Originally Posted by
yanrair
On previous models opposing stab movement with yoke automatically jammed the stab and stopped it moving. Consider... Much more dangerous than MACS is a fault called runaway stabiliser where the STAB runs out of control until it hits the stops and we all say the Lord’s Prayer and goodbye.Except that this is not what happens. Pilot sees /senses/feels/ hears (it’s loud) stab runaway and counteracts initially with control yoke , THUS STOPPING THE STAB , MOVING. Then , applies opposite stab trim , and finally if none of this succeeds, STAB OFF switches. And from years of watching pilots do this NNP procedure they usually get it stopped PDQ. And if it runs a long way out of trim, they trim it back manually. It’s never been a problem. Are you now saying that the trim was permitted due to a tech issue to run so far forward nose down as to result in a vertical dive - AND THERE WAS NOTHING WE COULD DO ABOUT IT? That’s what I’m getting from a lot of this feedback. Have I misunderstood. Because I’ve heard nothing from any source so far to suggest that this plane was unfliable.
Talk tomorrow. It’s late here.
Yanrair
Pilot trim via yoke mounted thumb switches stops MCAS stab command and runs stab in direction of pilot trim input. On the other hand, non-MCAS runaway stab may or may not continue to run if opposed by thumb switch trim or column cutout (i.e., large column in opposite direction). Nature of runaway depends on where in the system the error that is the cause is located. In the worst case, non-MCAS runaway can only be stopped by stab cutout switch activation. It is for that reason that timely identification and shutdown via stab cutout switches of a runaway that cannot be arrested by other means is critical.
Timeliness of stab cutout shutdown of misbehaving MCAS is not as critical as long as the crew continues to oppose it via pilot trim inputs to keep close to pitch trim while diagnosis of need for stab cutout switch activation is made. Lion Air pilot on accident flight was able to manage pitch control for many minutes in presence of MCAS responding to errant AOA with repeated airplane nose down trim commands that had to be walked back to trim via thumb switch commands. It was only after control was transferred to the right seat and the F/O failed to provide sufficient airplane nose up trim to fully counter each MCAS increment of airplane nose down trim that the stabilizer move progressively more airplane nose down to the point where full aft column was not sufficient to keep the nose up. As has been noted in earlier posts, increasing airspeed once in the dive made the situation worse as elevator authority was reduced due to actuation blowback.