Originally Posted by
Ian W
MCAS is not a stall protection system. It is there to meet the requirements of 14 CFR 25.173 that the pull force on the control column should be linear with speed and AoA. With the Max the pull force gets lighter as the AoA gets close to stall but it is not a stall prevention device. Apparently, Boeing was more concerned about handling problems at high AoA in steep turns somewhere most pilots would not take a 73.
Thanks Ian. This confusion is very widespread in the current general (outside pprune) consensus of current MAX issues. Not only in the general media, but even in articles/content generated by commercial pilots and industry members (including current NG pilots), MCAS is almost ubiquitously described as an antistall system operating somewhat like a stick pusher operating on the stabilizer instead. This is required because of a putative dangerous pitch up tendency at high AoA.
We are very unlikely to get the actual MAX stability data from Boeing but the Cm versus alpha data is more likely to be something on the left (similar to that suggested in Bjorn's corner at Leeham news). But many reasonably technical writers are describing something more like on the right. This seems unlikely because certification requires the aircraft to be safely flyable without stability augmentation.
Probable 737MAX behaviour versus typical media description
This seems to be a double edged sword for Boeing going forward. There is a lot of semi-informed consensus now that the re-engine was totally unacceptable because the unaugmented plane is dangerously unstable, and will remain so even with an improved MCAS2. But demonstrating that the unaugmented plane is fairly safe given pilot knowledge of it's decreased stability margin (over the NG) makes a lethal failure of MCAS1 look even worse, as it the problem it was needed to solve was more certification and sales critical than safety critical.