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: 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.