Originally Posted by
Gilles Hudicourt
How about a case where the pilots, after having been trained about the perils of spurious MCAS activations, would get an airspeed indication failure (unreliable airspeed) followed by real stall warning (generated by the AOA vane), and a justified MCAS activation, but that they would misinterpret as a spurious MCAS activation. They would do the runaway trim drill, meaning trim against the down stab, and turn off the stabilizer switches and pull on the stick.
Would they be able to recover from the stall that would follow ?
from a fellow in Airliners.net
quote:
I'm beginning to picture this scenario:
- * you have a malfunctioning AOA sensor, which basically by random misfortune happens to be the one feeding STS and MCAS for this flight (the other one works just fine, but is being ignored)
* you get no warning of the failure, because your cheap-o third-world airline did not purchase the AOA disagree warning option
* instead, you get stick shaker and stall warning
* so you push a bit of nose down (and perhaps add a little thrust) to build a safety speed margin above an airspeed you're suddenly no longer sure about
* MCAS - which you may or may not know about, certainly not through reading the FCOM - silently kicks in and gives you 2.5°/sec of nose down trim for 10 seconds - speed increases quickly
* you may have noticed - amid the cacophony of alarms, perhaps while trying to run an unreliable airspeed checklist - the uncommanded trim movement, but then it stops on its own - so no, this is not a runaway stab trim occurence
* you pull back on the yoke to avoid overspeeding, and probably trim nose up to alleviate the effort - but not for a full 10 seconds: stab trim is still nose down and speed is not back down to where it started
* after 10 seconds MCAS is back in action, and gives you another 10 seconds of nose down trim - speed increases further
* you're now pulling back on the yoke with all your might, but blowback kicks in and despite your best efforts the elevator looses authority
At this point you have just
one chance of living through the day by executing this exact sequence of actions:
1)
do not heed the backseater pilot deadheading on the jump seat behind you, yelling to cutoff stab trim! At least not yet: if you do, you will not have enough elevator authority to climb out of the dive you're currently in; you need electric trim because hand cranking the manual trim wheel will not be fast enough, and you need both hands to apply as much nose up elevator as you can in the meanwhile
2) use electric trim switch to trim nose up
and continue to pull back on the yoke (something they taught you since PPL should not be done: first stick, then trim)
3) if you aren't a smoking hole in the ground by now, get speed under control and back to level flight eventually
4)
NOW you must cutoff the stab trim to avoid riding the rollercoaster again
Creepy, and sobering
unquote:
as I am only rated on airbus I cannot comment but this scenario makes a lot of sense to me.
if it happened somehow like this 95% of us superpilots could have been caught in this trap.