5 ms·

Boeing 737 Max 8 has a different software system. That software is now a focus of investigators. The Max 8 is outfitted with bigger, more fuel-efficient engines

by kappi 8y ago

Boeing 737 Max 8 has a different software system. That software is now a focus of investigators. The Max 8 is outfitted with bigger, more fuel-efficient engines than earlier 737s, and the weight and positioning of those engines shifted the plane's center of gravity forward, increased the potential for the nose to pitch up after take-off. To counteract this risk, Boeing developed software known as the Maneuvering Characteristics Augmentation System, or MCAS. Max 8s come equipped with a sensor that reads the plane's angle relative to the wind flow, prompting MCAS to automatically trigger the plane's nose to angle downward if it gets a specific reading. However, problems could arise if the MCAS system gets erroneous sensor readings. The system automatically pushes the plane's nose down, potentially surprising pilots who are unfamiliar with the system and overriding their commands.This is what investigators believe happened to Lion Air Flight 610 before it crashed in October more details https://www.cbsnews.com/news/boeing-737-max-8-boeing-737-800-how-are-the-planes-different/ https://www.cbsnews.com/news/boeing-737-max-8-boeing-737-800...

subhro 8y ago
>The Max 8 is outfitted with bigger, more fuel-efficient engines than earlier 737s, and the weight and positioning of those engines shifted the plane's center of gravity forward, increased the potential for the nose to pitch up after take-off. Umm, a little confused here, if the center of gravity shifted forward, why will the pitch up tendency increase?
- erokar 8y ago
  It happens when thrust is added abruptly, in e.g. a go-around.
  - subhro 8y ago
    Still does not explain the relationship between forward shifting CG and abrupt thrust.
    - CydeWeys 8y ago
      The center of thrust is shifted forward when the engines are shifted forward.
    - salawat 8y ago
      It's hard to describe, but here goes. The Center of thrust is offset down and somewhat in front of the CG, this means high thrust imparts a pitch up moment. Small, but present. The bigger problem is the nacelles shape though, they create body lift at high AoA; imagine a vacuum bubble trying to suck the wing/nacelle backwards forming in the airstream, "pulling" them back. Combine both of these characteristics, and you have an airframe demonstrating near stall positive pitch instability. I.e. you get a moment that if left uncountered will reinforce itself until the plane ends up stalling; behavior completely unprecedented in the airframe. So in order to "counter" this behavior an automated system was introduced which supposedly restored the pilot relative flight characteristics in line with the old airframe's specs. I'm probably oversimplifying, but it's the closest I can get to a visualization that makes some sense.
- cjbprime 8y ago
  The pitch up tendency is only increased at high angles of attack, at which point the engine nacelles themselves add extra lift, possibly inducing a stall when you're already near one.
- aphextron 8y ago
  >Umm, a little confused here, if the center of gravity shifted forward, why will the pitch up tendency increase? Because the center of thrust is also moved forward, and its effect is much greater than that offset by the center of mass. The 737 airframe was never designed to handle such powerful engines.
  - gsnedders 8y ago
    It's also a vastly longer aircraft than originally designed: the -100/-200 are 29m/30.53m long. The MAX 8 is 39.47m long. (And the MAX 10 will be 43.8m.)
  - teleclimber 8y ago
    The center of thrust moving forward would not have any effect on the pitching moment.
    - aphextron 8y ago
      >The center of thrust moving forward would not have any effect on the pitching moment. Sure it would. Here's a better explanation than I care to make: https://www.quora.com/Where-should-the-center-of-mass-lift-and-thrust-be-on-a-jet https://www.quora.com/Where-should-the-center-of-mass-lift-a... "If the Center of Thrust does not act through the Center of Gravity there will be a moment causing the aircraft to pitch, yaw or roll about the axes of the aircraft and the moment(s) must be counter balanced by design (including provision for trimming when the thrust is varied)." Boeing chose to counteract that newly induced pitch moment with software, so that they could sell the MAX 8 to airlines as not requiring any new training or type ratings. And now nearly 400 people are dead because of a marketing decision.
      - teleclimber 8y ago
        If you move an engine forwards (or backwards) you are moving it along its thrust vector. Therefore the moment resulting from the distance between the thrust vector and the center of mass is unchanged. Source: Physics 101
- gilbetron 8y ago
  I think it is actually that shifting the gravity forward makes the flight control surfaces more effective because they are now further away from the CG, but that's just from years of developing flight sims for the Navy, and no real knowledge ;) It also could be with the engines moved forward from the wings a bit, their thrust tends to rotate the air frame "up".
  - subhro 8y ago
    You are right. Airframes are designed to be nose heavy. The wings tends to fly up, whereas the elevators tend to fly down. In a dead stick condition, the airplane is designed to fly with a pitch down attitude so you don't lose airspeed and have enough airflow over the wings to continue generate enough lift to glide. If the CG shifts forwards, it is actually a good thing. Source: I am a pilot.
    - TimTheTinker 8y ago
      It’s also that the engine nacelles themselves begin to produce lift (and a consequent pitch-up moment) at high AoA.
      - subhro 8y ago
        Oh well, I did not know that. Do you have any citation that explains this further?
- sangnoir 8y ago
  It's not the center of gravity that causes pitch-up - it's additional lift by engine nacelles at higher angles of attack; this lift increases the AoA further due to the position of the engines relative to the wing. This feedback loop leads to a stall when the angle gets high enough
  - Gibbon1 8y ago
    They should have put spoilers on the nacelles instead of trying to fix the planes bad aerodynamics in software.
    - iamaelephant 8y ago
      Yeah you would know.
      - Gibbon1 8y ago
        How do you know that I don't know. Please explain.
  - subhro 8y ago
    Probably not completely true. An airfoil stalls when the critical angle of attack is exceeded. For most airfoils, this sits at around 17°. For all commercial aircrafts (I think 727 onwards) there is a mechanism called stick pusher that will not allow an airfoil to go over critical AoA. I am not sure if this has been overridden somwhere in the 737 Max 8 though.
    - tostr 8y ago
      Are you saying 17deg from horizontal? Let's ignore wind and other influences. A normal plane can not climb more steeply than 17deg? I'm not implying you're wrong, just very surprised to see this low a number. I would have guessed much closer to something like 45deg, maybe even more.
      - subhro 8y ago
- olivil 8y ago
  This is all in relation to the wing as the point of rotation.
erokar 8y ago
If this Reddit comment is true it's pretty damning. https://www.reddit.com/r/flying/comments/b08h03/737_max_mega.. https://www.reddit.com/r/flying/comments/b08h03/737_max_mega.... Claims intermittent fluctuations in AoA can activate the MCAS. So no need for the AoA indicator to be faulty then, means the risk of false MCAS intervention is higher than previously believed.
- aerovistae 8y ago
  That doesn’t link to a comment, it links to the parent post
- erokar 8y ago
  Correct link: https://www.reddit.com/r/flying/comments/b08h03/737_max_mega_thread/eidycdx/ https://www.reddit.com/r/flying/comments/b08h03/737_max_mega...
- throwaway427 8y ago
  You keep posting this Reddit comment in every single one of these threads. Maybe lay off for a bit?
iscrewyou 8y ago
Regarding your point of bigger engines, I understood it differently. The bigger engines provide a lot of thrust and with the new position of the engines, it’s a lot easier for the plane to stall. So the plane was outfitted with a sensor and software that reads the angle of attack and it corrects it in case the plane reaches the factor of safety angle. If the sensor malfunctions, the software would get wrong readings and try to correct something that doesn’t need correction. It moves the plane nose down. Pilots correct. Process repeats. Hence the elevation fluctuations seen in Lion Air and Ethiopian Airlines.
jahewson 8y ago
> Max 8s come equipped with a sensor that reads the plane's angle relative to the wind flow Just the one sensor?
- dlgeek 8y ago
  One on each side, but only one per flight computer (flight computers are redundant, but only one is active at a time).
  - 0xffff2 8y ago
    Additionally, there is an optional feature to add a cockpit warning if the two sensors disagree, which could have helped the pilots diagnose the issue in the case of the Lion Air crash. The Lion Air plane did not have this option; not sure about the Ethiopian plane.
clarkmoody 8y ago
Aircraft engineering is a highly coupled discipline. Change one part of the system, and multiple other systems might need to adjust.
Luc 8y ago
Hmmm, this 'comment' is just some copied paragraphs from a different news article, i.e. http://kcfj570.com/2019/03/12/how-is-the-boeing-737-max-8-different-from-737s/ http://kcfj570.com/2019/03/12/how-is-the-boeing-737-max-8-di...
- youeseh 8y ago
  Awesome! Saves me time from clicking on the link and finding a paywall, or even hunting for the pertinent information :). [edit: should've at least given credit though].
- MagnitudeFC 8y ago
  busted!
- e40 8y ago
  And I'm pretty sure I saw it in another thread here on HN (not the same person, I just checked this user's comment history).
trhway 8y ago
> The Max 8 is outfitted with bigger, more fuel-efficient engines than earlier 737s, and the weight and positioning of those engines shifted the plane's center of gravity forward, increased the potential for the nose to pitch up after take-off. To counteract this risk, Boeing developed software known as the Maneuvering Characteristics Augmentation System, or MCAS. reminds a typical enterprise software project - deep serious issues stemming from screwed up basic architecture are hastily patched up by software patches. Sounds like it was a money saving architectural decision as development of more deep modifications to the airframe to allow for those engines to be positioned safely would definitely cost more than a software patch. And after the Lion crash the Boeing is doing patches for the patch ...
- SilasX 8y ago
  I'm excited to hear the software gore on this one.
throwaway427 8y ago
It's also possible that that software system isn't triggered the same way on AA and SWA 737s as it is on the other airlines...
dmead 8y ago
i said this in another thread and was downvoted. oh well.
dang 8y ago
We've banned this account for plagiarizing from another source. If you don't want to be banned, you're welcome to email hn@ycombinator.com and give us reason to believe this won't happen again—and also that you'll follow https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html in the future.
- kappi 8y ago
  Don't understand the need for banning. I forgot to copy the link of news article.. Anybody can easily see that it is an official or news language and not a comment from me. It is copied from the below bigger article https://www.cbsnews.com/news/boeing-737-max-8-boeing-737-800-how-are-the-planes-different/ https://www.cbsnews.com/news/boeing-737-max-8-boeing-737-800...
  - dang 8y ago
    Ok, we'll unban you. In the future, please use quotation marks or some other standard indicator (e.g. italics, which you can get by enclosing text in asterisks) to indicate that you're quoting.

by kappi 8y ago

subhro 8y ago

>The Max 8 is outfitted with bigger, more fuel-efficient engines than earlier 737s, and the weight and positioning of those engines shifted the plane's center of gravity forward, increased the potential for the nose to pitch up after take-off. Umm, a little confused here, if the center of gravity shifted forward, why will the pitch up tendency increase?

erokar 8y ago
It happens when thrust is added abruptly, in e.g. a go-around.
- subhro 8y ago
  Still does not explain the relationship between forward shifting CG and abrupt thrust.
  - CydeWeys 8y ago
    The center of thrust is shifted forward when the engines are shifted forward.
  - salawat 8y ago
    It's hard to describe, but here goes. The Center of thrust is offset down and somewhat in front of the CG, this means high thrust imparts a pitch up moment. Small, but present. The bigger problem is the nacelles shape though, they create body lift at high AoA; imagine a vacuum bubble trying to suck the wing/nacelle backwards forming in the airstream, "pulling" them back. Combine both of these characteristics, and you have an airframe demonstrating near stall positive pitch instability. I.e. you get a moment that if left uncountered will reinforce itself until the plane ends up stalling; behavior completely unprecedented in the airframe. So in order to "counter" this behavior an automated system was introduced which supposedly restored the pilot relative flight characteristics in line with the old airframe's specs. I'm probably oversimplifying, but it's the closest I can get to a visualization that makes some sense.
cjbprime 8y ago
The pitch up tendency is only increased at high angles of attack, at which point the engine nacelles themselves add extra lift, possibly inducing a stall when you're already near one.
aphextron 8y ago
>Umm, a little confused here, if the center of gravity shifted forward, why will the pitch up tendency increase? Because the center of thrust is also moved forward, and its effect is much greater than that offset by the center of mass. The 737 airframe was never designed to handle such powerful engines.
- gsnedders 8y ago
  It's also a vastly longer aircraft than originally designed: the -100/-200 are 29m/30.53m long. The MAX 8 is 39.47m long. (And the MAX 10 will be 43.8m.)
- teleclimber 8y ago
  The center of thrust moving forward would not have any effect on the pitching moment.
  - aphextron 8y ago
    >The center of thrust moving forward would not have any effect on the pitching moment. Sure it would. Here's a better explanation than I care to make: https://www.quora.com/Where-should-the-center-of-mass-lift-and-thrust-be-on-a-jet https://www.quora.com/Where-should-the-center-of-mass-lift-a... "If the Center of Thrust does not act through the Center of Gravity there will be a moment causing the aircraft to pitch, yaw or roll about the axes of the aircraft and the moment(s) must be counter balanced by design (including provision for trimming when the thrust is varied)." Boeing chose to counteract that newly induced pitch moment with software, so that they could sell the MAX 8 to airlines as not requiring any new training or type ratings. And now nearly 400 people are dead because of a marketing decision.
    - teleclimber 8y ago
      If you move an engine forwards (or backwards) you are moving it along its thrust vector. Therefore the moment resulting from the distance between the thrust vector and the center of mass is unchanged. Source: Physics 101
gilbetron 8y ago
I think it is actually that shifting the gravity forward makes the flight control surfaces more effective because they are now further away from the CG, but that's just from years of developing flight sims for the Navy, and no real knowledge ;) It also could be with the engines moved forward from the wings a bit, their thrust tends to rotate the air frame "up".
- subhro 8y ago
  You are right. Airframes are designed to be nose heavy. The wings tends to fly up, whereas the elevators tend to fly down. In a dead stick condition, the airplane is designed to fly with a pitch down attitude so you don't lose airspeed and have enough airflow over the wings to continue generate enough lift to glide. If the CG shifts forwards, it is actually a good thing. Source: I am a pilot.
  - TimTheTinker 8y ago
    It’s also that the engine nacelles themselves begin to produce lift (and a consequent pitch-up moment) at high AoA.
    - subhro 8y ago
      Oh well, I did not know that. Do you have any citation that explains this further?
sangnoir 8y ago
It's not the center of gravity that causes pitch-up - it's additional lift by engine nacelles at higher angles of attack; this lift increases the AoA further due to the position of the engines relative to the wing. This feedback loop leads to a stall when the angle gets high enough
- Gibbon1 8y ago
  They should have put spoilers on the nacelles instead of trying to fix the planes bad aerodynamics in software.
  - iamaelephant 8y ago
    Yeah you would know.
    - Gibbon1 8y ago
      How do you know that I don't know. Please explain.
- subhro 8y ago
  Probably not completely true. An airfoil stalls when the critical angle of attack is exceeded. For most airfoils, this sits at around 17°. For all commercial aircrafts (I think 727 onwards) there is a mechanism called stick pusher that will not allow an airfoil to go over critical AoA. I am not sure if this has been overridden somwhere in the 737 Max 8 though.
  - tostr 8y ago
    Are you saying 17deg from horizontal? Let's ignore wind and other influences. A normal plane can not climb more steeply than 17deg? I'm not implying you're wrong, just very surprised to see this low a number. I would have guessed much closer to something like 45deg, maybe even more.
    - subhro 8y ago
olivil 8y ago
This is all in relation to the wing as the point of rotation.

erokar 8y ago

If this Reddit comment is true it's pretty damning. https://www.reddit.com/r/flying/comments/b08h03/737_max_mega.. https://www.reddit.com/r/flying/comments/b08h03/737_max_mega.... Claims intermittent fluctuations in AoA can activate the MCAS. So no need for the AoA indicator to be faulty then, means the risk of false MCAS intervention is higher than previously believed.

aerovistae 8y ago
That doesn’t link to a comment, it links to the parent post
erokar 8y ago
Correct link: https://www.reddit.com/r/flying/comments/b08h03/737_max_mega_thread/eidycdx/ https://www.reddit.com/r/flying/comments/b08h03/737_max_mega...
throwaway427 8y ago
You keep posting this Reddit comment in every single one of these threads. Maybe lay off for a bit?

iscrewyou 8y ago

Regarding your point of bigger engines, I understood it differently. The bigger engines provide a lot of thrust and with the new position of the engines, it’s a lot easier for the plane to stall. So the plane was outfitted with a sensor and software that reads the angle of attack and it corrects it in case the plane reaches the factor of safety angle. If the sensor malfunctions, the software would get wrong readings and try to correct something that doesn’t need correction. It moves the plane nose down. Pilots correct. Process repeats. Hence the elevation fluctuations seen in Lion Air and Ethiopian Airlines.

jahewson 8y ago

> Max 8s come equipped with a sensor that reads the plane's angle relative to the wind flow Just the one sensor?

dlgeek 8y ago
One on each side, but only one per flight computer (flight computers are redundant, but only one is active at a time).
- 0xffff2 8y ago
  Additionally, there is an optional feature to add a cockpit warning if the two sensors disagree, which could have helped the pilots diagnose the issue in the case of the Lion Air crash. The Lion Air plane did not have this option; not sure about the Ethiopian plane.

clarkmoody 8y ago

Aircraft engineering is a highly coupled discipline. Change one part of the system, and multiple other systems might need to adjust.

Luc 8y ago

Hmmm, this 'comment' is just some copied paragraphs from a different news article, i.e. http://kcfj570.com/2019/03/12/how-is-the-boeing-737-max-8-different-from-737s/ http://kcfj570.com/2019/03/12/how-is-the-boeing-737-max-8-di...

youeseh 8y ago
Awesome! Saves me time from clicking on the link and finding a paywall, or even hunting for the pertinent information :). [edit: should've at least given credit though].
MagnitudeFC 8y ago
busted!
e40 8y ago
And I'm pretty sure I saw it in another thread here on HN (not the same person, I just checked this user's comment history).

trhway 8y ago

> The Max 8 is outfitted with bigger, more fuel-efficient engines than earlier 737s, and the weight and positioning of those engines shifted the plane's center of gravity forward, increased the potential for the nose to pitch up after take-off. To counteract this risk, Boeing developed software known as the Maneuvering Characteristics Augmentation System, or MCAS. reminds a typical enterprise software project - deep serious issues stemming from screwed up basic architecture are hastily patched up by software patches. Sounds like it was a money saving architectural decision as development of more deep modifications to the airframe to allow for those engines to be positioned safely would definitely cost more than a software patch. And after the Lion crash the Boeing is doing patches for the patch ...

SilasX 8y ago
I'm excited to hear the software gore on this one.

throwaway427 8y ago

It's also possible that that software system isn't triggered the same way on AA and SWA 737s as it is on the other airlines...

dmead 8y ago

i said this in another thread and was downvoted. oh well.

dang 8y ago

We've banned this account for plagiarizing from another source. If you don't want to be banned, you're welcome to email hn@ycombinator.com and give us reason to believe this won't happen again—and also that you'll follow https://news.ycombinator.com/newsguidelines.html https://news.ycombinator.com/newsguidelines.html in the future.

kappi 8y ago
Don't understand the need for banning. I forgot to copy the link of news article.. Anybody can easily see that it is an official or news language and not a comment from me. It is copied from the below bigger article https://www.cbsnews.com/news/boeing-737-max-8-boeing-737-800-how-are-the-planes-different/ https://www.cbsnews.com/news/boeing-737-max-8-boeing-737-800...
- dang 8y ago
  Ok, we'll unban you. In the future, please use quotation marks or some other standard indicator (e.g. italics, which you can get by enclosing text in asterisks) to indicate that you're quoting.