Ethiopian Airlines 737 MAX Investigation Preliminary Report • Hacker News Board

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#181

Post by phire » Thu, Apr 04, 2019, 8:55 PM UTC

rootusrootus wrote: post #155

ams6110 wrote: post #32

Earlier quoted context omitted.

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…

They made two small trim up commands. But looking at the flight data graph, the stablizer on the bearly moves. The motor was probably stugling against the aerodynamic forces too.

Prehaps they didn't think it moved at all and decided they stablizer was jammed. 5 seconds later, MCAS commands down and because that's in the same direction as the aerodynamic forces it moved a lot easier.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#182

Post by makomk » Thu, Apr 04, 2019, 8:57 PM UTC

davepage wrote: post #43

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…

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)…

The thing is, it looks like they did try and use the manual trim switch to get the trim correct prior to pulling the breaker - they just didn't do it for quite long enough to fully correct for the misbehaving MCAS system's actions. If they hadn't I suspect the plane would've crashed several minutes earlier than it did. And remember, if they take more than a few seconds to consider whether the trim is correct before pulling the breaker, the MCAS system activates again, undoes their manual trimming, and tries to kill everyone on board.

This entire report is frankly terrifying.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#183

Post by phire » Thu, Apr 04, 2019, 9:00 PM UTC

fgonzag wrote: post #17

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…

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.

Sure, but that's assuming the plane was trimmed somewhat correctly to start with. The only reason the pilots had problems manually moving the trim was because the electric motor moved it far out of the correct trim range.

Also, it's not zero power. A ram air turbine deploys and provides hydraulic pressure to move the controls. And there are batteries to keep a small number of essential flight instruments powered.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#184

Post by msbarnett » Thu, Apr 04, 2019, 9:00 PM UTC

treis wrote: post #164

msbarnett wrote: post #132

Earlier quoted context omitted.

They're at 12,000-14,000 feet altitude at 05:43ish, when the co-pilot reports the non-functional manual trim. So ~6,000-7,000 height. Still damn close to the ground to be comfortable letting the plane dive.

But high enough to re-enable a system you know is malfunctioning? I guess we will have to wait for the final report but the pilot's actions are perplexing here. Even if we accept that they can't trim with the wheel, why enable electric trim and then not use it? Why not enable it, trim to where you want, and then use the cut out switch again?

> But high enough to re-enable a system you know is malfunctioning?

They couldn't land with the stabilizers mis-trimmed. If the wheel wouldn't budge, re-enabling it and trimming electronically was the only option likely to occur to them, since the only other possible option (release the column, let the plane nose down so that the forces on the screw ease and they can trim manually) is completely counter-intuitive and was removed from Boeing's documentation and simulator training 30-40 years ago and was not covered in the FAA/Boeing MCAS directive.

> Even if we accept that they can't trim with the wheel, why enable electric trim and then not use it?

They did use it after re-enabling it. They just didn't re-disable it within 5 seconds of their last input manual electronic trim command, so the MCAS ran again a final time.

Whether that's because they just didn't get to the switches in time, weren't aware they had such a short window to do so (the FAA/Boeing bulletin does a piss poor job of communicating this), or were just so thoroughly overwhelemed by everything that was going on is hard to say at this point.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#185

Post by javagram » Thu, Apr 04, 2019, 9:01 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.

Indeed there are many ways to design this better.

The 737 max was designed to not be any different to fly than previous 737s, so adding a button to disable this system was not allowed as I understand it.

As these two tragic accidents have made clear that specification was pretty wrongheaded...

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#186

Post by cjbprime » Thu, Apr 04, 2019, 9:02 PM UTC

ls612 wrote: post #161

Domenic_S wrote: post #117

Earlier quoted context omitted.

Why is it possible to set the trim to insane? Is there a legitimate scenario where that would be used?

If the AoA was truly way too high then insane trim would be required to correct that. MCAS and similar systems came about in response to AF 447 where the pilots entered a high AoA situation and stalled the plane into the Atlantic.

Is it necessarily true that you have to use the stabilizer for MCAS? Would implementing a stick pusher instead fail to avoid some stalls?

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#187

Post by VBprogrammer » Thu, Apr 04, 2019, 9:02 PM UTC

cjbprime wrote: post #131

3JPLW wrote: post #94

Earlier quoted context omitted.

Is it still possible at or near VMO? And when the trim is maxed out? I don't think I've seen an authoritative answer on that, and that's the situation they faced.

Another commenter pointed out that the captain is pulling back as hard as he possibly can on the yoke; releasing it to turn a trim wheel instead may have been just as deadly.

If my reading of the graphs is correct they are above Vmo for most of the episode. I'm surprised the flight surfaces weren't torn from their hinges.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#188

Post by paulmd » Thu, Apr 04, 2019, 9:02 PM UTC

holler wrote: post #74

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…

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"

Because right after takeoff is the single most common point at which you might stall the aircraft. What's more, at that point it is likely to be a power-on stall, which is the specific situation that MCAS is intended to fix (because the 737 MAX has a tendency to pitch upwards in power-on scenarios).

Really there should just have been a separate cutout for MCAS itself, but that would have triggered retraining requirements, so Boeing wanted to avoid that. But very simply, there should just be an 'off' switch to disable this system if it starts to run away, distinct from the whole runaway trim procedure.

(as well as 2-of-3 redundancy on the sensors, the lack of redundancy is seriously exacerbating the rate of incorrect MCAS system activations)

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#189

Post by phire » Thu, Apr 04, 2019, 9:06 PM UTC

danaliv wrote: post #118

Am I mistaken or is there a missing item in the chronology where the pilots reactivated the electric stabilizer trim? The timeline mentions stab trim being switched to cutout, and if you look at the FDR graphs there's a (relatively) long period of more-or-less stable trim after that. But then there's an automatic trim down, which is only possible if the electric stab trim is switched back on, and there's no mention o…

The pilots didn't make verbal mention of re-enabling the switches and the position of the switches isn't recorded in the flight data recorder.

We have no way of knowing when, or why those switches were flipped.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#190

Post by ams6110 » Thu, Apr 04, 2019, 9:08 PM UTC

msbarnett wrote: post #168

ams6110 wrote: post #121

Earlier quoted context omitted.

Pilots should have known that MCAS is inactive with flaps extended, per information from Boeing.

Nope. The Boeing/FAA directive re: MCAS does not instruct them to extend flaps. Further, there's other things going on in the cockpit due to the malfunctioning AoA sensor that's the root of all this. Specifically one of the effects is that they get an Unreliable Airspeed Indication, and run that checklist. That checklists memory items include maintaining the current flap configuration (in this case, flaps up). The Bo…

You're correct that the AD and runaway trim procedure does not instruct them to extend flaps. What I was addressing was the statement upthread that "Flaps out supposedly would have kept MCAS off but the pilots weren't expected to know that."

If I'm a 737 MAX pilot, after the Lion Air crash I'm learning absolutely everything I can about MCAS. And from information Boeing provided I would learn that it operates with autopilot off, flaps up, at high indicated AoA.

You mention unreliable airspeed, that was actually their first problem. Flaps were extended at that point, as according to the narrative they were not retracted until after the AoA disagree, stick shaker, and airspeed disagree. In fact it almost reads like they were intending to continue the flight, engage autopilot, and climb to 32,000 ft with the stick shaker going the whole time. That seems very odd to me.