Ethiopian Airlines 737 MAX Investigation Preliminary Report • Hacker News Board

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#171

Post by msbarnett » Thu, Apr 04, 2019, 8:41 PM UTC

JRKrause wrote: post #159

cjbprime wrote: post #113

Earlier quoted context omitted.

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.

They did so repeatedly.

The bulletin completely fails to note that if you don't time the STAB CUTOUT immediately after you use manual electronic control but instead the MCAS acts again before you cutout, you're left trying to manually trim the stabilizers in a situation where the elevators are putting so much force on the jackscrew that manual stabilizer trim may be impossible.

That's what got this flight.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#172

Post by robryan » Thu, Apr 04, 2019, 8:43 PM UTC

cjbprime wrote: post #111

brucemoose wrote: post #25

Earlier quoted context omitted.

Layman question -- Does the MCAS system take current height from terrain into account when forcing the nose down?

Nope. (And arguably it shouldn't: a stall at low altitude is much worse than a stall at high altitude, because you lack recovery time. Most fatal stalls begin below 1000ft AGL, which is where this plane was. So your stall avoidance system shouldn't shutdown just because you're at low altitude: that's when you want to avoid stalls.)

Would there be different change in altitude characteristics in a nose up vs nose down situation?

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#173

Post by Rooster61 » Thu, Apr 04, 2019, 8:44 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"

I just took a look at the take-off procedure in the checklist, and it lists that one should extend flaps before and during take-off/ascent. The checklist says that one should start retracting the flaps at 1000'AGL, the altitude where they seem to have started having problems.

MCAS is disabled while flaps are extended. It would have activated the second they retracted the flaps.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#174

Post by Rooster61 » Thu, Apr 04, 2019, 8:44 PM UTC

cjbprime wrote: post #137

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"

You'll 100% for sure crash if you stall at 400ft above ground. MCAS is designed to prevent stalls.

Not at take-off. See above comment about flaps.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#175

Post by rootusrootus » Thu, Apr 04, 2019, 8:45 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?

Arguably a fundamental problem with these planes is that they have a bit too much automation, which runs directly contrary to the historical attitude of Boeing that 'pilots are always in control'. So this is a surprising change for a long-time Boeing pilot.

If you want to see what it's like with far more automation, look at Airbus. They have a fundamentally different philosophy on computer controlled aviation. They also don't have a perfect record, there have been Airbus crashes due to malfunctioning automation.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#176

Post by peteradio » Thu, Apr 04, 2019, 8:46 PM UTC

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…

Lol top comment. Seriously who sees this and is like NO I must downvote. Its a great comment, imagine a plane all of the sudden just fighting to crash you into the ground, what a horror show.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#177

Post by JRKrause » Thu, Apr 04, 2019, 8:50 PM UTC

makomk wrote: post #152

JRKrause wrote: post #73

Earlier quoted context omitted.

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.

Refer to the graph on Appendix 1, the automatic AND commands begin about 5 minutes before the crash. They counter them partially with the manual ANU electric trim for the first 1.5 minutes before issuing STAB CUTOUT.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#178

Post by Zak » Thu, Apr 04, 2019, 8:51 PM UTC

linuxftw wrote: post #141

Zak wrote: post #54

Earlier quoted context omitted.

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

During otherwise normal flight conditions, the nose might unexpectedly pitch up. MCAS is designed to prevent that when AoA > somevalue to prevent stalling.

During normal flight conditions, the nose does not pitch up beyond the pilot's ability to control with the elevator, and the pilot doesn't fly especially close to the critical angle of attack.

MCAS is for an unusual combination of conditions where both elevator and trim might be needed to reduce the angle of attack. An example would be if the pilot set a fairly high nose-up trim to maintain the slow speed of final approach, then decided to go around (abort the landing), applied full thrust, and fully raised the flaps too early.

A pilot would definitely know that using trim is an option if the elevator was not producing enough nose-down pitch, but a pilot who has put the aircraft into the situation where a stall is imminent during a passenger flight has already made a series of errors.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#179

Post by VBprogrammer » Thu, Apr 04, 2019, 8:53 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 graphs record the AND trim movement even if the electric trim is disabled. You can see that by comparing the pitch trim absolute position to the manual and automatic electronic trim graphs.

Re: Ethiopian Airlines 737 MAX Investigation Preliminary Report

#180

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

PunchTornado wrote: post #149

Zak wrote: post #54

Earlier quoted context omitted.

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

I don't understand, either you need the MCAS to fly the plane or you don't. If a 737 pilot can operate a 737 MAX only with MCAS, then the pilot shouldn't be able to disable it, right? What if he disables it and then the plane is stalling? There is no MCAS to help.

You don't need MCAS to fly the plane. You don't need ABS to drive a car. Both are designed to counteract erroneous user input based on the readings of fairly simple sensor systems that cannot understand the whole situation and are themselves capable of false readings.