I am not and never was a 737 pilot, neither was I a test pilot, however I did do many CofA air tests on VC10s, 707s and 747 aircraft, all of which involved stall tests. So may I try to disentangle some of the issues that have been brought up on this thread.
First of all, STS and MCAS are entirely different systems, although they do meet similar needs. STS is not an auto trim system. Indeed, it normally functions opposite to trim. And it is not designed to "prevent a stall condition." It is there to ensure adequate longitudinal stability
before the stall. The Boeing 737 MAX FCOM states:-
The Speed Trim System (STS) is a speed stability augmentation system designed to improve flight characteristics during operations with a low gross weight, aft center of gravity and high thrust when the autopilot is not engaged. The purpose of the STS is to return the airplane to a trimmed speed by commanding the stabilizer in a direction opposite the speed change. The STS monitors inputs of stabilizer position, thrust lever position, airspeed and vertical speed and then trims the stabilizer using the autopilot stabilizer trim. As the airplane speed increases or decreases from the trimmed speed, the stabilizer is commanded in the direction to return the airplane to the trimmed speed. This increases control column forces to force the airplane to return to the trimmed speed. As the airplane returns to the trimmed speed, the STS commanded stabilizer movement is removed.
This is also well described here in the PPRuNe Tech Log by FCeng84:-
B-737 Speed Trim System
For the stall, the basic requirements (in laymans’ language) are that the aircraft should have acceptable flight characteristics before, at and after the stall. By this I mean it has (a) acceptable longitudinal stability as the speed decays before the stall; (b) adequate stall warning (either from natural aerodynamic buffet or artificial devices such as a stick shaker); (c) clear identification of the stall itself (either by the nose dropping at the break point or, in the absence of this, a stick pusher); and (d) no unusual or undesirable characteristics at or immediately after the stall such as a pitch-up, a severe wing drop or entry into a spin.
In the case of T-tail aircraft, where it is possible to become locked into a deep stall, it is essential that the stick pusher functions at or before the stall and has a phase advance which makes it push early in a dynamic situation - sufficiently early to prevent an ‘over-swing’ of pitch to too high an AoA.
When Boeing designed the 737 MAX with the bigger diameter LEAP engines it was necessary to position the nacelles further forward than before. Unfortunately, when at high AoAs, the nacelles generate a certain amount of aerodynamic lift which degrades the longitudinal stability. Thus, it was decided to incorporate the MCAS. It is separate from the STS and fulfils a similar but different function by running the stabliser nose down to ensure adequate longitudinal stability at high AoAs. Unfortunately a description of this system was not included in any of the Boeing manuals available to pilots. There is a good description here:-
https://leehamnews.com/2018/11/14/bo...to-the-pilots/
Silverstrata has quoted part of the note below in response to an earlier post by me:-
MCAS (Maneuvering Characteristics Augmentation System) is implemented on the 737 MAX to enhance pitch characteristics with flaps UP and at elevated angles of attack. The MCAS function commands nose down stabilizer to enhance pitch characteristics during steep turns with elevated load factors and during flaps up flight at airspeeds approaching stall. MCAS is activated without pilot input and only operates in manual, flaps up flight. The system is designed to allow the flight crew to use column trim switch or stabilizer aislestand cutout switches to override MCAS input. The function is commanded by the Flight Control computer using input data from sensors and other airplane systems.
The MCAS function becomes active when the airplane Angle of Attack exceeds a threshold based on airspeed and altitude. Stabilizer incremental commands are limited to 2.5 degrees and are provided at a rate of 0.27 degrees per second. The magnitude of the stabilizer input is lower at high Mach number and greater at low Mach numbers. The function is reset once angle of attack falls below the Angle of Attack threshold or if manual stabilizer commands are provided by the flight crew. If the original elevated AOA condition persists, the MCAS function commands another incremental stabilizer nose down command according to current aircraft Mach number at actuation.
I emphasise again, the MCAS is not an anti-stall device. It neither detects the stall nor acts as any kind of stick pusher. It is there to maintain the correct longitudinal stability
before the stall in order to meet the certification requirements.
FAA Part 25 regulations require that there be a minimum stick force per knot when speed is changed from the trim condition. The MCAS is a design solution (a software fix?) applied to the automatics to increase the required force, giving a degree of "artificial stability" by trimming the stabiliser nose down when hand flying. The paragraphs on longitudinal stability are 5.2.2.1.2, 7.2.1.1.4 and 7.2.2.2.3. See here:-
https://www.faa.gov/documentLibrary/...r/AC_25-7D.pdf
The relevant point in para 7.2.1.1.4 says (and I quote):-
The average gradient of the stick force versus speed curves for each test configuration may not be less than 1 lb for each 6 knots for the appropriate speed ranges specified in § 25.175. Therefore, after each curve is drawn, draw a straight line from the intersection of the curve and the required maximum speed to the trim point. Then draw a straight line from the intersection of the curve and the required minimum speed to the trim point. The slope of these lines must be at least 1 lb for each 6 knots. The local slope of the curve must remain stable for this range.
It contains a diagram which is similar to this one in Fig 1 in this Boeing document:-
https://www.boeing.com/commercial/ae...y/fo01txt.html
Finally, if anyone wants to know more about trimming, may I recommend this article by Captain Alex Fisher:-
https://www.skybrary.aero/bookshelf/books/2627.pdf
I apologise for the length of this post. If any of you think I am talking Bollux, please feel free to fire a suitable broadside! As I said at the beginning, I never flew 737s and, as you will see from my age, I am now somewhat out-of-date.