Originally Posted by
Tomaski
The degree of difficulty of using the manual trim is not so much a function of absolute trim position as it is how far from neutral trim the stab is - you don't have to hit the stops of the trim range to have a problem. I'm guessing that 2-3 units out of trim would be enough to create difficulty and 4+ units out would fall into the nearly impossible to move range (without something like the yo-yo maneuver to relieve pressure). It's really a matter of the total forces on the jackscrew/nut mechanism which, again, are a function of elevator (not stab) position, airspeed, and altitude. And again, manual trim can be safely used in all areas of the certified flight envelope as long as the stab is kept reasonably close to neutral trim (no forward or back pressure on the control column). Despite concerns to the contrary, I am not aware of any circumstances of aerodynamic loading that prevents the Main Electric Trim from being able to reposition the stab no matter how far out of trim it is. In the early months of the investigations there were some suggestions that the electric trim motor might have stalled in one or both of the MAX accidents, but none of the official investigating agencies have ever cited this as an issue. If this were an actual concern, it probably would have been included in the laundry list of deficiencies that needed to be corrected before the MAX can return to service.
Thanks for your detailed answer. I see your point about how far from neutral trim being the important number. I can’t remember from my ATPL theory how the pull force in an out of trim situation changes with speed, is it squared?