My understanding is that the horizontal stabilizer position is determined by the sum of the yolk ("steering wheel") position and the trim wheel position. MCAS might actually spin the trim wheel. In any case, MCAS trimmed the plane way out of whack, such that even with the yolk all the way back, it wasn't flyable. The only way to fix the trim was to turn back on the electric motors on the trim system (including its MCAS subsystem) or else crank the trim wheel like mad by hand and hope their hands are fast enough to get the plane sanely trimmed before the plane lost too much altitude.
Edit: I see inamberclad mentioned an additional detail that the trim wheels get stuck when the yolk is pulled all the way back. I wasn't aware of this, but it makes the pilots' actions make much more sense.
That doesn't sound right. It sounds like it might be correct on an Airbus, but not a Boeing. On a 737, yoke movement mechanically controls the elevator and the trim system mechanically (and with a motor to help) controls the stabilizer. There is no connection between the yoke and the stabilizer on a Boeing. Does that make sense?
It's not just that the trim wheels don't work when the yoke (not yolk) is pulled, but that it can require too much torque to move them -- it's a purely mechanical connection with the motor off -- when the elevator is deflected in opposition to the stabilizer.
Comments
My understanding is that the horizontal stabilizer position is determined by the sum of the yolk ("steering wheel") position and the trim wheel position. MCAS might actually spin the trim wheel. In any case, MCAS trimmed the plane way out of whack, such that even with the yolk all the way back, it wasn't flyable. The only way to fix the trim was to turn back on the electric motors on the trim system (including its MCAS subsystem) or else crank the trim wheel like mad by hand and hope their hands are fast enough to get the plane sanely trimmed before the plane lost too much altitude.
Edit: I see inamberclad mentioned an additional detail that the trim wheels get stuck when the yolk is pulled all the way back. I wasn't aware of this, but it makes the pilots' actions make much more sense.
That doesn't sound right. It sounds like it might be correct on an Airbus, but not a Boeing. On a 737, yoke movement mechanically controls the elevator and the trim system mechanically (and with a motor to help) controls the stabilizer. There is no connection between the yoke and the stabilizer on a Boeing. Does that make sense?
It's not just that the trim wheels don't work when the yoke (not yolk) is pulled, but that it can require too much torque to move them -- it's a purely mechanical connection with the motor off -- when the elevator is deflected in opposition to the stabilizer.
NB: "yoke", not "yolk".