Rogue Boeing 737 Max planes ‘with minds of their own’ | 60 Minutes Australia

Discussion in 'The Watering Hole' started by Winston, May 14, 2019 at 5:31 PM.

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  1. May 16, 2019 at 12:52 AM #31

    Flyfalcons

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    The problem is, by the time that the pilots realized what the problem was and disconnected the electric trim, the aircraft was so far out of trim that 1) they could not maintain control of the aircraft, and 2) the excessive loads on the stabilizer prevented the pilots from physically being able to move the manual trim wheel. An expat pilot on a professional pilot board that I belong to had trained the Ethiopian captain, and said that he was an excellent pilot by any world standard. I just can't get behind the idea that had this been an American crew, that the aircraft would have been recoverable.
     
  2. May 16, 2019 at 12:52 AM #32

    UhClem

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    A pitch up moment creates an angular acceleration which increases the angular rate which eventually leads to a high AoA.

    So you would prefer to wait until the AoA becomes extreme (the pilots are asleep or something) to do anything rather than counteract the forces that might lead to it?

    Remember that the problem was supposed to be a pitch up moment caused by the engine position. Counter that force and it cannot create a high AoA. And the plane handles like any other 737 with no tendency to pitch up.
     
  3. May 16, 2019 at 2:59 AM #33

    SpaceManMat

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  4. May 16, 2019 at 5:28 AM #34

    BEC

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    Flyfalcons has it exactly right. MCAS is on the airplane to counteract a decrease in column force as the nose comes up, and this was known from wind tunnel and/or CFD studies (I don’t remember which) before the airplane was built. A close former colleague of mine actually ran that trade study for our group. It is also true that the intent was to make the airplane feel, to the flight crew, like the Next Generation airplanes in order to minimize NG to MAX transition training required.

    What I did not know, having left the company in October of 2016 was the bit about flight testing leading to a huge increase in the authority given the system (as OverTheTop describes).

    This was exactly the intent as I understood it...and may well have been part of the rationale for not having any specific mention of the system in the Crew Ops manuals or the NG to MAX transition training at the outset. Of course we now know that was not correct.

    Quite likely, sadly.

    I expect the CFM-56 nacelle generates some lift at high angles of attack but the LEAP engine nacelle generates more. The pitch up moment manfests itself as decreased column force with increased angle of attack past a certain point.

    Fly-by-wire elevators would have unraveled so many pieces of the certification basis that it was not even considered. Some of the reasons are as you suggest. As I mentioned above, the effect was known before the airplane flew, but apparently the magnitude of it was not if this increase of authority given to MCAS during flight test I have heard about in several places (besides here) is correct.

    I would like to ask you again: would you recognize intermittent down trim being put in uncommanded as a runaway and therefore do the runaway stab memory procedure? I honestly wonder, because this is the other place that, in my opinion, a grave error was made in the implementation (as opposed to the intent/purpose) of MCAS as it was put on the airplane and was another part of the apparent rationale for not explicitly training for the new function.
     
  5. May 16, 2019 at 1:48 PM #35

    mach7

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    This is a very interesting question.

    I know how/when speed trim works. That is the only time on a 737 that I will accept the trim to move without any input
    from me. When speed trim activates it does so at a very slow rate (.2 deg/sec I believe) and all the flight instruments are working properly.

    We do train for unreliable airspeed and runaway trim. We also train on partial instrument failures and in the past we have done the failure of the flying pilots AOA vane.

    All of these situations are very distracting but the aircraft can be flown very precisely despite them.

    Sooo, would I recognizes it? I'm thinking I might let the 1st movement go by thinking its speed trim. But a quick cross check
    of the flight instruments would show me that I had another issue.
    Honestly I would probably transition to the Unreliable airspeed check list initially, before returning to the runaway stabilizer. This is assuming that the trim was moving intermittently. At the 2nd un-commanded stab trim movement I would be in the runaway trim procedure.

    If the trim was continuously moving It would be the runaway trim procedure immediately.

    This discussion is purely hypothetical, knowing what I know now I would disconnect the trim right away and deal with that path 1st.
     
  6. May 16, 2019 at 5:52 PM #36

    BEC

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    Thanks, Mark. I appreciate the perspective of someone who flies 737s as this aspect has been nagging at me ever since I heard of the Lion Air crash.

    The action of MCAS as it was implemented had an interval between automatic trim inputs - I have seen several different values for this but the one that sticks in my mind is 30 seconds - so it's not the same to this model-airplane-only pilot (but retired Boeing engineer) as "the trim just goes until the mechanical stop" kind of thing that I think of when I hear the term "runaway" and knowing some of how the stab trim system on a 737 functions. Hence the question.

    Again, thanks for the thoughtful answer.
     
  7. May 16, 2019 at 10:07 PM #37

    OverTheTop

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    I have been following this issue across many media streams since it happened. It is definitely great to get pilot input on this discussion. Thanks! :)
     
  8. May 16, 2019 at 10:37 PM #38

    boatgeek

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    Was there a system on the 737NG that would automatically pitch the plane one way or the other? I'm trying to wrap my brain around how much change this system really was. If there wasn't anything on the prior versions of the 737 that would push the plane up or down (absent throttle changes or pilot/autopilot commands), then it seems like a bigger change even if it was intended to just make the Max feel like the older versions.

    Reading news coverage is a little eye-opening about how much stuff Boeing outsources. I'm not necessarily expecting them to be programming everything themselves, but this seems like a pretty big system to farm out, and I wonder about the QC/testing on the software if they didn't figure out that the AOA Disagree light wasn't coming on.
     
  9. May 16, 2019 at 11:00 PM #39

    mikec

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    The autopilot can control the stabilizer trim on the 737NG. There's a separate switch to inhibit autopilot trim, leaving the manual trim switches still functioning. Apparently on the Max these switches were relabeled and it's not clear to me if there is a way to kill MCAS control of trim and leave manual electric trim operating. https://www.737ng.co.uk/B_NG-Flight_Controls.pdf
     
  10. May 17, 2019 at 1:39 AM #40

    mach7

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    According to the preliminary accident report the Ethiopian crew did use the trim cutout switches and it did stop the MCAS from further trim movement. Once they reset the trim cutout switches MCAS was able to continue to move the stabilizer.
    The procedure is to throw both the autopilot and manual trim switches. Speed trim works through the autopilot trim system, I assume MCAS does also but I don't know this to be the case.
     

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