Originally Posted by
Aloha_KSA
As a long-time 737 driver I'll just chime-in a few points. Regarding the trim wheel ability to move the stab at high speed from full deflection: I seriously doubt that there would ever be too much force on the elevator that the trim wheel could not move it. There is a lot of leverage in the jack screw, and the turns of the trim wheel make very minute changes to the angle of the stab.
But know that moving from full deflection to neutral would take a painfully long time. On the -200 we used to wind the trim full fore and aft as part of the preflight checks. (We stopped doing that on the NG.) . Using the motor-driven trim this took about a minute to go full forward, full aft, then back to about 4 units. It is not physically possible to wind the trim wheel that fast for that long manually, especially when your aircraft is lurching about like a rodeo bull. Also note that it doesn't take much movement to change the aircraft's attitude significantly, and the aircraft is very controllable using manual trim. Back in the cargo days I once did a 20 minute flight using only the rudder and stab trim and manual power (on the -200) from after flaps up to 10000' on descent. That's right - I didn't touch the yoke, autopilot-off from climb-out to descent. It was very controllable and stable. (That was a good training exercise, too.)
My First Point: If we don't catch this mis-trim early, un-doing it manually will take a very long time and maybe more time than is available when your aircraft is only 1000' AGL. AND to use the trim wheel for more than small changes, one has to fold the handle out. A handle that has injured many a knee in simulator sessions because combined with the trim motor's speedy rotation of the wheel, it can leave one with a permanent limp.
Second: For all the arm-chair Monday morning QB's who are saying: "Oh, they should have recognized it immediately and disconnected the trim:"...
I am a human factors researcher working in aviation safety, a long time lurker on pprune (5+ years), and this is finally the post which I feel the need to commend so much that I have created an account. This post, which takes into account human psychology and environmental factors, should be required reading for anyone following any of the recent developments regarding the MAX's safety record.
I am personally shocked at how many posters here truly believe that the pilots should be clairvoyant when presented with a cacophony or warnings and little time to react. It has been covered in other posts but both the Lion Air and Ethiopian crashes presented the pilots with a series of stimuli that did not match the stimuli presented in (most carrier's) training for stabilizer runaway; it is thus not expected for the pilots to immediately reach for the cutout switches. Many of the other proposed actions by the crew(s), which some posters seem to consider common sense, again fail to consider human performance limitations in cognitively demanding situations.
Boeing may have a lot to answer for in terms of design philosophy, flight manual omissions, training deficiencies, etc.. However, I think another critical realization the professional aviation community should draw from these recent tragedies is that basic human-automation interaction principles are still not being given adequate consideration by manufacturers.