[msbarnett]

No, the wheel has an extendable crank handle and is meant for manual use precisely when the electronic stabilizer trim is cutout.

But aerodynamic forces can conspire to make it very hard or physically impossible for human force to rotate it if the elevator is being used to combat the stabilizer at high speeds. See here for more discussion of the physics behind it: https://www.satcom.guru/2019/04/stabilizer-trim-loads-and-ra...

[cjbprime]

Thanks, that makes sense. When the report says that the pilots found manual trim impossible after STAB TRIM CUTOUT, do you think it's referring to an attempt at manual electric trim (expected to fail) or both pilots manually moving the trim wheels with the handle?

[msbarnett]

My (non-expert) reading is that the co-pilot couldn't move the wheel, while the pilot was engaged in fighting to to keep the stick pulled back (a Swedish pilot tried this scenario in a simulator last week and literally had to keep both arms locked around the stick towards the end).

Counter-intuitively, letting go of the stick in brief increments might have been the correct move. Letting the nose pitch down would take force off the jackscrew and let both pilots crank hard on the stabilizer. Boeing manuals once covered this, but apparently they haven't since the 1980s, and their directive after the Lion Air accident made no mention of the necessity of such a procedure. Also they were only 7,000 feet above the ground so whether or not they'd have recovered in time is hard to say. Quite possibly the MCAS had already doomed the flight.

[cjbprime]

> they were only 7,000 feet above the ground

No, it's worse: the origin airport is at 7000ft elevation. They only had around 1000ft height AGL for all of the flight. So I agree that releasing the elevator at all seems surely suicidal.

[msbarnett]

They're at 12,000-14,000 feet altitude at 05:43ish, when the co-pilot reports the non-functional manual trim. So ~6,000-7,000 height.

Still damn close to the ground to be comfortable letting the plane dive.

[cjbprime]

Oh! I think this is new information in the report -- the FlightRadar24 ADS-B data never shows them above 8500ft altitude, ~1000ft AGL. FlightRadar24 must have missed the final few minutes?

https://www.flightradar24.com/blog/flightradar24-data-regard...

[treis]

But high enough to re-enable a system you know is malfunctioning?

I guess we will have to wait for the final report but the pilot's actions are perplexing here. Even if we accept that they can't trim with the wheel, why enable electric trim and then not use it? Why not enable it, trim to where you want, and then use the cut out switch again?

[msbarnett]

> But high enough to re-enable a system you know is malfunctioning?

They couldn't land with the stabilizers mis-trimmed. If the wheel wouldn't budge, re-enabling it and trimming electronically was the only option likely to occur to them, since the only other possible option (release the column, let the plane nose down so that the forces on the screw ease and they can trim manually) is completely counter-intuitive and was removed from Boeing's documentation and simulator training 30-40 years ago and was not covered in the FAA/Boeing MCAS directive.

> Even if we accept that they can't trim with the wheel, why enable electric trim and then not use it?

They did use it after re-enabling it. They just didn't re-disable it within 5 seconds of their last input manual electronic trim command, so the MCAS ran again a final time.

Whether that's because they just didn't get to the switches in time, weren't aware they had such a short window to do so (the FAA/Boeing bulletin does a piss poor job of communicating this), or were just so thoroughly overwhelemed by everything that was going on is hard to say at this point.

[phire]

The flight data graph shows no manual electric trim inputs at that time.

So must be manual trim wheel movements.