "I give up - you pilot this thing."

So, again: What's this thing good for?

Pitch feel in high AoA attitudes.

Funny you should say that, "cancels" is nuanced:
"We will replace the orders with Boeing's Max 10s and 787s," Garuda President Ari Ashkara told the Nikkei Asian Review on Wednesday.

In a high AoA attitude. Singular. Because the "revised" software will activate MCAS ONCE, pitch the nose down a smaller amount than previously...and then shut. off.

So, again: What's this thing good for?

Yep, several here have tied themselves in knots justifying things. I'm not satisfied.