Ethiopian Airlines 737 MAX Investigation Preliminary Report • Hacker News Board

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#151

Post by linuxftw » Thu, Apr 04, 2019, 8:23 PM UTC

Gibbon1 wrote: post #79

fgonzag wrote: post #52

Earlier quoted context omitted.

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 think the issue is the difference between the force needed to trim the aircraft and the acceptable control gain between the yoke and the elevator. I think with light aircraft trim is what you think of it. Slightly centers the yoke so the aircraft will fly level without control input. I think what's going on with the 737-MAX is the needed trim is way beyond what's needed to fly the plane. I'm now wondering is the MA…

> Meaning once the aircraft pitches up the pilot doesn't have enough authority to push the nose back down.

Or, alternatively, the pitch up happens so quickly and severely the risk of stall is too great above a certain AoA and the pilot won't have adequate time to respond.

If either is the case, it implies the aircraft is not safe to fly without MCAS operational, and possibly not at all.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#152

Post by makomk » Thu, Apr 04, 2019, 8:25 PM UTC

JRKrause wrote: post #73

fgonzag wrote: post #62

Earlier quoted context omitted.

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…

The manual electric trim inputs override the MCAS inputs[0], so they could have kept applying the appropriate trim regardless of the MCAS malfunction. [0] Appendix 1, 05:40:30 (You can see the AND command being canceled by the pilots manual electric ANU command)

It only overrides the MCAS inputs when they're actively adjusting the trim. So, they did try and correct the trim with the manual inputs, and five seconds after the last push of a trim adjust button the malfunctioning MCAS activated and adjusted the trim to something far worse than when they'd started. If I'm reading this right, the plane hit the ground killing everyone on board less than 30 seconds later.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#153

Post by rob74 » Thu, Apr 04, 2019, 8:26 PM UTC

weaksauce wrote: post #29

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.

The planes when setup correctly have a pretty good glide slope actually. When there is no power the pilots can deploy the apu to restore basic power for flight controls.

I assume you mean the Ram Air Turbine (https://en.wikipedia.org/wiki/Ram_air_turbine)?

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#154

Post by pdonis » Thu, Apr 04, 2019, 8:27 PM UTC

linuxftw wrote: post #139

pdonis wrote: post #134

Earlier quoted context omitted.

> You can't both require MCAS and be okay with disabling it at the most critical portion of a flight where the system is needed most. If MCAS activates when it isn't supposed to, which is what happened here, disabling it is what you want to do--to stop it from doing an incorrect thing. The plane was not in a situation where MCAS was needed; it was perfectly flyable without MCAS in the flight regime it was in. The pro…

> The plane was not in a situation where MCAS was needed; As far as I can tell, there is no takeoff (pitch up) situation where MCAS is not needed on the MAX 800. It's required for safe operation of the plane, otherwise the nose might pitch up unexpectedly into a stall. As I said, the system can't both "be required for safe operation of the plane" and "it's safe to disable MCAS while it's malfunctioning during takeoff…

> As far as I can tell, there is no takeoff (pitch up) situation where MCAS is not needed on the MAX 800.

Reference, please? I have not seen anything that says MCAS is required for safe operation at a normal angle of attack for takeoff and climb to altitude. Note that the key parameter is angle of attack, not pitch angle; if the plane is climbing the angle of attack is lower than the pitch angle, because the plane is climbing so the relative wind is coming from somewhat above the horizontal direction.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#155

Post by rootusrootus » Thu, Apr 04, 2019, 8:27 PM UTC

ams6110 wrote: post #32

hencq wrote: post #18

Earlier quoted context omitted.

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…

Interesting, that when they turned the electric trim back on, they only momentarily blipped the switches to trim up, and the stabilizer did move. Then five seconds later MCAS pitched down again and it was over. Wonder why didn't they trim up more when they turned the electric trim back on. On the surface it appears that was their last chance to salvage the situation.

That's what I think is most interesting. Given the position they found themselves in, going against the directive from Boeing to leave the trim cutout for the duration of the flight, means they were desperate to get the trim back up. So why did they not immediately make use of the reenabled electric trim control to counteract MCAS for as long as it took? They had kept the plane flying up until reenabling the electric trim, but that one action pushed it over the edge and could not be reversed? Confusing.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#156

Post by PunchTornado » Thu, Apr 04, 2019, 8:28 PM UTC

pdonis wrote: post #134

linuxftw wrote: post #46

Earlier quoted context omitted.

Many people have this opinion. You can't both require MCAS and be okay with disabling it at the most critical portion of a flight where the system is needed most.

> You can't both require MCAS and be okay with disabling it at the most critical portion of a flight where the system is needed most. If MCAS activates when it isn't supposed to, which is what happened here, disabling it is what you want to do--to stop it from doing an incorrect thing. The plane was not in a situation where MCAS was needed; it was perfectly flyable without MCAS in the flight regime it was in. The pro…

God, the people on that flight should thank that off duty pilot. So close to death.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#157

Post by fuckyah » Thu, Apr 04, 2019, 8:28 PM UTC

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…

"After the autopilot disengaged, the DFDR recorded an automatic aircraft nose down (AND) trim command four times without pilot’s input."

This seems to be key imo, why is there no master switch to completely turn off any computer assisted steering?

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#158

Post by linuxftw » Thu, Apr 04, 2019, 8:30 PM UTC

rootusrootus wrote: post #120

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.

But I just quoted 3 other sensors were reading off so...?

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#159

Post by JRKrause » Thu, Apr 04, 2019, 8:32 PM UTC

cjbprime wrote: post #113

simion314 wrote: post #85

Earlier quoted context omitted.

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.

This bulletin notes that electric stabilizer can be used to neutralize the control column prior to STAB CUTOUT, which it appears the Ethiopian crew did not do.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#160

Post by robryan » Thu, Apr 04, 2019, 8:33 PM UTC

javagram wrote: post #138

holler wrote: post #74

Earlier quoted context omitted.

this may be a naive question but with all of the advanced technology on these planes, why isn't there a check for altitude before any automated command runs which says "hey we just took off, are 400ft above ground, MCAS is saying nose down, we will crash if that command runs, CANCEL"

MCAS system is supposed to stop the plane from stalling. The faulty AoA sensor is causing the plane to think it’s about to stall and therefore MCAS works as designed to prevent the pilot from putting the plane into a stall. Of course the design is wrong because the plane is actually pointing toward the ground and MCAS thinks it’s pointing to the sky...

What about a single button to turn MCAS off and return to trim to a neutral position? Or a setting where the pilot is telling the system to ignore the sensor input.

Maybe a bit like a self driving car planes will eventually have a system where they can use visual markers as a check on what sensors are reporting.