What’s sad is that, reading the pprune.org discussions of the first Max crash, it’s mentioned that it would have been possible to recover the aircraft by disabling the autopilot and MCAS systems.
How is it that current 737 Max pilots weren’t made aware of this emergency procedure in the meantime, while awaiting the software updates?
A lot of junk in the PPrune thread, but some good info on the MCAS, etc. It’s not necessarily needed for flight, but was rather added due to certification requirements around control forces and angle-of-attack:
There is a cert requirement that as AOA increases, the nose up pilot command required must not decrease. This is demonstrated at fixed thrust levels so there is no change in thrust pitching moment. The 737MAX issue here that gives rise to the need for MCAS is that as AOA increases the lift provided by the engine cowling that is so large and mounted so far forward of the wing causes a nose up pitching moment that results is a decrease in the column pull needed to maintain a steady positive AOA rate. That characteristic is not compliant with the requirements. MCAS comes active during this maneuver putting in nose down stabilizer that must be countered by the column. The net effect of engine cowling lift and MCAS nose down stabilizer as AOA increases is that the column needed to complete the maneuver does not decrease part way through the range of AOA for which characteristics must be demonstrated. 737MAX without MCAS fails the cert demo. 737MAX with MCAS passes the cert demo.
Also, the MCAS is getting the attention because it commanded the control inputs that led to the crash, but the root cause is faulty angle-of-attack sensors, which are what caused the MCAS to trim the elevators down in the first place. Similar sensor issues (though airspeed, not AoA) resulted in the AF447 crash.