Originally Posted by
Bergerie1
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:-
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.
I would have to respectfully disagree that the MCAS is likely there to comply with FAA 25.175, along with the STS. Note the stick force requirement in that section is not at airspeed less than 1.3VSR. It seems more likely that is there to comply with 25.203 (stall characteristics), so the media is not too far off in calling it an "antistall system".
From what is currently out there (including the Leeham news article), the (-MCAS) MAX has objectionable handling characteristics near the stall. At high AoA the nacelles effectively extend the wing forward, moving the neutral point forward also and degrading the static margin and longitudinal stability. However the problem doesn't really fit neatly into the certification regulations, as although the simultaneous pitch up (made worse by likely high power/thrust line effects) is like what an unstable aircraft would do, it is in fact an uncommanded pitch up alongside a transition into a less longitudinally stable (possibly unstable) state. This is likely to result in a stall unless promptly attended to manually or automatically.
Section 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.
The abnormal nose-pitch in the regulation is
during the stall, a possibility if the tail stalls before the wing (as can occur if COG is behind the neutral point). What the -MCAS MAX does is (likely) a bit different, it pitches up
before the stall, and it may (appropriately) pitch down when the stall occurs. It would be hard to argue that by this it is compliant though and no augmentation is necessary. However the degraded longitudinal stability after the uncommanded pitch up would likely also fail 25.203.
Although MCAS it is not an antistall system
per se, it effectively functions as one. It should be harder to achieve an intentional 1g stall in a MAX with MCAS operating than a NG.