Ethiopian Airlines 737 MAX Investigation Preliminary Report • Hacker News Board

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#191

Post by cjbprime » Thu, Apr 04, 2019, 9:09 PM UTC

rootusrootus wrote: post #163

linuxftw wrote: post #151

Earlier quoted context omitted.

> 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.

MCAS is not an emergency system, it is a trim control system designed to make the plane behave predictably in an otherwise unstable flight regime. If you wanted it to move fast, you would use a stick pusher; moving the trim takes many seconds.

I wonder why Boeing didn't use a stick pusher, then.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#192

Post by VBprogrammer » Thu, Apr 04, 2019, 9:09 PM UTC

msbarnett wrote: post #75

davepage wrote: post #43

Earlier quoted context omitted.

It does not appear to be "precisely" the procedure, in that sufficient nose up trim was not selected by the yoke manual trim switch prior to pulling the trim cutouts. And then they did not work the mechanical trim wheel hard enough to reset the residual nose down trim. And, the crew left the power setting at climb throughout almost the entire sequence, so they oversped the airframe. And, when they (against procedure)…

> 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 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.

The idea that a pilot would knowingly leave thrust at climb power while in modestly level flight at low altitude and expect not to overspeed is very strange to me, checklist or no checklist.

I think they were too busy fighting the controls to worry about that.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#193

Post by crimsonalucard » Thu, Apr 04, 2019, 9:19 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…

Even if this wasn't successfully done they would still be at fault. A plane is suppose to operate without have to have to do this, the procedures are a safety protocol. For the MCAS and safety protocol to fail is a double whammy.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#194

Post by jessmay » Thu, Apr 04, 2019, 9:19 PM UTC

1. Using the input from a single AoA sensor as the single source of truth.

2. Not ticking the box on the AoA DISAGREE alert option on the order sheet.

3. Not using electronic override to neutral before STAB TRIM CUTOUT.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#195

Post by crimsonalucard » Thu, Apr 04, 2019, 9:24 PM UTC

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…

Perhaps a beeping red indicator light/sound asking the pilot if he needs to override MCAS rather than a silent correction.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#196

Post by msbarnett » Thu, Apr 04, 2019, 9:24 PM UTC

js2 wrote: post #122

davepage wrote: post #43

Earlier quoted context omitted.

It does not appear to be "precisely" the procedure, in that sufficient nose up trim was not selected by the yoke manual trim switch prior to pulling the trim cutouts. And then they did not work the mechanical trim wheel hard enough to reset the residual nose down trim. And, the crew left the power setting at climb throughout almost the entire sequence, so they oversped the airframe. And, when they (against procedure)…

You can still point back to Boeing for not recommending flight simulator training of this failure scenario. One thing that I've wondered though as I've read more about MCAS. MCAS can't kick in till the flaps are fully retracted. Usually when something goes wrong in response to an action, the response is to undo the action. Why isn't the procedure or even the pilot instinct to re-extend the flaps when the nose down oc…

> One thing that I've wondered though as I've read more about MCAS. MCAS can't kick in till the flaps are fully retracted. Usually when something goes wrong in response to an action, the response is to undo the action. Why isn't the procedure or even the pilot instinct to re-extend the flaps when the nose down occurs just after having retracting the flaps?

When the two AoA sensors began to disagree during takeoff, the left and right side airspeeds started disagreeing as a result. That leads the pilots to execute the Unreliable airspeed indicator checklist memory items, which include levelling off for troubleshooting and leaving the flaps in their current configuration (retracted), and to keep thrust high to avoid stalling due to the unreliable airspeed indicators.

It's precisely at this point that MCAS now kicks in and starts nosing them down. Extending flaps will cut it back out but: 1) they're now instinctually fighting the stabilizer with elevators to stay out a nose dive, 2) the checklist they were running when this started says not to extend flaps, 3) the Boeing/FAA directive tells them to hit the STAB CUTOUT, so they do that, but 4) the elevator/stabilizer fight + the high speed from the thrust being set to climb due to the checklist they were running when this started results in so much force on the manual stabilizer crank that the co-pilot can't turn it by hand and now they're so engaged in this stabilizer fight and trying to get the plane trimmed for landing that the flaps are the last thing on their mind, and the Boeing/FAA directive never instructed them to try re-extending them, airspeed disagree be damned. They're completely out of the manual and FAA/Boeing directive at this point. Nothing covers this (anymore, 30 year manuals would have had a procedure to ease the forces on the wheel here).

So lacking any other means of trimming for landing, they turn the electronic stabilizers back on, and trim electronically, which works...but they don't cut it again within 5 seconds (the FAA/Boeing directive didn't really spell out how crucial this timing is, or even consider that manual trim would have been made impossible by everything else going on), and the MCAS runs again, and the dive becomes fatal.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#197

Post by jpatokal » Thu, Apr 04, 2019, 9:26 PM UTC

ec109685 wrote: post #145

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…

For comparatively trivial things like what to do when we get too much traffic on servers, engineers do “game day” testing. How does anyone expect pilots to run through these crazy scenarios and perform flawlessly unless they have done it in a simulator many times?

Um, they do? Endless simulator drills of various emergency scenarios is a large part of becoming a pilot and staying certified.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#198

Post by fgonzag » Thu, Apr 04, 2019, 9:28 PM UTC

robryan wrote: post #160

javagram wrote: post #138

Earlier quoted context omitted.

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.

Seriously, a MCAS indicator light (if MCAS is actively issuing commands), a big red button below it called "Disable MCAS" , and a MCAS disabled light (so pilots actually remember when it is disabled) should be more than enough to prevent this.

Besides adding the AoA disagree light AND having MCAS use a triple AoA sensor vote to determine the AoA angle.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#199

Post by VBprogrammer » Thu, Apr 04, 2019, 9:32 PM UTC

rootusrootus wrote: post #112

dsfyu404ed wrote: post #20

Earlier quoted context omitted.

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?

I doubt this is possible. If it was it would be a certification requirement because it would make any trim runaway incident trivial.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#200

Post by aaron695 » Thu, Apr 04, 2019, 9:33 PM UTC

ummonk wrote: post #140

rdiddly wrote: post #119

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 aircr…

Why is that getting downvoted? What the heck.

> knowing some greedy bunch of assholes were effectively that malevolent entity, whose will was expressed via the aircraft.

If this it true then link? When was this confirmed? Who are the assholes?

Else this is just Reddit spam that contributes less than nothing.