Originally Posted by
MechEngr
There's no drastic pitch-up. How does this get started and why is it repeated? Comparing the plan form to the NG and the differences are difficult to make out.Slight changes in configuration tend not to make drastic changes in performance. More than that, the Max has a thrust line closer to the CG than any other 737, so the pitch coupling should be less or about the same. The larger engine can produce more thrust, but unless the plane is a lot heavier than previous planes the only way to use that thrust is in uncomfortable sea level take-offs or in normal rate departures from high-hot-humid airports. The size increases the coupling of fuel burn to air movement for an increase in thrust specific fuel consumption, aka, fuel efficiency. So, they keep the thrust the same and burn less fuel.
I'm sure the change in factor was not yelled from the roof tops because no one had a fault-tree analysis linking the AoA system to the MCAS output to a definitive emergency outcome. Had they passed along the change in rate without anyone having done the analysis, it would have not raised any questions, just like it did not in the first place. The more aggressive change should have acted to draw the attention of the flight crew to the more rapid pitch trim change and then to the cause of it, rather than being a slower and less obvious one; it appears in the first and second MCAS events that this is exactly what happened. In the third event, the pilots seemed unaware of the change in trim for a relatively long time and did not notice they were left with a large increment.
So far no one has volunteered just how the control forces for pitch mis-trim can be so low so the pitch trim error is unrecognized and so high they cannot be overcome.
Don't think of it as an issue of "thrust line" nor "thrust to weight" but rather nacelle lift farther forward and higher than the NG. This in fact causes a pitch up moment force behind the curve.