Ethiopian Airlines 737 MAX Investigation Preliminary Report • Hacker News Board

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#11

Post by wereHamster » Thu, Apr 04, 2019, 6:07 PM UTC

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 that "the pilot is always in control". That doesn't seem to be the case anymore, if by cutting the power to certain subsystems you can't control the airplane anymore.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#12

Post by [deleted] » Thu, Apr 04, 2019, 6:08 PM UTC

intrasight wrote: post #9

I really feel as we're entering an era where full automation is the only solution. This current paradigm of the plane telling the pilots "I don't know what to do here - handing it back to you" doesn't make much sense. Do we just need better multi-modal and redundant sensors so the plane isn't flying blind?

[deleted]

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#13

Post by fgonzag » Thu, Apr 04, 2019, 6:09 PM UTC

dsfyu404ed wrote: post #6

fgonzag wrote: post #5

Earlier quoted context omitted.

So MCAS put them in a hard to recover position which required electric trim to recover, and as soon as they turned on the electric trim MCAS turned on again and forced them into a crash? Then this is much worse than initially suggested, it means a pilot can be aware of MCAS and still be put in a extremely dangerous position due to the extreme control authority of the module.

Per the article a combination of things put them in that situation. Not just MCAS. Why is everyone so wedded to the narrative that this is the fault of MCAS and nothing else? Complex systems that have millions of man hours spent making them safer don't usually have back to back failures because of one faulty subsystem. We've incrementally refined commercial air travel to the point where single source accidents are ba…

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. MCAS both caused the situation, and made fixing it impossible, so it was the root cause of the crash.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#14

Post by Rooster61 » Thu, Apr 04, 2019, 6:09 PM UTC

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.

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.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#15

Post by jessaustin » Thu, Apr 04, 2019, 6:10 PM UTC

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.

...was that an issue with older 737 models...

737 is a common plane. If there were a systemic problem it would have been discovered long ago. (In fact I think several such problems were discovered and fixed long ago.) Look how quickly this new problem was discovered by the public, with both the manufacturer and primary regulator working to hide it.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#16

Post by Rooster61 » Thu, Apr 04, 2019, 6:11 PM UTC

fgonzag wrote: post #13

dsfyu404ed wrote: post #6

Earlier quoted context omitted.

Per the article a combination of things put them in that situation. Not just MCAS. Why is everyone so wedded to the narrative that this is the fault of MCAS and nothing else? Complex systems that have millions of man hours spent making them safer don't usually have back to back failures because of one faulty subsystem. We've incrementally refined commercial air travel to the point where single source accidents are ba…

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.…

> The point still stands, the root cause was MCAS

Agreed. One could argue that the AoA reading mismatch was the root cause, but the plane alerted the pilots to the discrepancy just as it was meant to (through stick shake and panel readouts).

They were aware of the issue, took measures to mitigate them, and MCAS still drove the plane into the ground.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#17

Post by fgonzag » Thu, Apr 04, 2019, 6:13 PM UTC

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…

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.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#18

Post by hencq » Thu, Apr 04, 2019, 6:13 PM UTC

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…

Yeah, from the report:

> At 05:40:35, the First-Officer called out “stab trim cut-out” two times. Captain agreed and First Officer confirmed stab trim cut-out.

[...]

> 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 05:43:04, the Captain asked the First Officer to pitch up together and said that pitch is not enough.

> At 05:43:11, about 32 seconds before the end of the recording, at approximately 13,4002 ft, two momentary manual electric trim inputs are recorded in the ANU direction. The stabilizer moved in the ANU direction from 2.1 units to 2.3 units.

> At 05:43:20, approximately five seconds after the last manual electric trim input, an AND automatic trim command occurred and the stabilizer moved in the AND direction from 2.3 to 1.0 unit in approximately 5 seconds. The aircraft began pitching nose down. Additional simultaneous aft column force was applied, but the nose down pitch continues, eventually reaching 40° nose down. The stabilizer position varied between 1.1 and 0.8 units for the remainder of the recording.

Seems this debunks the initial implicit commentary how this was due to 'third world' pilots and couldn't happen in the US.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#19

Post by crooked-v » Thu, Apr 04, 2019, 6:13 PM UTC

dsfyu404ed wrote: post #6

fgonzag wrote: post #5

Earlier quoted context omitted.

So MCAS put them in a hard to recover position which required electric trim to recover, and as soon as they turned on the electric trim MCAS turned on again and forced them into a crash? Then this is much worse than initially suggested, it means a pilot can be aware of MCAS and still be put in a extremely dangerous position due to the extreme control authority of the module.

Per the article a combination of things put them in that situation. Not just MCAS. Why is everyone so wedded to the narrative that this is the fault of MCAS and nothing else? Complex systems that have millions of man hours spent making them safer don't usually have back to back failures because of one faulty subsystem. We've incrementally refined commercial air travel to the point where single source accidents are ba…

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.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#20

Post by dsfyu404ed » Thu, Apr 04, 2019, 6:14 PM UTC

fgonzag wrote: post #13

dsfyu404ed wrote: post #6

Earlier quoted context omitted.

Per the article a combination of things put them in that situation. Not just MCAS. Why is everyone so wedded to the narrative that this is the fault of MCAS and nothing else? Complex systems that have millions of man hours spent making them safer don't usually have back to back failures because of one faulty subsystem. We've incrementally refined commercial air travel to the point where single source accidents are ba…

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 but on an aircraft with a different trim system it wouldn't be "hurr durr" levels of bad.