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Ethiopian Airlines 737 MAX Investigation Preliminary Report

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Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#51

Post by ams6110 »

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…

Yes, pilots selecting "up" on the trim switch should stop and override MCAS. But it also "resets" MCAS so that it will pitch down again on its next cycle. So you'd have to be constantly correcting it, or switch the electric trim off entirely (i.e. treat it as runaway trim).

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#52

Post by fgonzag »

msbarnett wrote: post #10
salex89 wrote: post #4

This isn't only MCAS issue anymore... If there are issues with manually rectifying trim after it went haywire because of any reason, that might be a problem. Interesting to find out was that an issue with older 737 models, just without severe runaway trim events.

> If there are issues with manually rectifying trim after it went haywire because of any reason, that might be a problem. It's not so much a "problem" as it is "physics". If the stabilizers are forcing the nose down, and the pilot pulls back on the stick (engaging the elevators) to pull the nose up, the forces exerted on the stabilizer are such that, in extreme cases, human force is insufficient to move the manual wh…

Layman question here:

Are there real scenarios where you want to have both control surfaces opposing each other so heavily? Are there real cases for having the trim trying to pitch the plane down while the pilot is trying to pitch up with the stabilizer?

Or should we limit the trim so it never actually opposes the pilots intentions? (Maybe set it to a neutral position and disengage auto function if this condition is detected?)

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#53

Post by weaksauce »

fgonzag wrote: post #36
Rooster61 wrote: post #7

> The crew performed runaway stabilizer checklist and put the stab trim cutout switch to cutout position and confirmed that the manual trim operation was not working. This is very damning to Boeing. It's precisely the procedure they prescribed and intended to mitigate possible MCAS caused trim runaway. In light of the fact that they put this in place as a justification for moving the engines forward without reclassif…

This means that Boeing is now 100% liable for the crash, as there was nothing the airline could've done in this scenario. It also means that the aircraft will have to be re-designed and re-classified before it ever flies again (it's now a very clear design error, not a training or communication error)

Thechnically they could have used the thumb switch and regained level flight to override the mcas system temporarily and then shut off the electrical system to the stab and trim manually thereafter. However, they didn’t know about the mcas system and the troubleshooting steps that Boeing has for that mode of failure were insufficient for this precise scenario.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#54

Post by Zak »

linuxftw wrote: post #42
Zak wrote: post #26

The design of this system, from what I've heard so far sounds insane . For some background, systems intended to automatically override pilot input upon detecting an impending stall have been used on large, complex aircraft for a long time. The most common mechanism is a stick pusher, which mechanically pushes forward on the stick to mimic what a pilot should do about an impending stall. The force of a typical stick p…

A compounding problem is, if the MCAS system is disable, now the flight characteristics are completely changed. Without additional training, there's no way for the pilots to know they have to trim down.

There's no way for pilots to know they have to trim down under what conditions?

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#55

Post by msbarnett »

rellui wrote: post #34
msbarnett wrote: post #10

Earlier quoted context omitted.

> If there are issues with manually rectifying trim after it went haywire because of any reason, that might be a problem. It's not so much a "problem" as it is "physics". If the stabilizers are forcing the nose down, and the pilot pulls back on the stick (engaging the elevators) to pull the nose up, the forces exerted on the stabilizer are such that, in extreme cases, human force is insufficient to move the manual wh…

From the way you describe it, if the stabilizer reaches the point where human force is insufficient to bring it back then they're toast since MCAS is tied into the electronic stabilizer. How did this scenario get missed during testing? You'd think they would do extra testing around the new changes.

> How did this scenario get missed during testing? You'd think they would do extra testing around the new changes.

Seems like systemic issues. Reportedly Boeing's submitted risk assessment of the MCAS was based on a single adjustment applied from it, not repeated cycles of application based on a failure of the AoA to recover to what it saw as "level".

The FSB noted the change to the cutout configuration as a difference, but didn't analyze how that could change anything during Autopilot or Speed Trim runaway situations. They didn't even make any note of the MCAS as a difference at all (possibly they were not informed of it?).

I suspect this all comes down to the fact that Boeing started with a conclusion -- The MAX series must not require new simulator time -- and that conclusion was the lens through which they analyzed (or didn't) everything else.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#56

Post by dmix »

sctb wrote: post #28
treis wrote: post #2

This shouldn't be marked as a dupe since it's the actual report. It sounds like they did everything right. There are two fateful moments: >At 05:41:46, the Captain asked the First-Officer if the trim is functional. The First-Officer has replied that the trim was not working and asked if he could try it manually. The Captain told him to try. At 05:41:54, the First-Officer replied that it is not working At this point t…

The report was the basis for this article and discussion yesterday: https://news.ycombinator.com/item?id=19570378 . From Hacker News' perspective, since the report itself was prominently linked in the article and its contents were discussed directly in the thread many times, it qualifies as a duplicate.

How is that the same? The BBC article doesn't contain even 20% of the information as the full report (including multiple charts).

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#57

Post by sctb »

dmix wrote: post #56
sctb wrote: post #28

Earlier quoted context omitted.

The report was the basis for this article and discussion yesterday: https://news.ycombinator.com/item?id=19570378 . From Hacker News' perspective, since the report itself was prominently linked in the article and its contents were discussed directly in the thread many times, it qualifies as a duplicate.

How is that the same? The BBC article doesn't contain even 20% of the information as the full report (including multiple charts).

You're absolutely right, it's not the same (in this sense “duplicate” is a misnomer—or at least should be applied to the thread or topic and not the submitted link).

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#58

Post by msbarnett »

fgonzag wrote: post #52
msbarnett wrote: post #10

Earlier quoted context omitted.

> If there are issues with manually rectifying trim after it went haywire because of any reason, that might be a problem. It's not so much a "problem" as it is "physics". If the stabilizers are forcing the nose down, and the pilot pulls back on the stick (engaging the elevators) to pull the nose up, the forces exerted on the stabilizer are such that, in extreme cases, human force is insufficient to move the manual wh…

Layman question here: Are there real scenarios where you want to have both control surfaces opposing each other so heavily? Are there real cases for having the trim trying to pitch the plane down while the pilot is trying to pitch up with the stabilizer? Or should we limit the trim so it never actually opposes the pilots intentions? (Maybe set it to a neutral position and disengage auto function if this condition is…

I'm not really an expert but my understanding is that the stabilizer will always out-muscle the elevator due to the physics of their respective positions on the aircraft.

In yet another change in the MAX series, there used to be what was called an aft column cutout switch which did more or less what it sounds like: engage when the stick was pulled back, cutting out autopilot commands to the stabilizer, so that it never attempted to outmuscle the pilot's input commands to the elevator.

This was removed in the MAX. Possibly this was to meet certification requirements: if the MCAS was needed to push down on the nose to meet the requirement that "During the approach to the stall, the longitudinal control pull force should increase continuously as speed is reduced from the trimmed speed to the onset of stall warning." If the MAX tends to pitch nose up at a certain point, the MCAS is intended to create a counter force against the pilots commands as it approaches that, rather than have the pilot pulling back at some point suddenly buck the nose upwards, then the MCAS needs to be active when the pilot is pulling back on the stick.

But it does argue that a LOT more thought needed to go into MCAS failure modes if it was necessary for it not to be cut out when the pilot is operating elevators.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#59

Post by fgonzag »

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.

But being able to actually utilize all your control surfaces without electrical power must be a basic requirement.

They could make the trim control wheel mechanically assisted, for example (since there are cases where electronic trim control is required because because of the amount of force needed to turn the wheel)

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#60

Post by Rooster61 »

dsfyu404ed wrote: post #35
Rooster61 wrote: post #14

Earlier quoted context omitted.

Doubtful. Older models don't have the oversized, forward-mounted engines, and thus don't have as much of an AoA problem as the MAX does in typical flight conditions. That's the reason the MAX should have been a separate classification with specific hands-on training.

The MAX is stable in normal flight. It's only when you are pitched up does the engine placement make it want to pitch up even more (and it is only a problem at a speed that is low enough that the normal stabilizer control doesn't have enough control authority to push it back) hence MCAS forcing the nose down every so often via the trim. A normal flight does include pitching up but it does not include pitching up at s…

> MCAS is specifically designed for an edge case

Source? That's the first I've heard that it's only for edge cases.

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