Originally Posted by
737 Driver
A MCAS failure presents itself as runaway stab trim. This uncommanded motion can be caused by a number of different malfunctions, MCAS being only one of them. The runaway stab trim procedure is agnostic as to the underlying cause of the the malfunction.
The procedure itself calls for disengagement of the Autopilot, use of Main Electric Trim (yoke trim switch) as necessary, disengagement of the Autothrottles, and if the runaway has not ceased, use of the stab trim cutout switches. Once all electric trim has been terminated, you would use manual trim as needed until landing.
Though it is not currently procedure, if you had a good idea that the problem really was MCAS (presumably because of an erroneous AOA or airspeed input), you could extend the flaps and restore the electric trim since MCAS only works with the flaps up. Technically, engaging the autopilot would also cause MCAS inputs to cease, however, our manuals contain warnings that the autopilot may not stay engaged with an erroneous AOA or airspeed.
I'm not sure if this has been discussed already and I've missed it, but is it possible for the electric stab trim to become stalled or overwhelmed by aerodynamic forces when there's a combination of a bit of speed and full stab deflection.