§ 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.
25.203 requires that the plane remains controllable, the controls don't reverse, and there can't be abnormal nose-up pitching.
The reports I've read say that on the MAX the stick forces get lighter when approaching a stall without MCAS, in narrow ranges of the flight envelope and CG. That's not really the same as abnormally pitching up is it?
Contrary to the hyperbole we've read in these threads, there's never been any documented evidence that the MAX pitches up, or is unstable or uncontrollable. Only that the stick forces lighten in certain corners of the envelope when nearing stall. So that would seem to make the bolded text above something that the MAX passes without problem, even without MCAS.
If anybody has different data, please provide corrections.
“Stick lightening” is a nice way of saying that the pressure is decreasing. In my books, a decrease in pressure is a movement in the negative direction. Thus, stick pressure is not remaining positive. I’m pretty sure stick lightening is not permissible under 25.203.
But hey, don’t take my word for 25.203 being applicable. Just google a few technical articles about MCAS, and you can read how 25.203 is cited in pretty much every single one. You guys seem to like Peter Lemme, so how about reviewing this in-depth article, where among others, he cites and bolds passages from 25.203: https://www.satcom.guru/2019/03/regulat ... s.html?m=1
Or, are they all wrong, these many sources citing 25.203? Are a few a.net members correct, and everyone else, including the authors of the Lion Air report, wrong?
My evidence may be a combination of my own logical surmising, combined with a whack of circumstantial evidence citing 25.203, but I feel that present a decent case. If anyone can prove me wrong, please do. I’m honestly happy to be corrected.
As to pitch-up tendency and instability, here’s R. John Hansman, an MIT professor of aeronautics,
discussing the MAX:
As I understand it, at high angles of attack the Nacelles — which are the tube shaped structures around the fans — create aerodynamic lift. Because the engines are further forward, the lift tends to push the nose up — causing the angle of attack to increase further. This reinforces itself and results in a pitch-up tendency which if not corrected can result in a stall. This is called an unstable or divergent condition. It should be noted that many high performance aircraft have this tendency but it is not acceptable in transport category aircraft where there is a requirement that the aircraft is stable and returns to a steady condition if no forces are applied to the controls.
(emphasis is mine)