5 ms·

MCAS wasn't to rectify a hardware electronics problem, it was for a dynamics/physics problem. The equivalent would be if, say, the car was designed to be overw

MCAS wasn't to rectify a hardware electronics problem, it was for a dynamics/physics problem. The equivalent would be if, say, the car was designed to be overweight on one side and susceptible to tipovers, and had software to automatically turn the wheels the other direction when it felt like it.

bumby 4y ago
When I said hardware, I meant in the broader airframe hardware, not specifically electronics hardware. The physics you mention is a result of the center-of-gravity of the airframe design. The broader point being, it became cheaper and more expedient (at least superficially) to try and fix a hardware problem with software. (Although Boeing did also create a cringey option to incorporate already existing redundant AOA sensors only as software option, rather than as a default)
- jaywalk 4y ago
  > The physics you mention is a result of the center-of-gravity of the airframe design. No, it's not. The larger size and more forward placement of the engines on the MAX caused it to pitch up in certain high angle-of-attack maneuvers, so MCAS was designed to automatically trim the aircraft to make it act more like the NG.
  - bumby 4y ago
    Yes, it is. But you don't have to take my word for it. >"heavier engines that are also cantilevered further forward on the wing to provide more ground clearance. That changes the center of gravity."[1] The "pitching up" is a direct result of the change in the center of gravity and thrust vector. [1] https://www.seattletimes.com/business/boeing-aerospace/faa-evaluates-a-potential-design-flaw-on-boeings-737-max-after-lion-air-crash/ https://www.seattletimes.com/business/boeing-aerospace/faa-e...
lisper 4y ago
> it was for a dynamics/physics problem Not exactly. The plane was (and still is) perfectly flyable without MCAS, but its handling characteristic were sufficiently different from the plane it was designed to replace that it required pilots to be retrained. That made its value proposition less attractive because of the extra training costs. MCAS was an attempt to make the new plane handle like the old one so pilots would not have to be retrained. And it failed catastrophically. The point is, it was a completely artificial problem produced by business considerations, not physics.
- WalterBright 4y ago
  > it was a completely artificial problem produced by business considerations Making the plane behave the same as a different airplane makes it safer. There are many examples of crashes due to pilots reacting to an emergency in a manner appropriate to the previous plane they were flying, rather than the current one. About 98% of the mass media reporting on MCAS was written by people who have no idea how airplanes work, and is hysterical nonsense.
  - lisper 4y ago
    Well, I'm a (private) pilot, so I like to think I have some clue about how airplanes work. It's true that, all else being equal, making an airplane's handling characteristics the same as a familiar predecessor improves safety. But clearly all else was not equal here.
    - WalterBright 4y ago
      There was nothing wrong with the MCAS concept. What was wrong was its reliance on a single sensor, such that a bad sensor made it misbehave. Other things wrong with it were it had too much authority, and it should have disabled itself if the pilot was countermanding it with the control column. The other thing wrong was the pilots not using the stab trim cutoff switch, which is supposed to be a "memory item" for them. I worked on the design of the 757 stabilizer trim system. The cutoff switch was always the backup for things going wrong with the trim. It's right there on the console within easy reach for a damn good reason. (Other systems could be turned off by overhead circuit breakers, but the stab trim cutoff was placed in a special priority position.) As for needing software at all, all jetliners have an active yaw damper to keep the pointy end forward. This is to counter a stability problem from having swept wings. Pilots of low&slow straight wing aircraft are often not familiar with this. A Cessna will be stable if you just let go of the controls. A swept wing jetliner, not without augmentation. The mass media also omits the reason for the MAX. The new engines gave it 15% less fuel burn. This is massive cost savings (and less pollution, too.)

bumby 4y ago

When I said hardware, I meant in the broader airframe hardware, not specifically electronics hardware. The physics you mention is a result of the center-of-gravity of the airframe design. The broader point being, it became cheaper and more expedient (at least superficially) to try and fix a hardware problem with software. (Although Boeing did also create a cringey option to incorporate already existing redundant AOA sensors only as software option, rather than as a default)

jaywalk 4y ago
> The physics you mention is a result of the center-of-gravity of the airframe design. No, it's not. The larger size and more forward placement of the engines on the MAX caused it to pitch up in certain high angle-of-attack maneuvers, so MCAS was designed to automatically trim the aircraft to make it act more like the NG.
- bumby 4y ago
  Yes, it is. But you don't have to take my word for it. >"heavier engines that are also cantilevered further forward on the wing to provide more ground clearance. That changes the center of gravity."[1] The "pitching up" is a direct result of the change in the center of gravity and thrust vector. [1] https://www.seattletimes.com/business/boeing-aerospace/faa-evaluates-a-potential-design-flaw-on-boeings-737-max-after-lion-air-crash/ https://www.seattletimes.com/business/boeing-aerospace/faa-e...

lisper 4y ago

> it was for a dynamics/physics problem Not exactly. The plane was (and still is) perfectly flyable without MCAS, but its handling characteristic were sufficiently different from the plane it was designed to replace that it required pilots to be retrained. That made its value proposition less attractive because of the extra training costs. MCAS was an attempt to make the new plane handle like the old one so pilots would not have to be retrained. And it failed catastrophically. The point is, it was a completely artificial problem produced by business considerations, not physics.

WalterBright 4y ago
> it was a completely artificial problem produced by business considerations Making the plane behave the same as a different airplane makes it safer. There are many examples of crashes due to pilots reacting to an emergency in a manner appropriate to the previous plane they were flying, rather than the current one. About 98% of the mass media reporting on MCAS was written by people who have no idea how airplanes work, and is hysterical nonsense.
- lisper 4y ago
  Well, I'm a (private) pilot, so I like to think I have some clue about how airplanes work. It's true that, all else being equal, making an airplane's handling characteristics the same as a familiar predecessor improves safety. But clearly all else was not equal here.
  - WalterBright 4y ago
    There was nothing wrong with the MCAS concept. What was wrong was its reliance on a single sensor, such that a bad sensor made it misbehave. Other things wrong with it were it had too much authority, and it should have disabled itself if the pilot was countermanding it with the control column. The other thing wrong was the pilots not using the stab trim cutoff switch, which is supposed to be a "memory item" for them. I worked on the design of the 757 stabilizer trim system. The cutoff switch was always the backup for things going wrong with the trim. It's right there on the console within easy reach for a damn good reason. (Other systems could be turned off by overhead circuit breakers, but the stab trim cutoff was placed in a special priority position.) As for needing software at all, all jetliners have an active yaw damper to keep the pointy end forward. This is to counter a stability problem from having swept wings. Pilots of low&slow straight wing aircraft are often not familiar with this. A Cessna will be stable if you just let go of the controls. A swept wing jetliner, not without augmentation. The mass media also omits the reason for the MAX. The new engines gave it 15% less fuel burn. This is massive cost savings (and less pollution, too.)