21 ms·

Ethiopian Airlines 737 MAX Investigation Preliminary Report

243 points by kunkurus 8y ago

treis 8y ago
This shouldn't be marked as a dupe since it's the actual report. It sounds like they did everything right. There are two fateful moments: >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 this point they were in stable flight and the trim wasn't that extreme. An inability to manually adjust trim is a huge design flaw unless it was pilot error. >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. They re-activated electric trim control but only slightly adjusted it. MCAS kicks in to send them nose down. There's no further pilot correction recorded. Again, unclear here if there is an airplane issue preventing them from adjusting trim or a pilot error.
- sctb 8y ago
  The report was the basis for this article and discussion yesterday: https://news.ycombinator.com/item?id=19570378 https://news.ycombinator.com/item?id=19570378. From Hacker News' perspective, since the report itself was prominently linked in the article and its contents were discussed directly in the thread many times, it qualifies as a duplicate.
  - dredmorbius 8y ago
    You'd just noted and moved comments to this submission from another, which is confusing. (emailed to hn@ also)
    - Rooster61 8y ago
      Agreed, an article based on the report is not the same as the actual report. Not to mention it pulled this off the front page, where it absolutely should be considering the traffic this issue has been garnering.
  - dmix 8y ago
    How is that the same? The BBC article doesn't contain even 20% of the information as the full report (including multiple charts).
    - sctb 8y ago
      You're absolutely right, it's not the same (in this sense “duplicate” is a misnomer—or at least should be applied to the thread or topic and not the submitted link).
      - Rooster61 8y ago
        If that's the case, it would be appreciated if this was moved back, or at least re-posted. It appears worthy of being visible based on the amount of discussion it generated in a short time. Moving it to this thread effectively threw a bucket on the fire. This document is a big deal.
        sctb 8y ago
        OK! We've put it back.
        Rooster61 8y ago
        Err...Not quite, its still under the poster of the bbc article, but the effort is appreciated nonetheless. You guys rock. :)
mhandley 8y ago
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 reliable) reads 365 knots. VMO is 340, so they're already in big trouble. 5 seconds later, MCAS kicked back in, and then it was all over. Very sad.
- fgonzag 8y ago
  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.
  - dsfyu404ed 8y ago
    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 basically nonexistent. I'm not saying that MCAS isn't mostly to blame but the fact that the maximum inputs from the pilot is still lesser than the control authority from the trim raises some serious questions MCAS or not. Had "extend flaps" been at the bottom of the checklist that would have disabled MCAS and likely prevented this crash.
    - quenstionsasked 8y ago
      MCAS issues nose-down, speed increases and nose pitches down. Nose never pitches back up. Plane crashes. Sure, maybe some complex combination of maneuvers could have helped them out of this but there's really everything pointing the blame at this one system. Pilots did what the boeing manual suggested (even included at the end of this report).
salex89 8y ago
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.
- msbarnett 8y ago
  > If there are issues with manually rectifying trim after it went haywire because of any reason, that might be a problem. It's not so much a "problem" as it is "physics". If the stabilizers are forcing the nose down, and the pilot pulls back on the stick (engaging the elevators) to pull the nose up, the forces exerted on the stabilizer are such that, in extreme cases, human force is insufficient to move the manual wheel. This is extremely (EDIT: No longer) well documented by Boeing going back decades, and it presents solutions of either: 1) reengaging the electronic stabilizer trim and trimming electronically back to a point where the forces are manageable, then cutting electronic trim again, or 2) relaxing the stick, letting the nose go the way the stabilizers want it to, which slackens the force, and makes cranking the wheel manually doable. Great if you're stabilizers are trimmed severely nose down at a height of 40,000 feet; less great at a height of 1,000 feet. (EDITED TO ADD: Apparently this information & procedure was removed from Boeing's manuals after the 1980s, and pilots no longer train on it, as the issues that led to its necessity were thought solved. The MCAS apparently has reintroduced the need for them.). This is compounded by a new change in the 737-MAX: in prior 737 series aircraft, there are two stab trim cutout switches: one to cut the autopilot's ability to command the stabilizer trim, the other to cut electronic power to the stabilizer trim adjustment entirely, so you could stop the autopilot from changing the trim but still retain electronic control and not need to worry about the manual crank forces. The two switches are MAIN ELEC and AUTO PILOT. In the MAX, this changed, and it's now an all or nothing setup: you have to kill electronic power to kill autopilot commands to the stabilizer. The two switches are now PRI and B/U. Both must be cut out to stop any stabilizer runaway, including MCAS runaway. This all seems to add up to Boeing's narrative of "The MCAS is safe, pilots just need to know to hit the stab trim cutout" being grossly insufficient. They must cut it at a moment where the stabilizer isn't grossly mistrimmed with respect to level flight, OR they may well need to re-engage it and race the MCAS to get into a close-to-proper trim setup prior to cutting out the stabtrim and assuming manual trim control.
Rooster61 8y ago
> 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 reclassifying the plane, I don't see how they can get around fault here. What a tragedy. I can't imagine the emotion those pilots must have felt after attempting to do exactly what they were told to do to keep the aircraft in the air, only to have the nose lurch back downwards. Godspeed, gentlemen. It wasn't your fault.
- smaili 8y ago
  Just out of curiosity, will there actually be any punishments handed out beyond just a slap on the wrist? Or is Boeing far too large of a corp to take on?
  - geofft 8y ago
    Incidentally, Boeing has received billions of dollars in illegal tax subsidies from the US (as in, anti-competitive with non-US manufacturers like Airbus): https://www.bbc.com/news/business-38131611 https://www.bbc.com/news/business-38131611 So it's hard to imagine fines or market penalties actually conveying any productive sense of punishment.
    - CompelTechnic 8y ago
      >This marks a victory for Airbus in a war without end. >Back in September the European aerospace giant, which employs 15,000 people in the UK, was on the receiving end when it was found that billions of euros in low interest loans amounted to illegal subsidies. >Boeing celebrated that moment as a comprehensive victory which would deal a mortal blow to Airbus and result in more US jobs. >The reality is that neither of these companies can exist without government subsidies. Both of these companies are strategic assets to the countries that have them. So no surprise here. I'm not even upset at it- it's realpolitik.
intrasight 8y ago
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 8y ago
  [deleted]
- crooked-v 8y ago
  This problem was one of too much automation, which decided that the best course of action was to fly the plane into the ground. In a less automated system, the pilots would have been able to enable electrical trim systems without also having their trim control overridden by MCAS and would have been able to recover flight control that way.
- rootusrootus 8y ago
  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.
Zak 8y ago
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 pusher is low enough that a pilot can overcome it with arm strength. A stick pusher, or a pilot pushing forward on the stick is not enough for the aerodynamics of the 737 MAX in certain circumstances. The position of the engines means the thrust tends to push the nose up, and at low speed, the elevator controlled by the stick alone is not enough to push the nose down when nearing a stall. Instead, the trim must be used, which moves the whole horizontal stabilizer rather than just the elevator. That's not part of the procedure on most aircraft, including other 737s, so regulators would not allow pilots rated for 737s to easily transfer their rating to the new aircraft, if they were willing to certify such an aircraft at all. MCAS is the solution to this problem. It automatically uses the trim to force the nose down with greater authority than the elevator alone could provide. While the concept is sound, the implementation strikes me as reckless. First, the system uses only one of the two angle of attack sensors to determine that the aircraft is nearing a stall. Such sensors have a fairly high rate of failure relative to the safe operation of a commercial aircraft. As far as I can tell, causing the system to use both sensors is a software change and all models of the aircraft had the required hardware. I can't imagine a good reason the system was designed this way, especially since Boeing has included MCAS on a previous aircraft, the tanker version of the 767, and it did use two sensors in that application. Second, activating the system automatically, and allowing it to contradict the pilot's attempt to pitch up using the elevator seems like a mistake. While stalls are almost always the result of pilot error, I'm inclined to think automated systems should let humans have the last word. A better design might have been to automatically trim nose-down in response to full forward pressure on the stick at low speed. That could be in combination with a stick pusher providing full forward pressure on the stick when an impending stall is detected in the absence of pilot input to the contrary.
- linuxftw 8y ago
  A compounding problem is, if the MCAS system is disable, now the flight characteristics are completely changed. Without additional training, there's no way for the pilots to know they have to trim down.
dao- 8y ago
I'm starting to think that disabling MCAS isn't even a sane option because then the pilots are exposed to unexpected aerodynamic characteristics due to the plane's weird design. Does this make sense?
- linuxftw 8y ago
  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.
  - pdonis 8y ago
    > 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 problem is that, if you disable MCAS (which requires disabling the entire electric stability trim system) without first using the electric trim system to get back to something close to neutral trim, you will be stuck with a lot of nose down trim that you can't remove fast enough with the manual trim wheel. It looks like that's the situation this flight got into. But if you first use the electrim trim system to get back to something close to neutral trim, and then, before MCAS has a change to mess it up again, disable the entire electric stability trim system, you're ok. How do we know? Because that's what the first Lion Air flight (the one the day before the one the crashed) did, because an off duty pilot that was riding in the jump seat figured out what was happening and how to deal with it and told the pilots what to do.
    - linuxftw 8y ago
      > 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." You have to pick one.
linuxftw 8y ago
> 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 'sensors' are known to fail, and it's an easy scapegoat. My question is, does the flight recorder have raw inputs from the sensor, or just what the flight computer is reporting from the sensor? Seems like the latter.
- rootusrootus 8y ago
  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.
  - linuxftw 8y ago
    But I just quoted 3 other sensors were reading off so...?
    - jellicle 8y ago
      The problem is all those other things are CALCULATED values off the AoA sensor. Literally everything depends on a single tube which is known to get clogged with ice, bugs etc. So for the 737 Max, a single clogged AoA sensor causes all these things immediately and simultaneously: -- all your airspeed and altitude readouts go haywire, at least on one side of the cockpit -- a bunch of alarms start going off and a bunch of lights start flashing, each indicating a different problem -- your control stick starts shaking -- a little-documented system immediately commands the nose straight down into the ground, and keeps commanding it until turned off (but turning it off effectively prevents you from counteracting the nose down command) Good luck.
ralph84 8y ago
This puts the nail in the coffin of a quick software fix getting the max back in the air anytime soon. To have any credibility the FAA is going to have to recertify the entire flight control system with all failure modes considered. And no scout's honor with Boeing this time.
jordache 8y ago
>The left Indicated Airspeed increased, eventually reaching approximately 458 kts and the right Indicated Airspeed reached 500 kts at the end of the recording. The last recorded pressure altitude was 5,419 ft on the left and 8,399 ft on the right. How can the sensors record such different readings?
- daddylonglegs 8y ago
  The reading from the pitot tubes (pressure sensors) for a particular airspeed varies with the sensors angle to the airstream so the airspeed value is adjusted based on the angle of attack (AoA). If the AoA sensor is on the fritz on one side the airspeed reading for that side changes. For redundancy, each pilot receives separate instrument readings from independent sensors. When the airspeed readings disagree a warning is activated and the pilots are expected to follow a prescribed and memorized procedure. The warning that the AoA sensors disagree was a paid option before this crash and the Lion Air crash.
  - mannykannot 8y ago
    While the changes in the right pressure altitude indicator track those of the radar altimeter reading, the left falls below it -- except, in the last 15 seconds, there are blips where the left pressure altitude jumps to match the right one. These correspond to the the times when the errant left AofA indication briefly matches that from the (apparently working) right AofA vane. The airspeed discrepancy has the same pattern: the left is reading low and also showing blips, where it equals the right reading, that also correspond to when the AofA indicators match. All this would be consistent with the left AofA reading being used to correct the left static pressure reading by raising it at high angles of attack, and vice-versa for the right AofA vane and static port (this assumes there is a right static port used by the right pressure-driven instruments, an a left port for the left ones.)
- sdenton4 8y ago
  Q: How do you use a barometer to accurately measure altitude? A: Drop it, and count the seconds until it hits the ground.
- 8y ago
Osiris 8y ago
Do we know of any documented situations in which MCAS actually did it's job correctly, that is, preventing a stall from too large AoA?
- laydn 8y ago
  This is a great question. I am not sure why it is not being asked more frequently.
- bbojan 8y ago
  Preventing a stall is not what the job of MCAS is. (edit: clearer language)
  - cjbprime 8y ago
    This language is not clear, at least to me. MCAS is absolutely an anti-stall system. There's some correlation with yoke handling, but it's still with the goal of affecting yoke handling in order to prevent stalls.
danaliv 8y ago
> Both engines were found buried in the ground at a depth of approximately 10 meters. My God.
- segmondy 8y ago
  fly into the ground at 40 degrees doing 600mph will do that.
deleted 8y ago
[deleted]
DougPhillips88 8y ago
The average Ethiopian IQ is 69. Just sayin'.
danaliv 8y ago
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 of that. I also can't help but wonder if exceeding Vmo was a factor. (I don't mean to sound as though I'm trying to assign any blame here. Obviously this was a confluence of problems, the most significant of which is the control system reacting to an erroneous 75° (!) AOA indication.)
- VBprogrammer 8y ago
  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.
  - danaliv 8y ago
    Right, I get that, but the last automatic AND actually moved the stabilizer.
    - VBprogrammer 8y ago
      Your right. They didn't mention it, probably because it's not indicated on the CVR at what point they decided to try turning it back on.
- phire 8y ago
  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.
- Torquer 8y ago
  I find it amazing that there is no mention in the report of this fact. The investigators obviously know that they turned the cutout switches back to normal mode. Assuming there really was no mention of it on the CVR strongly suggests that it was the very inexperienced first officer who did this (the switches are on his side) without getting concurrence from the captain. If you carefully study the graphs in the report, the plane was really not in bad shape up until this point. They were 7000 feet above ground and climbing. They were going too fast, but again, this is the fault of the crew. If they would have not flipped the cutout switch and taken stock in the situation and backed the throttles down, it is quite possible this flight would have ended very differently.
rdiddly 8y ago
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 aircraft. EDIT: You guys can fucking blow me. Moderators ban my account please.
- ummonk 8y ago
  Why is that getting downvoted? What the heck.
  - aaron695 8y ago
    > 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.
    - geofft 8y ago
      The "greedy bunch of assholes" are the Boeing execs who optimized for profit over the lives of those pilots and their passengers. (I thought that was pretty clear from the comment, fwiw.) I think it's been pretty thoroughly established at this point that an underlying design constraint in the MCAS was that it was easier for Boeing to get a 737-with-modifications approved than a new airplane, despite the engines being configured rather differently from existing 737s, and the MCAS was a computerized hack to avoid instability caused by the design of the newer engines (specifically, the newer engines would risk pitching up and stalling if you flew a 737 MAX like a normal 737, so they added the MCAS to counteract that, and the problem in both crashes was the MCAS unnecessarily activated and pitched the plane down when the plane was actually flying fine). Certainly lots of other things went wrong too, but had they instead just built a new plane design, the problem wouldn't have arisen in the first place. (I can find links about this if you're curious but it seems to be the consensus interpretation of what happened.)
ummonk 8y ago
It appears that Boeing's directive was simply wrong, and doomed them. The right response to faulty MCAS is not to do a stab trim cutout and then manually retrim, but to first use the manual electric trim switches to fix the trim and then immediately do a stab trim cutout. This is mentioned at the end of the directive, but reads more like an afterthought when it should be the standard procedure.
- VBprogrammer 8y ago
  In my opinion the pilots here have attempted to follow that checklist as well as could be expected (bearing in mind that their lives depended on it). Having to trim to neutral before disengaging the trim would have contradicted the normal trim runaway checklist. We're now talking about pilots having to have abnormal levels of skill and possibly even strength to fly this aircraft. That's an untenable state of affairs. Boeing have really screwed this one up.
jessmay 8y ago
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.
- elsonrodriguez 8y ago
  Boeing should be raked over the coals for even allowing item 1 to be a valid order configuration, and item 2 for being an order option.
toobulkeh 8y ago
Can someone explain to me how $BA is up nearly 3% after this news? Closed 395.86 USD +11.12 (2.89%)
aeternus 8y ago
> From 05:40:42 to 05:43:11 (about two and a half minutes), the stabilizer position gradually moved in the AND direction from 2.3 units to 2.1 units. During this time, aft force was applied to the control columns which remained aft of neutral position. The left indicated airspeed increased from approximately 305 kt to approximately 340 kt (VMO). The right indicated airspeed was approximately 20-25 kt higher than the left. This part is very interesting. This aircraft nose down (AND) trim occurred while the electric trim system was disabled. This means that either the pilots were moving the manual trim wheel in the wrong direction, or that aerodynamic forces were causing significant AND trim.
spockz 8y ago
Imagine being the one who was forced to develop this. Regardless of whether one did see something like this coming and raised objections or missed it, I would be unable to continue to function. Do we know how teams cope with this?
YjSe2GMQ 8y ago
Aircraft crashes attract a lot of attention, and probably for a good reason. But just to keep folks' internal perception of risk - aircraft safety has improved a lot: https://www.edge.org/images/airtravelsafety.png https://www.edge.org/images/airtravelsafety.png
dboreham 8y ago
Is there a demand for freight aircraft of this size? Conversation to freighters might be the best option for these planes.
teh_klev 8y ago
I usually enjoy HN discussions, but this is a pitiful and sad thread. I couldn't be bothered trawling past the responses to the top comment with back seat drivers, and "ooh but I read the 737's flight manual, they fucked up". Likely 99% of the commentariate here are neither 737 Max pilots, let alone even licensed to fly commercial passenger jets, or even have a pilots license. And no, as much as I love the guy, watching Mentour Pilot does not make you an expert. They have fuck all knowledge about, or even experience, about the sudden cockpit workload that was imposed on these pilots until the point of hull loss. Have some respect. Oh, and you know what, this was Ethiopia's national carrier, I expect these flyers were at the top of their profession and as good a pilot as any other national carrier,
- bronco21016 8y ago
  Can’t upvote enough. As someone who actually does fly airplanes for a living it’s disturbing to me the amount of ‘expert’ arm chair quarterbacks that have come out on HN in the wake of these two incidents. Immediately after the Ethiopian crash there were 200+ comment threads every few hours full of experts speculating about all the things that went wrong. People who were flat out wrong could not be told otherwise. I had to stop even engaging.
  - teh_klev 8y ago
    Thanks for the supportive comment. I thought I was going to get a polite but firm message from dang asking me not to drink-and-hackernews :)
djmips 8y ago
Imagine MCAS was never created. Instead pilots had to deal with the nose up stalls themselves. Would that have ended up being the outcome? Instinctively I feel like pilots could deal with the situation where MCAS was supposed to help, especially a lot better than when MCAS is dooming these two planes. What do you think?
godson_drafty 8y ago
To avoid FAA scrutiny, Boeing developed and deployed MCAS covertly. This was done because they feared scrutiny would have led to huge losses in the face of competition from Airbus. Had Boeing management just laid all their cards on the table, potentially the FAA could have still approved the design, with small but crucial improvements. These changes might have included: using both AOA sensors, using a 3rd AOA sensor, not switching between left and right sensors each flight. Instead, the covert nature of the development and deployment of this feature allowed flaws in its design to go unchecked.

243 points by kunkurus 8y ago

This shouldn't be marked as a dupe since it's the actual report. It sounds like they did everything right. There are two fateful moments: >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 this point they were in stable flight and the trim wasn't that extreme. An inability to manually adjust trim is a huge design flaw unless it was pilot error. >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. They re-activated electric trim control but only slightly adjusted it. MCAS kicks in to send them nose down. There's no further pilot correction recorded. Again, unclear here if there is an airplane issue preventing them from adjusting trim or a pilot error.

sctb 8y ago
The report was the basis for this article and discussion yesterday: https://news.ycombinator.com/item?id=19570378 https://news.ycombinator.com/item?id=19570378. From Hacker News' perspective, since the report itself was prominently linked in the article and its contents were discussed directly in the thread many times, it qualifies as a duplicate.
- dredmorbius 8y ago
  You'd just noted and moved comments to this submission from another, which is confusing. (emailed to hn@ also)
  - Rooster61 8y ago
    Agreed, an article based on the report is not the same as the actual report. Not to mention it pulled this off the front page, where it absolutely should be considering the traffic this issue has been garnering.
- dmix 8y ago
  How is that the same? The BBC article doesn't contain even 20% of the information as the full report (including multiple charts).
  - sctb 8y ago
    You're absolutely right, it's not the same (in this sense “duplicate” is a misnomer—or at least should be applied to the thread or topic and not the submitted link).
    - Rooster61 8y ago
      If that's the case, it would be appreciated if this was moved back, or at least re-posted. It appears worthy of being visible based on the amount of discussion it generated in a short time. Moving it to this thread effectively threw a bucket on the fire. This document is a big deal.
      - sctb 8y ago
        OK! We've put it back.
        Rooster61 8y ago
        Err...Not quite, its still under the poster of the bbc article, but the effort is appreciated nonetheless. You guys rock. :)

mhandley 8y ago

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 reliable) reads 365 knots. VMO is 340, so they're already in big trouble. 5 seconds later, MCAS kicked back in, and then it was all over. Very sad.

fgonzag 8y ago
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.
- dsfyu404ed 8y ago
  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 basically nonexistent. I'm not saying that MCAS isn't mostly to blame but the fact that the maximum inputs from the pilot is still lesser than the control authority from the trim raises some serious questions MCAS or not. Had "extend flaps" been at the bottom of the checklist that would have disabled MCAS and likely prevented this crash.
  - quenstionsasked 8y ago
    MCAS issues nose-down, speed increases and nose pitches down. Nose never pitches back up. Plane crashes. Sure, maybe some complex combination of maneuvers could have helped them out of this but there's really everything pointing the blame at this one system. Pilots did what the boeing manual suggested (even included at the end of this report).

salex89 8y ago

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.

msbarnett 8y ago
> If there are issues with manually rectifying trim after it went haywire because of any reason, that might be a problem. It's not so much a "problem" as it is "physics". If the stabilizers are forcing the nose down, and the pilot pulls back on the stick (engaging the elevators) to pull the nose up, the forces exerted on the stabilizer are such that, in extreme cases, human force is insufficient to move the manual wheel. This is extremely (EDIT: No longer) well documented by Boeing going back decades, and it presents solutions of either: 1) reengaging the electronic stabilizer trim and trimming electronically back to a point where the forces are manageable, then cutting electronic trim again, or 2) relaxing the stick, letting the nose go the way the stabilizers want it to, which slackens the force, and makes cranking the wheel manually doable. Great if you're stabilizers are trimmed severely nose down at a height of 40,000 feet; less great at a height of 1,000 feet. (EDITED TO ADD: Apparently this information & procedure was removed from Boeing's manuals after the 1980s, and pilots no longer train on it, as the issues that led to its necessity were thought solved. The MCAS apparently has reintroduced the need for them.). This is compounded by a new change in the 737-MAX: in prior 737 series aircraft, there are two stab trim cutout switches: one to cut the autopilot's ability to command the stabilizer trim, the other to cut electronic power to the stabilizer trim adjustment entirely, so you could stop the autopilot from changing the trim but still retain electronic control and not need to worry about the manual crank forces. The two switches are MAIN ELEC and AUTO PILOT. In the MAX, this changed, and it's now an all or nothing setup: you have to kill electronic power to kill autopilot commands to the stabilizer. The two switches are now PRI and B/U. Both must be cut out to stop any stabilizer runaway, including MCAS runaway. This all seems to add up to Boeing's narrative of "The MCAS is safe, pilots just need to know to hit the stab trim cutout" being grossly insufficient. They must cut it at a moment where the stabilizer isn't grossly mistrimmed with respect to level flight, OR they may well need to re-engage it and race the MCAS to get into a close-to-proper trim setup prior to cutting out the stabtrim and assuming manual trim control.

Rooster61 8y ago

> 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 reclassifying the plane, I don't see how they can get around fault here. What a tragedy. I can't imagine the emotion those pilots must have felt after attempting to do exactly what they were told to do to keep the aircraft in the air, only to have the nose lurch back downwards. Godspeed, gentlemen. It wasn't your fault.

smaili 8y ago
Just out of curiosity, will there actually be any punishments handed out beyond just a slap on the wrist? Or is Boeing far too large of a corp to take on?
- geofft 8y ago
  Incidentally, Boeing has received billions of dollars in illegal tax subsidies from the US (as in, anti-competitive with non-US manufacturers like Airbus): https://www.bbc.com/news/business-38131611 https://www.bbc.com/news/business-38131611 So it's hard to imagine fines or market penalties actually conveying any productive sense of punishment.
  - CompelTechnic 8y ago
    >This marks a victory for Airbus in a war without end. >Back in September the European aerospace giant, which employs 15,000 people in the UK, was on the receiving end when it was found that billions of euros in low interest loans amounted to illegal subsidies. >Boeing celebrated that moment as a comprehensive victory which would deal a mortal blow to Airbus and result in more US jobs. >The reality is that neither of these companies can exist without government subsidies. Both of these companies are strategic assets to the countries that have them. So no surprise here. I'm not even upset at it- it's realpolitik.

intrasight 8y ago

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 8y ago
[deleted]
crooked-v 8y ago
This problem was one of too much automation, which decided that the best course of action was to fly the plane into the ground. In a less automated system, the pilots would have been able to enable electrical trim systems without also having their trim control overridden by MCAS and would have been able to recover flight control that way.
rootusrootus 8y ago
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.

Zak 8y ago

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 pusher is low enough that a pilot can overcome it with arm strength. A stick pusher, or a pilot pushing forward on the stick is not enough for the aerodynamics of the 737 MAX in certain circumstances. The position of the engines means the thrust tends to push the nose up, and at low speed, the elevator controlled by the stick alone is not enough to push the nose down when nearing a stall. Instead, the trim must be used, which moves the whole horizontal stabilizer rather than just the elevator. That's not part of the procedure on most aircraft, including other 737s, so regulators would not allow pilots rated for 737s to easily transfer their rating to the new aircraft, if they were willing to certify such an aircraft at all. MCAS is the solution to this problem. It automatically uses the trim to force the nose down with greater authority than the elevator alone could provide. While the concept is sound, the implementation strikes me as reckless. First, the system uses only one of the two angle of attack sensors to determine that the aircraft is nearing a stall. Such sensors have a fairly high rate of failure relative to the safe operation of a commercial aircraft. As far as I can tell, causing the system to use both sensors is a software change and all models of the aircraft had the required hardware. I can't imagine a good reason the system was designed this way, especially since Boeing has included MCAS on a previous aircraft, the tanker version of the 767, and it did use two sensors in that application. Second, activating the system automatically, and allowing it to contradict the pilot's attempt to pitch up using the elevator seems like a mistake. While stalls are almost always the result of pilot error, I'm inclined to think automated systems should let humans have the last word. A better design might have been to automatically trim nose-down in response to full forward pressure on the stick at low speed. That could be in combination with a stick pusher providing full forward pressure on the stick when an impending stall is detected in the absence of pilot input to the contrary.

linuxftw 8y ago
A compounding problem is, if the MCAS system is disable, now the flight characteristics are completely changed. Without additional training, there's no way for the pilots to know they have to trim down.

dao- 8y ago

I'm starting to think that disabling MCAS isn't even a sane option because then the pilots are exposed to unexpected aerodynamic characteristics due to the plane's weird design. Does this make sense?

linuxftw 8y ago
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.
- pdonis 8y ago
  > 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 problem is that, if you disable MCAS (which requires disabling the entire electric stability trim system) without first using the electric trim system to get back to something close to neutral trim, you will be stuck with a lot of nose down trim that you can't remove fast enough with the manual trim wheel. It looks like that's the situation this flight got into. But if you first use the electrim trim system to get back to something close to neutral trim, and then, before MCAS has a change to mess it up again, disable the entire electric stability trim system, you're ok. How do we know? Because that's what the first Lion Air flight (the one the day before the one the crashed) did, because an off duty pilot that was riding in the jump seat figured out what was happening and how to deal with it and told the pilots what to do.
  - linuxftw 8y ago
    > 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." You have to pick one.

linuxftw 8y ago

> 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 'sensors' are known to fail, and it's an easy scapegoat. My question is, does the flight recorder have raw inputs from the sensor, or just what the flight computer is reporting from the sensor? Seems like the latter.

rootusrootus 8y ago
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.
- linuxftw 8y ago
  But I just quoted 3 other sensors were reading off so...?
  - jellicle 8y ago
    The problem is all those other things are CALCULATED values off the AoA sensor. Literally everything depends on a single tube which is known to get clogged with ice, bugs etc. So for the 737 Max, a single clogged AoA sensor causes all these things immediately and simultaneously: -- all your airspeed and altitude readouts go haywire, at least on one side of the cockpit -- a bunch of alarms start going off and a bunch of lights start flashing, each indicating a different problem -- your control stick starts shaking -- a little-documented system immediately commands the nose straight down into the ground, and keeps commanding it until turned off (but turning it off effectively prevents you from counteracting the nose down command) Good luck.

ralph84 8y ago

This puts the nail in the coffin of a quick software fix getting the max back in the air anytime soon. To have any credibility the FAA is going to have to recertify the entire flight control system with all failure modes considered. And no scout's honor with Boeing this time.

jordache 8y ago

>The left Indicated Airspeed increased, eventually reaching approximately 458 kts and the right Indicated Airspeed reached 500 kts at the end of the recording. The last recorded pressure altitude was 5,419 ft on the left and 8,399 ft on the right. How can the sensors record such different readings?

daddylonglegs 8y ago
The reading from the pitot tubes (pressure sensors) for a particular airspeed varies with the sensors angle to the airstream so the airspeed value is adjusted based on the angle of attack (AoA). If the AoA sensor is on the fritz on one side the airspeed reading for that side changes. For redundancy, each pilot receives separate instrument readings from independent sensors. When the airspeed readings disagree a warning is activated and the pilots are expected to follow a prescribed and memorized procedure. The warning that the AoA sensors disagree was a paid option before this crash and the Lion Air crash.
- mannykannot 8y ago
  While the changes in the right pressure altitude indicator track those of the radar altimeter reading, the left falls below it -- except, in the last 15 seconds, there are blips where the left pressure altitude jumps to match the right one. These correspond to the the times when the errant left AofA indication briefly matches that from the (apparently working) right AofA vane. The airspeed discrepancy has the same pattern: the left is reading low and also showing blips, where it equals the right reading, that also correspond to when the AofA indicators match. All this would be consistent with the left AofA reading being used to correct the left static pressure reading by raising it at high angles of attack, and vice-versa for the right AofA vane and static port (this assumes there is a right static port used by the right pressure-driven instruments, an a left port for the left ones.)
sdenton4 8y ago
Q: How do you use a barometer to accurately measure altitude? A: Drop it, and count the seconds until it hits the ground.
8y ago

Osiris 8y ago

Do we know of any documented situations in which MCAS actually did it's job correctly, that is, preventing a stall from too large AoA?

laydn 8y ago
This is a great question. I am not sure why it is not being asked more frequently.
bbojan 8y ago
Preventing a stall is not what the job of MCAS is. (edit: clearer language)
- cjbprime 8y ago
  This language is not clear, at least to me. MCAS is absolutely an anti-stall system. There's some correlation with yoke handling, but it's still with the goal of affecting yoke handling in order to prevent stalls.

danaliv 8y ago

> Both engines were found buried in the ground at a depth of approximately 10 meters. My God.

segmondy 8y ago
fly into the ground at 40 degrees doing 600mph will do that.

deleted 8y ago

[deleted]

DougPhillips88 8y ago

The average Ethiopian IQ is 69. Just sayin'.

danaliv 8y ago

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 of that. I also can't help but wonder if exceeding Vmo was a factor. (I don't mean to sound as though I'm trying to assign any blame here. Obviously this was a confluence of problems, the most significant of which is the control system reacting to an erroneous 75° (!) AOA indication.)

VBprogrammer 8y ago
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.
- danaliv 8y ago
  Right, I get that, but the last automatic AND actually moved the stabilizer.
  - VBprogrammer 8y ago
    Your right. They didn't mention it, probably because it's not indicated on the CVR at what point they decided to try turning it back on.
phire 8y ago
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.
Torquer 8y ago
I find it amazing that there is no mention in the report of this fact. The investigators obviously know that they turned the cutout switches back to normal mode. Assuming there really was no mention of it on the CVR strongly suggests that it was the very inexperienced first officer who did this (the switches are on his side) without getting concurrence from the captain. If you carefully study the graphs in the report, the plane was really not in bad shape up until this point. They were 7000 feet above ground and climbing. They were going too fast, but again, this is the fault of the crew. If they would have not flipped the cutout switch and taken stock in the situation and backed the throttles down, it is quite possible this flight would have ended very differently.

rdiddly 8y ago

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 aircraft. EDIT: You guys can fucking blow me. Moderators ban my account please.

ummonk 8y ago
Why is that getting downvoted? What the heck.
- aaron695 8y ago
  > 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.
  - geofft 8y ago
    The "greedy bunch of assholes" are the Boeing execs who optimized for profit over the lives of those pilots and their passengers. (I thought that was pretty clear from the comment, fwiw.) I think it's been pretty thoroughly established at this point that an underlying design constraint in the MCAS was that it was easier for Boeing to get a 737-with-modifications approved than a new airplane, despite the engines being configured rather differently from existing 737s, and the MCAS was a computerized hack to avoid instability caused by the design of the newer engines (specifically, the newer engines would risk pitching up and stalling if you flew a 737 MAX like a normal 737, so they added the MCAS to counteract that, and the problem in both crashes was the MCAS unnecessarily activated and pitched the plane down when the plane was actually flying fine). Certainly lots of other things went wrong too, but had they instead just built a new plane design, the problem wouldn't have arisen in the first place. (I can find links about this if you're curious but it seems to be the consensus interpretation of what happened.)

ummonk 8y ago

It appears that Boeing's directive was simply wrong, and doomed them. The right response to faulty MCAS is not to do a stab trim cutout and then manually retrim, but to first use the manual electric trim switches to fix the trim and then immediately do a stab trim cutout. This is mentioned at the end of the directive, but reads more like an afterthought when it should be the standard procedure.

VBprogrammer 8y ago
In my opinion the pilots here have attempted to follow that checklist as well as could be expected (bearing in mind that their lives depended on it). Having to trim to neutral before disengaging the trim would have contradicted the normal trim runaway checklist. We're now talking about pilots having to have abnormal levels of skill and possibly even strength to fly this aircraft. That's an untenable state of affairs. Boeing have really screwed this one up.

jessmay 8y ago

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.

elsonrodriguez 8y ago
Boeing should be raked over the coals for even allowing item 1 to be a valid order configuration, and item 2 for being an order option.

toobulkeh 8y ago

Can someone explain to me how $BA is up nearly 3% after this news? Closed 395.86 USD +11.12 (2.89%)

aeternus 8y ago

> From 05:40:42 to 05:43:11 (about two and a half minutes), the stabilizer position gradually moved in the AND direction from 2.3 units to 2.1 units. During this time, aft force was applied to the control columns which remained aft of neutral position. The left indicated airspeed increased from approximately 305 kt to approximately 340 kt (VMO). The right indicated airspeed was approximately 20-25 kt higher than the left. This part is very interesting. This aircraft nose down (AND) trim occurred while the electric trim system was disabled. This means that either the pilots were moving the manual trim wheel in the wrong direction, or that aerodynamic forces were causing significant AND trim.

spockz 8y ago

Imagine being the one who was forced to develop this. Regardless of whether one did see something like this coming and raised objections or missed it, I would be unable to continue to function. Do we know how teams cope with this?

YjSe2GMQ 8y ago

Aircraft crashes attract a lot of attention, and probably for a good reason. But just to keep folks' internal perception of risk - aircraft safety has improved a lot: https://www.edge.org/images/airtravelsafety.png https://www.edge.org/images/airtravelsafety.png

dboreham 8y ago

Is there a demand for freight aircraft of this size? Conversation to freighters might be the best option for these planes.

teh_klev 8y ago

I usually enjoy HN discussions, but this is a pitiful and sad thread. I couldn't be bothered trawling past the responses to the top comment with back seat drivers, and "ooh but I read the 737's flight manual, they fucked up". Likely 99% of the commentariate here are neither 737 Max pilots, let alone even licensed to fly commercial passenger jets, or even have a pilots license. And no, as much as I love the guy, watching Mentour Pilot does not make you an expert. They have fuck all knowledge about, or even experience, about the sudden cockpit workload that was imposed on these pilots until the point of hull loss. Have some respect. Oh, and you know what, this was Ethiopia's national carrier, I expect these flyers were at the top of their profession and as good a pilot as any other national carrier,

bronco21016 8y ago
Can’t upvote enough. As someone who actually does fly airplanes for a living it’s disturbing to me the amount of ‘expert’ arm chair quarterbacks that have come out on HN in the wake of these two incidents. Immediately after the Ethiopian crash there were 200+ comment threads every few hours full of experts speculating about all the things that went wrong. People who were flat out wrong could not be told otherwise. I had to stop even engaging.
- teh_klev 8y ago
  Thanks for the supportive comment. I thought I was going to get a polite but firm message from dang asking me not to drink-and-hackernews :)

djmips 8y ago

Imagine MCAS was never created. Instead pilots had to deal with the nose up stalls themselves. Would that have ended up being the outcome? Instinctively I feel like pilots could deal with the situation where MCAS was supposed to help, especially a lot better than when MCAS is dooming these two planes. What do you think?

godson_drafty 8y ago

To avoid FAA scrutiny, Boeing developed and deployed MCAS covertly. This was done because they feared scrutiny would have led to huge losses in the face of competition from Airbus. Had Boeing management just laid all their cards on the table, potentially the FAA could have still approved the design, with small but crucial improvements. These changes might have included: using both AOA sensors, using a 3rd AOA sensor, not switching between left and right sensors each flight. Instead, the covert nature of the development and deployment of this feature allowed flaws in its design to go unchecked.