Originally Posted by
FCeng84
Hans - your patience is amazing. You are correct about the relationship between 737MAX engines and need for MCAS. It has nothing to do with the thrust pitching moment!!!
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.
That is a very clear explanation - the most clear of anything I've read. Well done FCeng84.
What is interesting, then, is that MCAS activates without column input. If the purpose of MCAS is to essentially negate the lift of the engine cowling - which causes lift when AOA increases - then one would think the system would only activate as a result of column pull. In other words - the system is to create a consistent feel on the column - so why have it activate absent deliberate pull on the column? Why doesn't the system require both an increase in AOA
and a pull on the column before it trims AND?