4 ms·

> It'd approach criminal liability if the FAA certified a plane they didn't know the level of instability of. The question isn't if they didn't know, it's if t

> It'd approach criminal liability if the FAA certified a plane they didn't know the level of instability of. The question isn't if they didn't know, it's if this is an unusual level of instability for an airliner. >We're talking 'stability that adheres to control forces required by regulation' rather than 'F-117 drop out of the sky' instability. Hopefully yes, but we don't know how much instability there is. If you assume the answer there's no point having the discussion.

mikeash 8y ago
Aerodynamically unstable aircraft cannot be flown purely by humans. They require a fly by wire system. The 737 is not fly by wire, thus we can pretty trivially conclude that the 737 is aerodynamically stable.
- pedrocr 8y ago
  That's pithy. MCAS is definitely fixing something.
  - mikeash 8y ago
    Of course, but it’s not fixing aerodynamic instability.
    - pedrocr 8y ago
      How does that change anything? If the instability is not aerodynamic and instead is runaway pitch up on power, how is that any different?
      - mikeash 8y ago
        You keep insisting that we don’t know if this airplane is unstable. My point is that we do know. If you want to argue that some other aspect of it makes it equally dangerous, be my guest. I’m just correcting that one thing.
        pedrocr 8y ago
        >You keep insisting that we don’t know if this airplane is unstable. Because we don't. You've only corrected a strawman, that we know this plane isn't a fully aerodynamic unstable design like an F-16. No one thought that. I never even specifically said aerodynamically unstable, just that we don't know the level of instability MCAS is correcting for to know if it is unusual for an airliner design. From what I had read I had assumed it was actually a power-on issue and not aerodynamic. But as it turns out the issue is indeed apparently aerodynamic from the engine shape generating more lift at higher angle of attack and thus leading to pitch up. And no, we don't know. We the public at least. We don't know how big the flight envelope is where MCAS is needed or how much of an issue it would be if MCAS didn't exist. From public information we don't know if without MCAS the plane wouldn't enter an unrecoverable pitch up attitude in normal flying conditions. That's not knowing if the plane isn't inherently unstable.
        mikeash 8y ago
        Just a few comments up, your response to the idea of being totally aerodynamically unstable was “hopefully yes.” Not “of course it’s stable, that’s obvious.” https://news.ycombinator.com/item?id=19556911 https://news.ycombinator.com/item?id=19556911 I’m not responding to a straw man, I’m responding to what you wrote. If it’s not what you meant, well, I can’t really do anything about that.
        deleted 8y ago
        [deleted]
        pedrocr 8y ago
        Well, quote the whole thing then: Hopefully yes, but we don't know how much instability there is. If you assume the answer there's no point having the discussion. I was making the point that we don't know what was actually certified. Having F-117 level instability is obviously not the case but apparently the instability it does have is actually aerodynamic so your argument doesn't even work. The actual opposite is happening. The non-fly-by-wire design was modified with an automatic control adjustment to fix an aerodynamic issue. All your argument proves is that the MAX didn't magically turn the 737 into an inherently unstable design which is not relevant to the issue of if the MCAS is covering up a large or small instability.
  - ams6110 8y ago
    Read mhandley's post upthread. It explains what MCAS does: it addresses a certification requirement for handling control forces at high angles of attack.

mikeash 8y ago

Aerodynamically unstable aircraft cannot be flown purely by humans. They require a fly by wire system. The 737 is not fly by wire, thus we can pretty trivially conclude that the 737 is aerodynamically stable.

pedrocr 8y ago
That's pithy. MCAS is definitely fixing something.
- mikeash 8y ago
  Of course, but it’s not fixing aerodynamic instability.
  - pedrocr 8y ago
    How does that change anything? If the instability is not aerodynamic and instead is runaway pitch up on power, how is that any different?
    - mikeash 8y ago
      You keep insisting that we don’t know if this airplane is unstable. My point is that we do know. If you want to argue that some other aspect of it makes it equally dangerous, be my guest. I’m just correcting that one thing.
      - pedrocr 8y ago
        >You keep insisting that we don’t know if this airplane is unstable. Because we don't. You've only corrected a strawman, that we know this plane isn't a fully aerodynamic unstable design like an F-16. No one thought that. I never even specifically said aerodynamically unstable, just that we don't know the level of instability MCAS is correcting for to know if it is unusual for an airliner design. From what I had read I had assumed it was actually a power-on issue and not aerodynamic. But as it turns out the issue is indeed apparently aerodynamic from the engine shape generating more lift at higher angle of attack and thus leading to pitch up. And no, we don't know. We the public at least. We don't know how big the flight envelope is where MCAS is needed or how much of an issue it would be if MCAS didn't exist. From public information we don't know if without MCAS the plane wouldn't enter an unrecoverable pitch up attitude in normal flying conditions. That's not knowing if the plane isn't inherently unstable.
        mikeash 8y ago
        Just a few comments up, your response to the idea of being totally aerodynamically unstable was “hopefully yes.” Not “of course it’s stable, that’s obvious.” https://news.ycombinator.com/item?id=19556911 https://news.ycombinator.com/item?id=19556911 I’m not responding to a straw man, I’m responding to what you wrote. If it’s not what you meant, well, I can’t really do anything about that.
        deleted 8y ago
        [deleted]
        pedrocr 8y ago
        Well, quote the whole thing then: Hopefully yes, but we don't know how much instability there is. If you assume the answer there's no point having the discussion. I was making the point that we don't know what was actually certified. Having F-117 level instability is obviously not the case but apparently the instability it does have is actually aerodynamic so your argument doesn't even work. The actual opposite is happening. The non-fly-by-wire design was modified with an automatic control adjustment to fix an aerodynamic issue. All your argument proves is that the MAX didn't magically turn the 737 into an inherently unstable design which is not relevant to the issue of if the MCAS is covering up a large or small instability.
- ams6110 8y ago
  Read mhandley's post upthread. It explains what MCAS does: it addresses a certification requirement for handling control forces at high angles of attack.