Originally Posted by
DaveReidUK
Er, no, it's much worse than that.
50 revolutions of the trim wheel is nowhere near end-to-end - that number of turns only moves the stab about 3 degrees (units).
This is probably a stupid idea, but is it actually necessary to control the stabilizer when there is a runaway? I assume that the pilots desperately cranking on that wheel are trying to return the stabilizer to neutral, rather than trying to replicate the anti-oscillation effects of the STS system. Would the plane still be flyable in any reasonable circumstance if some independent backup system returned the stabilizer to the "neutral" position on failure?
I imagine a secondary motor in the tail (perhaps with an independent power supply and battery if you want to really put a stake through Murphy) whose one job is to return the stabilizer to neutral whenever the pilots switch off the electric trim (using either one of those handy switches.) Located right at the jackscrew it would surely have more chances of moving a mechanically (or aerodynamically) "stuck" stabilizer than any system of wires and wheels running the entire length of the plane.
Obviously this negates MCAS but that is going to have to be solved another way.
The idea is not perfect, it introduces another motor freeze or go haywire, but it could be designed so that a haywire "return to neutral" motor can be overcome by the electric trim. The worst case seems to be that a failed backup motor could jam the jackscrew in a bad position, although without MCAS is the stabilizer ever going to be in a position that can't be counteracted by the elevator?
Expensive, but probably less than a billion and no new training required.