Originally Posted by
MickG0105
To borrow an image from Gums, when the crew looks to wind off some of the automatically commanded nose down trim, HAL says, 'I'm afraid I can't allow that, Dave.'
MickG - careful now. This is not at all a matter of HAL saying "I'm afraid I can't allow that, Dave." What is happening here is that MCAS has inserted a nose down stabilizer motion increment in response to its understanding that the airplane is at high AOA. The pilot has then manually re-positioned the stabilizer. MCAS is in no way preventing the pilot from moving the stabilizer in either direction as much as the pilot wants. MCAS is further waiting five seconds to allow the pilot to make sure that the pitch trim situation is to the liking of the pilot. If the pilot is not satisfied with the pitch trim and makes another manual trim input, MCAS sits it out for another five seconds. Only after that does MCAS make a new assessment as to whether or not the conditions exist (flaps up, proper speed range, and high indicated AOA) to trigger MCAS to insert a limited increment of nose down stabilizer motion. The design assumption here is that when a pilot makes pitch trim inputs, those inputs will be in the direction to trim the airplane such that each time MCAS makes its assessment as to whether or not to add more nose down, it is starting from a relatively trimmed condition. The pilot maintains highest priority for stabilizer control throughout and MCAS only moves the stabilizer a limited increment each time that the pilot finishes trimming manually.
While it does not present itself as continuously running run-away automatic stabilizer motion, with the airplane flying at a relatively steady flight condition repeated instances of the system moving the airplane away from the trim condition that the pilot has established manually should be recognized as improper automatic stabilizer operation and disabled via the cutout switches.