Comment by sokoloff | Hacker News Reader

Comment by sokoloff | Hacker News Reader

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IMO, no. There is a released fact (via photographs) that the jackscrew on the stab trim was run to the extreme. That's strong evidence to suggest that the non-normal checklist for stab trim runaway was not correctly followed.

The aviation minister's comment today does not carry weight in excess of the photographic, factual evidence, IMO. There are too many "loopholes" that she could be exploiting without leaving her statement technically incorrect.

The good thing, such as it is, is that the facts are almost surely going to emerge here. The CVR transcript, FDR dump, and analysis of whether there's a mechanism for the electric trim to run even with the stab trim cutout switches in cutout and whether that can exceed the crew's ability to stop and reverse the trim setting manually will be telling. I do admit that the minister has greater access to that information than you or I do at this time. It is possible, but not decided, that she's had time to review all that information, knows with certainty the crew is blameless, and has chosen to not make a specific enough statement to support that for any number of valid reasons.

The explanation put forward (which seems reasonable) is that the cutouts were disengaged because the aerodynamic forces were stronger than could be overcome by the manual control, so they had to turn the electrics back on in an attempt to move it (which allowed the MCAS to continue pushing the stabilizer to the extreme)

>The explanation put forward (which seems reasonable) is that the cutouts were disengaged because the aerodynamic forces were stronger than could be overcome by the manual control, so they had to turn the electrics back on in an attempt to move it (which allowed the MCAS to continue pushing the stabilizer to the extreme)

But the only way it would end at the total extreme is if they were within 2.5 degrees of max when they re-enabled electric trim and they crashed within 5 seconds. Otherwise, they should have been able to at least get some adjustment.

>Boeing’s anti-stall software on a doomed Ethiopian Airlines jet re-engaged up to four times after the crew initially turned it off due to suspect data from an airflow sensor, two people familiar with the matter have said.

https://www.theguardian.com/business/2019/apr/03/ethiopian-a...

4 cycles should have gotten them to 7-10 degrees away from the extreme. So something else was going on.

Ah. They'd have had to "roller coaster" the airplane (pitch up, then move the elevator surface towards neutral [so a net nose down]) to get the trim to move manually [the last step on the checklist]. Repeat as needed until the stab is trimmed within a range where it can move manually in unaccelerated flight.

That's an interesting possibility, which if they didn't know about it (I don't blame them), they may have felt they had no choice but to go "off checklist" and re-engage the electric trim system. I'd expect that would be in the context of "PF commands nose up pitch trim continuously while PNF engages the trim cutout to normal and monitors."

That's a tougher hand dealt the crew than I previously contemplated and if that's what they did, it's one for which I will owe them a retraction.

They never had the altitude to do that...

It's a shame that MCAS behavior wasn't fully explained after the first crash. If it had, they might have remembered that flaps disable MCAS. If I had to go "off checklist", I'd go: stab trim off -> flaps 1st detent -> stab trim on -> trim to neutral -> stab trim off for duration of flight -> flaps up -> manual flight for the duration. Still, that's all while you're pulling on the column like mad, all the indicators are lit, all the annunciations are going on, the stick shaker is going, you're going like a bat out of hell, and the ground is coming up at you. In the final seconds of flight, there was so much nose-down that they were pulling -2g.

Another thing we know now about MCAS is that it's locked out for 5 seconds after each trim switch operation. So, if you never have stab trim engaged for longer than 5 seconds a shot, you should be able to trim up the plane.

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