Originally Posted by
WillFlyForCheese
I've been thinking about this - and don't believe it can be an accurate statement of the purposeful design of the system. Here's why . . .
MCAS will, when data input dictates, trim AND. It will do so for 9 seconds or so - and then take a 5 second break. When it's taking its break - it's not fulfilling the purpose of 25.173 - right?
If the purpose of the system is to compensate for stick feel - it wouldn't take a break - because the lift generated by the cowlings doesn't take a break.
If tdracer is still around on this topic - I'd like to know how the designed 5-second stand down plays into the purposeful design of the system. It just doesn't make sense to me.
As the trim moves AND, the pull back feel of the column will become heavier. So in that sense, some AND trim can neutralize the tendency for a lighter feel at higher AoA, bringing the feel into certification compliance. Once the trim has moved enough to offset the pitch-up moment from the engine nacelles, it does not need to continue to move AND and MCAS would, in normal circumstances, stop its trim input. The problem in the two accidents is that the AoA was wildly divergent from normal range due to presumably damaged or missing AoA vane. That is what triggered MCAS to make the full 9-second/2.5 degree AND stab movement. In ordinary manual light, with correct AoA data, MCAS would presumably be making much smaller inputs.