Originally Posted by
capngrog
I recall having seen/read something about the MCAS that described it as a "background" feature that pilots would be unaware of under normal circumstances. The article/report went on to describe how a feature of the (MCAS) operation was that the console (aisle pedestal)-mounted trim wheels would not move when MCAS was activated. This had me scratching my head in that the trim wheels are mechanically connected to the jackscrew gearbox and some new engineering would be required to allow jackscrew movement without trim wheel movement. At any rate, I've been trying to find the article with, thus far, no success. Today I had lunch with a friend (retired B-727 Captain) who mentioned that he had also seen the article/report. I've seen several descriptions on how runaway stabilizer trim is dealt with in the B-737, and the techniques have been described as thus: 1) Counter trim using yoke mounted trim switches; 2) Grab and hold the manual trim wheel(s); move the STAB trim cut out switches to cut out. This afternoon, Aviation Week had an online article entitled: "The Boeing 737 MAX MCAS Explained' which cited only two means of stopping runaway MCAS operation: 1) Counter trim using yoke mounted trim switches; 2) move the STAB trim cut out switches to cut out. No mention was made of grabbing the trim wheels. The online article further stated: "The MAX flight-control law changes from speed trim to the MCAS because the MCAS reacts more quickly to AOA changes".
My question is: Will MCAS activation be reflected in movement of the trim wheels?
It should and it does. We can see what happened to PK-LQP's fateful flight. "Some sort of mechanism" had trimmed the plane nose down shown also by the rotating wheel plus its noisy winding sound, so the pilot countered it by manual trim up by rewinding that same rotating wheel the opposite way. This thing happened repeatedly, but it can be stopped and countered ALL THE TIME. It didn't behave like the classic runaway trim, i.e. continuous and unstoppable. So, for the time being, the pilot focused on other problems with the plane while asking the ATC permission to RTB. During this problem-solving, trying to quickly going onto memory items and perhaps consulting the NNC book, the counter trim by PF was either insufficient or being temporarily neglected.
Either the CAPT, PF or both which were occupied on solving the puzzle suddenly realized that all this time the ghost in the machine had been busy and still continued incessantly bringing the plane's nose down to the end of the jackscrew, it's all too late for them to recover.