The pilots did things correctly until 05:43:00 but then they made the mistake of not counteracting MCAS after 05:43:20. And as stated above, I suspect one pilot acted alone to turn on electrical trim because there was no verbal discussion or callout or confirmation. (This would then be a CRM issue.) And whoever switched it on may not have realized that MCAS would come back. And finally neither pilot noticed the trim moving back down.
For me, that's the main mystery. And that's another thing in common with the Lion Air flight.
Clearly, the pilots of both flights had understood that they could counter the nose down/MCAS with manual electric trim imputs. It was working. And then suddenly they stopped doing it! In the Lion Air flight, it's already incomprehensible. But we know they didn't know what MCAS was. So we can still admit that, at one point, they tried something else "without realizing" that stopping to counter the MCAS would crash the plane. But here in the second crash? They deactivate the switch 35 secondes after the first MCAS imput, which is rather fast! Both pilots verbally reported "trim cut-out". And indeed, the MCAS stopped immediately. To me, it is obvious that they knew perfectly well that the MCAS was causing the problem, and they were following Boeing's procedure. Unfortunately, it appears that the aircraft was not controllable with manual trim controls because of its high speed.
From that point on, a rational behaviour, considering that slowing down the aircraft was risky at only 1500 m above ground level with unreliable airspeed, was to reactivate the electric trim and to manually counter each MCAS activation every 10 seconds with the manual electric trim, as the Lion Air pilots did for quite some time. And that's precisely what they seemed to have done right after the switch was reactivated, since a manual electric trim has been recorded. And then, they suddenly stopped! Why??! After having understood that the problem came from the MCAS, after having noticed that disabling the switch stopped the MCAS, it simply seems impossible to me that a pilot could have "forgotten" that reactivating the switch would reactivate the MCAS. Especially after the Lion Air accident! At that time, those two pilots probably had MCAS only in their mind!
We can put forward 99 hypotheses on why this sudden stop of counter-triming against MCAS had happened,had it happened in a single accident only. But twice the exact same omission?
Are there any technical reasons that could explain why, after a certain time / a certain number of uses / a certain configuration, the manual electric trim button stops working? For example, that the button breaks, to take a rather simplistic example?
Last edited by sergegva
on Thu Apr 04, 2019 9:54 pm, edited 2 times in total.