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Ethiopian Airlines 737 MAX Investigation Preliminary Report

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Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#111

Post by cjbprime »

brucemoose wrote: post #25
mhandley wrote: post #3

Indeed looks like they did disable MCAS using the stabilizer trim cutout switches, but the aircraft was trimmed nose down when they did so. Aerodynamic loading prevented manual trimming using the trim wheels. They eventually turned electric trim back on, as they didn't really have any other options, but didn't trim back enough, and left it turned on. At this point the right airspeed indicator (which should be more re…

Layman question -- Does the MCAS system take current height from terrain into account when forcing the nose down?

Nope. (And arguably it shouldn't: a stall at low altitude is much worse than a stall at high altitude, because you lack recovery time. Most fatal stalls begin below 1000ft AGL, which is where this plane was. So your stall avoidance system shouldn't shutdown just because you're at low altitude: that's when you want to avoid stalls.)

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#112

Post by rootusrootus »

dsfyu404ed wrote: post #20
fgonzag wrote: post #13

Earlier quoted context omitted.

It's true there were aggravating factors, like the inability to manually rectify trim, which (would seem to me) would be a aerodynamic or design issue. The point still stands, the root cause was MCAS, without it this would never have happened. MCAS would not have pitched them into a dive, and even if they were pitched into a dive by some non MCAS system, they would have been able to turn on electric trim and recover.…

Likewise had Boeing designed a control surfaces system such that the pilot always has more control authority than the trim this also would not have happened. I didn't say that MCAS wasn't the lions share root cause here. It's just not as simple as the "hurr durr, MCAS bad" that a lot of people keep trying to portray the more nuanced statements of various regulatory agencies as being equivalent to. MCAS might be bad b…

Are there any airliners from any manufacturer where the pilot always has enough authority to override full trim?

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#113

Post by cjbprime »

simion314 wrote: post #85
ams6110 wrote: post #41

Earlier quoted context omitted.

Post Lion Air, they should have known that.

Why exactly? if Boeing did not updated the manuals and checklists how would pilots know that? from internet forums? Should a pilots ignore the checklists and try steps he read on the internet ? I could not find if Boeing updated their manuals/checklists after the first crash.

Yes, Boeing issued this service bulletin after Lion Air:

http://www.avioesemusicas.com/wp-content/uploads/2018/10/TBC...

And the FAA made it an emergency airworthiness directive in response.

The steps on it were broadly followed by the Ethiopian flight.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#114

Post by WalterBright »

cjbprime wrote: post #89
msbarnett wrote: post #75

Earlier quoted context omitted.

> And, the crew left the power setting at climb throughout almost the entire sequence, so they oversped the airframe. I believe that's correct given that they had an Unreliable Airspeed Indicator warning (due to the malfunctioning AoA sensor), per the memory items on the UAI checklist. It makes the forces on the trim wheel much worse, which explains why they couldn't turn it, but it's the correct checklist response t…

I think they couldn't turn the trim wheel because STAB TRIM CUTOUT unpowers it.

The trim wheel is mechanically connected via cables to turn the jackscrew.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#115

Post by msbarnett »

cjbprime wrote: post #106
msbarnett wrote: post #101

Earlier quoted context omitted.

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…

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?

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.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#117

Post by Domenic_S »

usrusr wrote: post #44
fgonzag wrote: post #17

Earlier quoted context omitted.

I thought all jets had to be capable of performing non powered glides (as in no electrical power, not just engine power). I guess that's no longer a requirement.

Being capable of performing a glide without electricity does not necessarily imply that a no-power glide can safely be entered from extreme conditions like diving too fast with trim set to insane.

Why is it possible to set the trim to insane? Is there a legitimate scenario where that would be used?

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#118

Post by danaliv »

Am I mistaken or is there a missing item in the chronology where the pilots reactivated the electric stabilizer trim?

The timeline mentions stab trim being switched to cutout, and if you look at the FDR graphs there's a (relatively) long period of more-or-less stable trim after that. But then there's an automatic trim down, which is only possible if the electric stab trim is switched back on, and there's no mention of that.

I also can't help but wonder if exceeding Vmo was a factor.

(I don't mean to sound as though I'm trying to assign any blame here. Obviously this was a confluence of problems, the most significant of which is the control system reacting to an erroneous 75° (!) AOA indication.)

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#119

Post by rdiddly »

Reading through the chronology in section 1.1, it feels like a movie where the aircraft is a malevolent entity that's trying to kill them. So if I were in the cockpit I'm sure it would've been some mix of fear and anger. My life's not in danger though, so I'm only left with the anger, on their behalf, knowing some greedy bunch of assholes were effectively that malevolent entity, whose will was expressed via the aircraft.

EDIT: You guys can fucking blow me. Moderators ban my account please.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#120

Post by rootusrootus »

linuxftw wrote: post #33

> Also, the airspeed, altitude and flight director pitch bar values from the left side noted deviating from the corresponding right side values. The left side values were lower than the right side values until near the end of the recording Hmm. Seems not just faulty AoA sensors? Perhaps some major malfunction in the flight computer? I've felt this entire time there's been a rush to claim 'faulty AoA sensors' because…

Blaming the sensors seems pretty plausible, because the expected failure rate of that sensor is roughly 1:100000 and there have been about a quarter million 737-MAX flights since the plane was introduced.
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