4 ms·

> One thing to note is the mention of a "Runaway Trim Checklist". The pilots may not have recognized (in time) that it was a runaway trim issue. Many such issu

> One thing to note is the mention of a "Runaway Trim Checklist". The pilots may not have recognized (in time) that it was a runaway trim issue. Many such issues involve trim being continuously applied. The trim wheel spins wildly. MCAS will trim in increments of 10s. So it will trim, pause, trim some more, pause. This may have confused the pilots. And they don't have much time to begin with – MCAS engages with autopilot off and flaps retracted. Most takeoffs have at least minimum flaps, so the plane will fly fine and then start misbehaving while still relatively close to the ground. > Most aircraft have 2-3 redundant systems for almost every control and feature. Yes. MAX 8 has two AOA sensors. Only one needs to misbehave for this issue to happen. But even with two, it is difficult to know which one is fault (for a machine). With three, it would be possible to tell. In any case, all of this is still speculation.

losvedir 8y ago
> MCAS will trim in increments of 10s. So it will trim, pause, trim some more, pause. No, not quite. By itself MCAS will only trim once, I believe. But if it's interrupted by a pilot manually trimming, then it will stop, wait 5 seconds, and then try to do its 10 second trim again. The characteristic up-down pattern from the Lion Air flight I think was from the latter scenario of MCAS starting to trim, the pilot manually trimming back (disabling MCAS for 5 seconds) and then it activating again. > Yes. MAX 8 has two AOA sensors. But I think the MCAS system only gets its input from one! (And it switches each flight.) If true, that's just appallingly bad.
- outworlder 8y ago
  > No, not quite. By itself MCAS will only trim once, I believe. Under normal operation, that seems to be the case. But note that we are operating with the assumption that the AoA sensor is faulty. So it may still think the plane is in a bad attitude. > But I think the MCAS system only gets its input from one! (And it switches each flight.) If true, that's just appallingly bad. As far as public information goes, that's correct.
- linuxftw 8y ago
  > No, not quite. By itself MCAS will only trim once, I believe. Unfortunately, we don't know this is actually true. Certainly, it's what Boeing wants us to believe, that their software is perfect in this regard. Might be completely broken. Where's the independent verification. And, allegedly, it's only supposed to trim just a little bit at each increment. If you look at the accounts, there are roller coaster-like drops. Seems to me it started nosing down really hard.
3JPLW 8y ago
> MCAS will trim in increments of 10s Fascinating. That sure seems to line up with the 20 second period found in the vertical profiles of both LionAir and this crash. https://www.nytimes.com/interactive/2019/03/13/world/boeing-737-crash-investigation.html https://www.nytimes.com/interactive/2019/03/13/world/boeing-...
ams6110 8y ago
In a turn, AOA will differ from one side of the plane to the other. You'd really need six AoA sensors, three per side, to be really confident.
- CydeWeys 8y ago
  Why would it differ from one side to the other? The wings are attached; they can't face differently into the wind from each other.
  - outworlder 8y ago
    Aircraft will also be banking in a turn, so aerodynamic forces will be different, which is what the AoA sensor relies on. So they are right. I'm wondering how big of a difference it is. In other words, would a comparison with some epsilon factor be enough?
    - deleted 8y ago
      [deleted]
    - CydeWeys 8y ago
      There's not just two AoAs though, one per wing; there's a whole gradient of different AoA values going down both wings, like how the speed of a turning record is continuously variable from the inner to outer edge. It helps to think of the two wings combined into a single long wing (like a B2 bomber, which does actually have this design), then imagine that banking into a turn -- it's different values along the entire surface. A single AoA value taken from the center of this long combined wing, i.e. the fuselage in a typical jetliner configuration, should be sufficient for the purposes of determining if the plane is about to stall. If you really need the AoA values at all points along the wings, you can calculate them by combining the center AoA sensor with the roll angle. This logic applies to airspeed as well (it varies continuously along the entire wing surface when the airplane is banked into a turn), and note that airplanes only have one cluster of pitot tubes, which are located on the fuselage.
      - 8y ago

losvedir 8y ago

> MCAS will trim in increments of 10s. So it will trim, pause, trim some more, pause. No, not quite. By itself MCAS will only trim once, I believe. But if it's interrupted by a pilot manually trimming, then it will stop, wait 5 seconds, and then try to do its 10 second trim again. The characteristic up-down pattern from the Lion Air flight I think was from the latter scenario of MCAS starting to trim, the pilot manually trimming back (disabling MCAS for 5 seconds) and then it activating again. > Yes. MAX 8 has two AOA sensors. But I think the MCAS system only gets its input from one! (And it switches each flight.) If true, that's just appallingly bad.

outworlder 8y ago
> No, not quite. By itself MCAS will only trim once, I believe. Under normal operation, that seems to be the case. But note that we are operating with the assumption that the AoA sensor is faulty. So it may still think the plane is in a bad attitude. > But I think the MCAS system only gets its input from one! (And it switches each flight.) If true, that's just appallingly bad. As far as public information goes, that's correct.
linuxftw 8y ago
> No, not quite. By itself MCAS will only trim once, I believe. Unfortunately, we don't know this is actually true. Certainly, it's what Boeing wants us to believe, that their software is perfect in this regard. Might be completely broken. Where's the independent verification. And, allegedly, it's only supposed to trim just a little bit at each increment. If you look at the accounts, there are roller coaster-like drops. Seems to me it started nosing down really hard.

3JPLW 8y ago

> MCAS will trim in increments of 10s Fascinating. That sure seems to line up with the 20 second period found in the vertical profiles of both LionAir and this crash. https://www.nytimes.com/interactive/2019/03/13/world/boeing-737-crash-investigation.html https://www.nytimes.com/interactive/2019/03/13/world/boeing-...

ams6110 8y ago

In a turn, AOA will differ from one side of the plane to the other. You'd really need six AoA sensors, three per side, to be really confident.

CydeWeys 8y ago
Why would it differ from one side to the other? The wings are attached; they can't face differently into the wind from each other.
- outworlder 8y ago
  Aircraft will also be banking in a turn, so aerodynamic forces will be different, which is what the AoA sensor relies on. So they are right. I'm wondering how big of a difference it is. In other words, would a comparison with some epsilon factor be enough?
  - deleted 8y ago
    [deleted]
  - CydeWeys 8y ago
    There's not just two AoAs though, one per wing; there's a whole gradient of different AoA values going down both wings, like how the speed of a turning record is continuously variable from the inner to outer edge. It helps to think of the two wings combined into a single long wing (like a B2 bomber, which does actually have this design), then imagine that banking into a turn -- it's different values along the entire surface. A single AoA value taken from the center of this long combined wing, i.e. the fuselage in a typical jetliner configuration, should be sufficient for the purposes of determining if the plane is about to stall. If you really need the AoA values at all points along the wings, you can calculate them by combining the center AoA sensor with the roll angle. This logic applies to airspeed as well (it varies continuously along the entire wing surface when the airplane is banked into a turn), and note that airplanes only have one cluster of pitot tubes, which are located on the fuselage.
    - 8y ago