Underscoring the difference between the speed-trim system on the 737 Next Generation (NG) and the MAX, Sinnett says: “Mechanically, on the NG there is a column cutoff switch that stops any automatic trim when the column is back to a certain spot. On the MAX, we still needed automatic trim when you got to that spot. MCAS differs from speed trim at elevated alpha because it bypasses that switch by design. To do that, it activates based on AOA rather than based on speed—which is what speed trim does. Speed trim is a function of airspeed, and MCAS is a function of angle-of-attack and Mach number, but it only triggers off AOA.”
The software is getting modified - should be good as long as both vanes don't get taken out by a flock and the algorithm to always leave 1.2 g authority in the elevator attains 10E-9 reliability.
What I haven't seen yet is a procedure to block MCAS in certain stick shaker cases, especially on rotation or early takeoff.