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I think both end up with blame on Boeing. If a discussion in world news isn't enough to prepare an experienced 737 pilot (ten years!) to counteract spurious MC

I think both end up with blame on Boeing. If a discussion in world news isn't enough to prepare an experienced 737 pilot (ten years!) to counteract spurious MCAS, then nothing is, at least to the level of safety expected in airline travel. A plausible explanation for (1) that I've seen discussed is that, after disabling the auto trim, it may be impossible to manually re-trim while the yoke's pulled back, without first relieving aerodynamic stress on the elevator -- in this case by pointing the nose even farther down temporarily, which no sane pilot is going to want to do close to the ground. (I don't know whether it could actually happen this way.)

honestoHeminway 8y ago
So basically the auto-trim gets the plane into a impossible to recover from position at start? Seems plausible.
VBprogrammer 8y ago
I think it's unlikely that this crash will be attributed to the same set of circumstances as the lion air crash. I don't find it credible that an experienced pilot would fail to catch what, in essence, is a trim runaway event. There is a reason trim runaway is a memory item on almost all aircraft. Even in the Lion air case the previous flight experienced difficulty with the system and dealt with it. This was before the publication and a great deal of publicity of the details around MCAS. I don't think we've heard anything yet to suggest this particular aircraft had a fault in previous flights. Another failure in the AoA sensor on a brand new aircraft seems unlikely. I don't find 1 particularly compelling, it would have been noted by now if it was impossible to trim the aircraft using the manual trim wheels. Unless this is very specific to the MAX. That said, I think it's entirely possible there is something else wrong in the control of the MAX.
- mattmcknight 8y ago
  >Another failure in the AoA sensor on a brand new aircraft seems unlikely. From another perspective, having one new sensor fail would seem to suggest that one should increase the prior probability for failure.
  - VBprogrammer 8y ago
    That's fair. It would be interesting to find out whether it was an existing sensor, used on other aircraft, and what the reliability is like. But I doubt that information is readily available.
- cjbprime 8y ago
  > I don't find 1 particularly compelling, it would have been noted by now if it was impossible to trim the aircraft using the manual trim wheels. It has been noted: 737 Flight Crew Training Manual, chapter Non-Normal Operations/Flight Controls, sub heading Manual Stabilizer trim: "Excessive air loads on the stabilizer may require effort by both pilots to correct mis-trim. In extreme cases it may be necessary to aerodynamically relieve the air loads to allow manual trimming. Accelerate or decelerate towards the in-trim speed while attempting to trim manually." In this case the excessive air load would be caused by the yoke control pulling the elevator up while the trim controlled stabilizer is doing the opposite. Like you say, it's hard to know whether this actually happened.
  - VBprogrammer 8y ago
    I've seen that before. It certainly doesn't say it's impossible. I need to have look in the certification requirement but I'm pretty sure it would be uncertifiable if the trim couldn't be neutralised.
    - cjbprime 8y ago
      > It certainly doesn't say it's impossible. What would it have to say? It says you might have to do X before Y, where Y is a thing that you need to do quickly while at low altitude to avoid dying and X is a thing that gives you even less time to do it in and makes the problem worse.
      - VBprogrammer 8y ago
        Ok let's look at it another way. Do you think it's even remotely likely that Boeing haven't had one of their 737 Max aircraft try this out at a safe altitude? They staked the future of the aircraft by saying they believe they are safe to fly. I've never suggested having a trim runaway close to the ground is trivial. It's certainly going to result in brown trousers when it happens, but it should be within the abilities of a well trained crew to handle. Following the outcry over Lion air I just don't believe a pilot on type wouldn't have known the procedures for getting the aircraft to a safe altitude and disabling the stab trim. As a pilot your hands are on the controls. You notice pretty instantly that your having to pull the nose up. You instinctively reach for the trim control on the yoke. This disables the MCAS system for 5 seconds. You get the pitch forces under control. A few seconds later MCAS decides you've still got the nose too high because of a faulty sensor and tries to trim it back down once more. How many times before you remember that fatal bug from that crash a few months ago. Maybe twice? Three times tops.
deleted 8y ago
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honestoHeminway 8y ago

So basically the auto-trim gets the plane into a impossible to recover from position at start? Seems plausible.

VBprogrammer 8y ago

I think it's unlikely that this crash will be attributed to the same set of circumstances as the lion air crash. I don't find it credible that an experienced pilot would fail to catch what, in essence, is a trim runaway event. There is a reason trim runaway is a memory item on almost all aircraft. Even in the Lion air case the previous flight experienced difficulty with the system and dealt with it. This was before the publication and a great deal of publicity of the details around MCAS. I don't think we've heard anything yet to suggest this particular aircraft had a fault in previous flights. Another failure in the AoA sensor on a brand new aircraft seems unlikely. I don't find 1 particularly compelling, it would have been noted by now if it was impossible to trim the aircraft using the manual trim wheels. Unless this is very specific to the MAX. That said, I think it's entirely possible there is something else wrong in the control of the MAX.

mattmcknight 8y ago
>Another failure in the AoA sensor on a brand new aircraft seems unlikely. From another perspective, having one new sensor fail would seem to suggest that one should increase the prior probability for failure.
- VBprogrammer 8y ago
  That's fair. It would be interesting to find out whether it was an existing sensor, used on other aircraft, and what the reliability is like. But I doubt that information is readily available.
cjbprime 8y ago
> I don't find 1 particularly compelling, it would have been noted by now if it was impossible to trim the aircraft using the manual trim wheels. It has been noted: 737 Flight Crew Training Manual, chapter Non-Normal Operations/Flight Controls, sub heading Manual Stabilizer trim: "Excessive air loads on the stabilizer may require effort by both pilots to correct mis-trim. In extreme cases it may be necessary to aerodynamically relieve the air loads to allow manual trimming. Accelerate or decelerate towards the in-trim speed while attempting to trim manually." In this case the excessive air load would be caused by the yoke control pulling the elevator up while the trim controlled stabilizer is doing the opposite. Like you say, it's hard to know whether this actually happened.
- VBprogrammer 8y ago
  I've seen that before. It certainly doesn't say it's impossible. I need to have look in the certification requirement but I'm pretty sure it would be uncertifiable if the trim couldn't be neutralised.
  - cjbprime 8y ago
    > It certainly doesn't say it's impossible. What would it have to say? It says you might have to do X before Y, where Y is a thing that you need to do quickly while at low altitude to avoid dying and X is a thing that gives you even less time to do it in and makes the problem worse.
    - VBprogrammer 8y ago
      Ok let's look at it another way. Do you think it's even remotely likely that Boeing haven't had one of their 737 Max aircraft try this out at a safe altitude? They staked the future of the aircraft by saying they believe they are safe to fly. I've never suggested having a trim runaway close to the ground is trivial. It's certainly going to result in brown trousers when it happens, but it should be within the abilities of a well trained crew to handle. Following the outcry over Lion air I just don't believe a pilot on type wouldn't have known the procedures for getting the aircraft to a safe altitude and disabling the stab trim. As a pilot your hands are on the controls. You notice pretty instantly that your having to pull the nose up. You instinctively reach for the trim control on the yoke. This disables the MCAS system for 5 seconds. You get the pitch forces under control. A few seconds later MCAS decides you've still got the nose too high because of a faulty sensor and tries to trim it back down once more. How many times before you remember that fatal bug from that crash a few months ago. Maybe twice? Three times tops.

deleted 8y ago

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