Cockpit voice recorder of Lion Air jet depicts pilots' frantic search for fix

Re:Cockpit voice recorder of Lion Air jet depicts pilots' fr... by mannykannot on Wednesday March 20, 2019 @08:52AM (#19441507)

This got my attention too. For at least 6 minutes, the crew were coping with the problem - the aircraft was holding altitude and speed, if erratically, and they must have been getting the trim back to approximately where it should have been, or else MCAS would have driven the stabilizer to the point where it overrode the elevator authority.

In about the last minute of that recording, the pattern changes: the pilot trim inputs decreased and became much shorter, and the stick forces changed from going up and down to a sustained, high, back-force.

The article says that near the end, the captain handed over control to the 1st officer so he could look for answers. I am speculating wildly here, but could that coincide with the change in the pattern of the pilot's responses? Could it be that the 1st officer did not attempt to re-trim, or did not do it completely? Did he try to trim manually, but perhaps could not because of the load on the stabilizer? [1]. IIRC, according to Dominic Gates, it would only take two cycles of MCAS intervention until the stabilizer had reached a point where full nose-up elevator could not keep the airplane level.

The chart was published in one of Dominic Gates' Seattle Times articles, but I am not in a position to find it right now...

[1] https://news.ycombinator.com/item?id=19400157

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Re:Cockpit voice recorder of Lion Air jet depicts pilots' fr... by VBprogrammer on Wednesday March 20, 2019 @09:07AM (#19441633)

Anyone who's ever piloted an aircraft would try to re-trim against pitch forces. It's inconceivable that the pilot for some reason just failed to do that. I'm not sure exactly what force it takes to get full nose up elevator at flying speed but it will be in the range of dozens of kilograms.

I personally wonder if there is potentially a thermal cut-off on the trim motor which could have been activated by this trimming and re-triming. Or even an outright failure of some part of the system.

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Re:Cockpit voice recorder of Lion Air jet depicts pilots' fr... by VBprogrammer on Wednesday March 20, 2019 @11:19AM (#19443037)

I just had a better look through the graphs in the report and I can see exactly what you are talking about. There is a clear pattern of trimming against the MCAS, then just before the speed increases and the altitude decreases the pattern changes. There are still nose up trim commands but much shorter. At that point the pitch trim position changes and never recovers to the same point where the aircrafts height and speed had previously been relatively stable.

Also notable is that engines spool-up about the same point to what looks like full takeoff thrust.

They must have identified an issue just after going flaps 0 because there is no other reason to go to back to flaps 5 (which incidentally disables MCAS). Why on earth they didn't drop flaps again and head back to the airport is a bit of a mystery to me.

I can't imagine the Captain handing control of a aircraft which was obviously a handful to a co-pilot and then looking straight down at the checklist, surely he'd have given guidance about the trim and waited to ensure the co-pilot was handling the aircraft?

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