By the time of the Ethiopian crash, 737 pilots everywhere knew all about MCAS and the procedure for disabling it. A preliminary report issued last week by Ethiopian Airlines indicates that after a few minutes of wrestling with the control yoke, the pilots on Flight 302 did invoke the checklist procedure, and moved the STAB TRIM switches to CUTOUT. The stabilizer then stopped responding to MCAS nose-down commands, but the pilots were unable to regain control of the airplane.
It’s not entirely clear why they failed or what was going on in the cockpit in those last minutes. One factor may be that the cutout switch disables not only automatic pitch trim movements but also manual ones requested through the buttons on the control yoke. The switch cuts all power to the electric motor that moves the stabilizer. In this situation the only way to adjust the trim is to turn the hand crank wheels near the pilots’ knees.
Wow, so disabling MCAS also disables all stabilizer commands?
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.
No, it says that moving the pitch trim switch to cutout disabled the trim buttons on the yoke. They should have still had manual pitch control the good old fashioned way (with the primary control inputs through the yoke), as well as the manual pitch trim wheels on either side of the throttle quadrant. However, it appears that due to an existing issue with the 737 design, they could not use the manual trim wheels for some reason while they were holding the yoke all the way back. That's a very bad situation to be in, as controlling an airplane that's severely out of trim can require huge amounts of force on the control inputs and could have just exhausted the pilots.
They don't have manual pitch control through the yoke (elevator) when the stabilizer is severely mistrimmed because the stabilizer is "stronger" than the elevator -- it's a much larger surface, the entire horizonal stabilizer moves. Yoke movement doesn't move the stabilizer.
The stabilizer trim switches enable/disable the electric stabilizer trim. There's a wheel with a foldout hand crank to manually control the stabilizer trim. Based on the preliminary report, the copilot was unable to turn the wheel which is why they turned the electric stabilizer trim back on.
One hypothesis is that because the pilot was fighting the extreme nose down stabilizer with nose up elevator the jackscrew that moves the stabilizer was so heavily loaded that the co-pilot didn't have enough strength to manually turn the trim wheel which is connected to the jackscrew by cables.
Another theory is that it was possible, but requires both pilots to turn their cranks together -- not possible if one is pulling back on their yoke and unable to let go without that being deadly.
Yes, all electric ones. The pilots have a purely mechanical method for moving the stabilizer but it seems like it wasn't possible in this particular situation, perhaps for aerodynamic reasons.
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Wow, so disabling MCAS also disables all stabilizer commands?
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".
No, it says that moving the pitch trim switch to cutout disabled the trim buttons on the yoke. They should have still had manual pitch control the good old fashioned way (with the primary control inputs through the yoke), as well as the manual pitch trim wheels on either side of the throttle quadrant. However, it appears that due to an existing issue with the 737 design, they could not use the manual trim wheels for some reason while they were holding the yoke all the way back. That's a very bad situation to be in, as controlling an airplane that's severely out of trim can require huge amounts of force on the control inputs and could have just exhausted the pilots.
They don't have manual pitch control through the yoke (elevator) when the stabilizer is severely mistrimmed because the stabilizer is "stronger" than the elevator -- it's a much larger surface, the entire horizonal stabilizer moves. Yoke movement doesn't move the stabilizer.
The stabilizer trim switches enable/disable the electric stabilizer trim. There's a wheel with a foldout hand crank to manually control the stabilizer trim. Based on the preliminary report, the copilot was unable to turn the wheel which is why they turned the electric stabilizer trim back on.
One hypothesis is that because the pilot was fighting the extreme nose down stabilizer with nose up elevator the jackscrew that moves the stabilizer was so heavily loaded that the co-pilot didn't have enough strength to manually turn the trim wheel which is connected to the jackscrew by cables.
Another theory is that it was possible, but requires both pilots to turn their cranks together -- not possible if one is pulling back on their yoke and unable to let go without that being deadly.
Yes, all electric ones. The pilots have a purely mechanical method for moving the stabilizer but it seems like it wasn't possible in this particular situation, perhaps for aerodynamic reasons.