Originally Posted by
AndyJS
"Boeing’s automatic trim for the 737 MAX was not disclosed to the Pilots" (Link to Leeham News article)
The Leeham News article is quite informative of why the MCAS is required on the MAX, and fills in some blanks. I originally thought heavier engines further forward should if anything bring COG forward (unless balanced by other modifications), creating a more longitudinally stable MAX. However the idea that at high AoA the nacelles become aerodynamic surfaces sufficient to longitudinally destabilise the aircraft is interesting. It's a big aside, but a similar problem was noted on the WWII Supermarine Spitfire. As development to more powerful engines dictated more and more propeller blade area, the non obvious effect of this as an aerodynamic surface way forward of the aerodynamic center caused degraded longitudinal stability margin.
So when you are at high AoA, your MAX may become pitch divergent (without MCAS). At the same time you are likely to be slow but with high thrust beneath the wings, pitching you upward into the stall.
One thing that keeps coming up considering how much importance the AoA signal now has in the MAX, is whether there is redundancy in the two vanes. I think the answer is not really, as many conditions (rolling, yawing!) will lead to expected AoA disagree between the two sides of the nose. A warning (i.e. stick shaker) based on a single high alpha vane is reasonable, but any kind of consensus signal from two vanes only with severe consequence for faulty data is quite problematic.
So the current two vanes are really only one sensor for the purposes of redundancy. I wonder whether to have such a high degree of reliance on a AoA signal you should incorporate three vanes on each side.