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I’m confused on this. I’ve read in multiple different places that

MCAS was required due to the easier stick forces at a high angle of attack and a high G force, and because of this, the Max didn’t meet FAA longitudinal stability

Others have said that

The Max has MCAS because they wanted it to feel like the NG

Confirming the easier forces, but excluding the FAA longitudinal stability possibility. Almost every source contradicts another one, and I’m unable to locate any info through the FAA site.

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The Boeing 737 MAX MCAS system is there ONLY to meet the FAA longitudinal stability requirements as specified in FAR Section 25.173, and in particular part (c) which mandates "stick force vs speed curve:, and also FAR Section 25.203 — "Stall characteristics".

FEDERAL AVIATION REGULATIONS Sec. 25.173 — Static longitudinal stability.

Sec. 25.173 — Static longitudinal stability.

Under the conditions specified in §25.175, the characteristics of the elevator control forces (including friction) must be as follows:

(a) A pull must be required to obtain and maintain speeds below the specified trim speed, and a push must be required to obtain and maintain speeds above the specified trim speed. This must be shown at any speed that can be obtained except speeds higher than the landing gear or wing flap operating limit speeds or V FC /M FC,whichever is appropriate, or lower than the minimum speed for steady unstalled flight.

(b) The airspeed must return to within 10 percent of the original trim speed for the climb, approach, and landing conditions specified in §25.175 (a), (c), and (d), and must return to within 7.5 percent of the original trim speed for the cruising condition specified in §25.175(b), when the control force is slowly released from any speed within the range specified in paragraph (a) of this section.

(c) The average gradient of the stable slope of the stick force versus speed curve may not be less than 1 pound for each 6 knots.

(d) Within the free return speed range specified in paragraph (b) of this section, it is permissible for the airplane, without control forces, to stabilize on speeds above or below the desired trim speeds if exceptional attention on the part of the pilot is not required to return to and maintain the desired trim speed and altitude.

FEDERAL AVIATION REGULATIONS Sec. 25.203 — Stall characteristics.

Sec. 25.203 — Stall characteristics.

(a) It must be possible to produce and to correct roll and yaw by unreversed use of the aileron and rudder controls, up to the time the airplane is stalled. No abnormal nose-up pitching may occur. The longitudinal control force must be positive up to and throughout the stall. In addition, it must be possible to promptly prevent stalling and to recover from a stall by normal use of the controls.

(b) For level wing stalls, the roll occurring between the stall and the completion of the recovery may not exceed approximately 20 degrees.

(c) For turning flight stalls, the action of the airplane after the stall may not be so violent or extreme as to make it difficult, with normal piloting skill, to effect a prompt recovery and to regain control of the airplane. The maximum bank angle that occurs during the recovery may not exceed—

(1) Approximately 60 degrees in the original direction of the turn, or 30 degrees in the opposite direction, for deceleration rates up to 1 knot per second; and

(2) Approximately 90 degrees in the original direction of the turn, or 60 degrees in the opposite direction, for deceleration rates in excess of 1 knot per second.

Here is some additional information: http://www.b737.org.uk/mcas.htm

MCAS is a longitudinal stability enhancement. It is not for stall prevention (although indirectly it helps) or to make the MAX handle like the NG (although it does); it was introduced to counteract the non-linear lift generated by the LEAP-1B engine nacelles at high AoA and give a steady increase in stick force as the stall is approached as required by regulation.

The LEAP engine nacelles are larger and had to be mounted slightly higher and further forward from the previous NG CFM56-7 engines to give the necessary ground clearance. This new location and larger size of nacelle cause the vortex flow off the nacelle body to produce lift at high AoA. As the nacelle is ahead of the C of G, this lift causes a slight pitch-up effect (ie a reducing stick force) which could lead the pilot to inadvertently pull the yoke further aft than intended bringing the aircraft closer towards the stall. This abnormal nose-up pitching is not allowable under 14CFR §25.203(a) "Stall characteristics". Several aerodynamic solutions were introduced such as revising the leading edge stall strip and modifying the leading edge vortilons but they were insufficient to pass regulation. MCAS was therefore introduced to give an automatic nose down stabilizer input during elevated AoA when flaps are up.

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  • $\begingroup$ I doubt the MAX saga has anything to do with static long stab. $\endgroup$ – JZYL Jan 6 at 13:13
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From publicly available sources, the first motive of the MCAS was to satisfy the stick force per G requirements, or maneuvering stability (not static longitudinal stability, which deals with speed stability). This is confusingly captured in 14 CFR 25.255(b) and (c) under Out-of-Trim Characteristics, but also applies broadly to buffet characterization at mid/high Mach (see AC 25-7C 25.251):

(b) In the out-of-trim condition specified in paragraph (a) of this section, when the normal acceleration is varied from + 1 g to the positive and negative values specified in paragraph (c) of this section—

(1) The stick force vs. g curve must have a positive slope at any speed up to and including VFC/MFC; and

(2) At speeds between VFC/MFC and VDF/MDF the direction of the primary longitudinal control force may not reverse.

(c) Except as provided in paragraphs (d) and (e) of this section, compliance with the provisions of paragraph (a) of this section must be demonstrated in flight over the acceleration range—

(1) −1 g to + 2.5 g; or

(2) 0 g to 2.0 g, and extrapolating by an acceptable method to −1 g and + 2.5 g.

This is only required for Mach 0.6 and above, since this is where transonic effect sets in. And indeed, according to the Seattle Times report, the shock formation induced from the pylon/wing junction made the maneuvering characteristics uncertifiable, hence the conceptualization of MCAS. Note that MCAS stands for Maneuvering Characteristics Augmentation System, and it originally required the detection of high load factor, as well as high AOA in one of the vanes, in order to activate.

Later on in the flight test program, there were further complications from the nacelles in the low-speed stall evaluation, which is correctly cited in @MikeSowsun's answer (required by 25.203). However, stall evaluation, unlike stick force/G evaluation, occurs at 1G (or 1.15G in turning stall); therefore, the G load activation requirement was removed, and MCAS became a single source activation feature.

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The MAX feels like the NG, which feels like the Classic, which feels like the -200. The flight control artificial forces are identical across the different versions, which share a type rating. No need for MCAS there.

Also, the MAX is longitudinally stable. MCAS was implemented for tackling some situations in extreme corners of the flight envelope, as mentioned in this answer and a few others.

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    $\begingroup$ Just one tiny note: any B737-900/-800/-700/-600/-500/-400/-300/-200/-100 is considered by BCA a “Classic” since this term applies to any airplane no longer in production. $\endgroup$ – Carlo Felicione Jan 5 at 18:43
  • $\begingroup$ @CarloFelicione Then please edit the 737 tag wiki on this site, and the Wikipedia pages for B737. $\endgroup$ – Koyovis Jan 7 at 3:02

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