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Ethiopian Airlines 737 MAX Investigation Preliminary Report

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Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#101

Post by msbarnett »

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.

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...

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#102

Post by kayfox »

proctor wrote: post #63
wereHamster wrote: post #11

Earlier quoted context omitted.

> Aerodynamic loading prevented manual trimming using the trim wheels. Are the trim wheels mechanically connected to the trim mechanism in the back? Is there no way to trim the horizontal stabilizer electrically without also enabling MCAS? What would happen if the pilots continually pressed the trim up switch on the yoke? That should've prevented MCAS from activating, right? I thought Boeing airplanes were built such…

this video[0] from "mentour pilot" on youtube explains the stabilizer system quite well. it seems the trim wheels are connected manually to the trim mechanism in the back, but when the trim stabilizers are in an extreme position the wind loading on them makes it very hard/impossible to rotate the trim wheels manually. from what i understand, one way to relieve/lessen the wind loading so that the manual wheels can be…

Its possible and I think it was demonstrated in one of Mentour Pilot's videos that both pilots can work together to manually trim. Boeing put notes in the manual that this may be needed and it would not break the manual trim system to apply that much force to the trim wheels.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#103

Post by Rooster61 »

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.

It cuts out any electronic input, namely autopilot influence and the electric trim control present on the yoke. The trim wheel is not electric, and is the intended control when electric stabilizer control is either unavailable or intentionally disabled.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#104

Post by cjbprime »

wereHamster wrote: post #11
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…

> Aerodynamic loading prevented manual trimming using the trim wheels. Are the trim wheels mechanically connected to the trim mechanism in the back? Is there no way to trim the horizontal stabilizer electrically without also enabling MCAS? What would happen if the pilots continually pressed the trim up switch on the yoke? That should've prevented MCAS from activating, right? I thought Boeing airplanes were built such…

> Is there no way to trim the horizontal stabilizer electrically without also enabling MCAS?

Correct. One wonders if this is a consequence of trying to maintain the type rating and therefore not being able to introduce any pilot training about the MCAS feature, and therefore not being able to introduce any control features for it.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#105

Post by Rooster61 »

daddylonglegs wrote: post #96
Rooster61 wrote: post #21

Earlier quoted context omitted.

> They must cut it at a moment where the stabilizer isn't grossly mistrimmed with respect to level flight, OR they may well need to re-engage it and race the MCAS to get into a close-to-proper trim setup prior to cutting out the stabtrim and assuming manual trim control. All while keeping the plane in the air, reading the manual, and attempting not to lose composure knowing that the same thing has happened to another…

> > They must cut it at a moment where the stabilizer isn't grossly mistrimmed with respect to level flight, OR they may well need to re-engage it and race the MCAS to get into a close-to-proper trim setup prior to cutting out the stabtrim and assuming manual trim control. > All while keeping the plane in the air, reading the manual, and attempting not to lose composure knowing that the same thing has happened to ano…

I don't remember where I read it, but my understanding is that the MAX simulator does NOT simulate MCAS behavior.

EDIT: Found it: https://www.reuters.com/article/us-ethiopia-airplane-simulat...

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#106

Post by cjbprime »

msbarnett wrote: post #101
cjbprime wrote: post #89

Earlier quoted context omitted.

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

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?

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#107

Post by [deleted] »

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.

[deleted]

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#108

Post by Gibbon1 »

dsfyu404ed wrote: post #66
Rooster61 wrote: post #60

Earlier quoted context omitted.

> MCAS is specifically designed for an edge case Source? That's the first I've heard that it's only for edge cases.

If the pilot, who hopefully has thicker skin than people on the internet, moves the buttons and levers properly MCAS should never have to do anything. It's only supposed to actually do anything when you let the nose pitch up a little too much (which the system will detect using the AoA sensors)[0]. It's like automatic braking in a new car, if you drive the way you're "expected to" you shouldn't be activating it often…

> when you let the nose pitch up a little too much

Or flying into windshear changes the angle of attack on you.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#109

Post by zepearl »

dsfyu404ed wrote: post #27
crooked-v wrote: post #19

Earlier quoted context omitted.

MCAS forced the trim to a point where (because of aerodynamics) it was impossible to manually change the trim even after disabling MCAS, and was designed in such a way that it was impossible to engage the electrical trim controls (which could have saved the plane in this case) without also reenabling MCAS.

Flaps out supposedly would have kept MCAS off but the pilots weren't expected to know that.

Maybe, when they started thinking about a plan B, they were anyway already much too fast to use flaps?

I guess that there is a max speed for flaps (maybe the computer even actively refuses to extend them if the airspeed is higher than X to avoid that wings are destroyed).

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