A 2018 FAA (Federal Aviation Administration directive advised pilots to handle MCAS (Maneuvering Characteristics Augmentation System) failure by disabling electric control of stabilizer trim using a pair of cutout switches. Pilots would then need to use a hand crank to move the stabilizer back to the desired position. It's noted that previous 737 models had separate switches to disable autopilot and electric stabilizer control, but the 737 MAX lacked this distinction.
Avionics engineer Peter Lemme explains how aerodynamic forces acting on the stabilizer and elevator in a nose-down situation would oppose pilots' attempts to correct the trim using their manual control.
A 1982 Boeing 737-200 Pilot Training Manual acknowledges this possibility, describing a series of maneuvers which can be used to relieve force on the controls and allow incremental correction of trim. However, it's suggested that the Ethiopian Airline plane had already gained too much speed and lost too much altitude for such a maneuver to be possible.
https://www.moonofalabama.org/2019/04/ethiopian-airline-crash-boeing-and-faa-advice-to-737-max-pilots-was-insufficient-and-flawed.html
https://www.satcom.guru/2019/04/stabilizer-trim-loads-and-range.html
(Score: 3, Informative) by shortscreen on Tuesday April 09 2019, @12:31PM (3 children)
Looking at one of the charts later, I saw that they didn't actually start losing altitude until the very end. So instead of "losing too much altitude" perhaps "never having gained enough altitude in the first place."
(Score: 1) by Coward, Anonymous on Tuesday April 09 2019, @03:54PM
Supposedly, they were heading towards mountains where the minimum level for safe flight is 14000 feet. So their altitude above ground may well have been dropping.
(Score: 2) by bob_super on Tuesday April 09 2019, @05:18PM
> So instead of "losing too much altitude" perhaps "never having gained enough altitude in the first place."
Well, unless you hit a tall or flying object, isn't any crash a matter of losing all your altitude without losing all your speed ?
(Score: 2) by RS3 on Thursday April 11 2019, @04:54PM
Thank you so much for your submission. Your links confirm what I've understood and surmised, and filled in some big gaps, esp. re: stabilizer trim wheel force required. A lot more to read and digest. I've only flown (piloted) small planes, and only a little bit. Until reading the articles you linked, I didn't realize that big plane designs allowed conditions which required enormous human pilot effort to control them. From the beginning of this tragedy I've been bothered by the existence of MCAS being kept secret from the pilots, and that turning electric trim OFF is the only way to disable MCAS. If the pilots could have turned MCAS off, but kept electric trim on, they likely would have been able to control the plane.
Reading about the physical force required to overcome aerodynamic loads which were imparted to the control columns and trim wheels, and looking at the flight envelope from Lion Air https://static.seattletimes.com/wp-content/uploads/2018/11/LionAir-BlackBox-WEB-1020x680.jpg [seattletimes.com], I have to wonder if the pilots were just physically exhausted, couldn't fight anymore, and maybe even blacked out in the final moments.