Boeing - Design Issues...

Discussion in 'Rules and Regulations / Flight Safety / Better Pil' started by TXFlyGuy, Apr 11, 2019.

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  1. Oct 29, 2019 #381

    Vigilant1

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    The "column cutout switch" is what is inhibited when MCAS is operating. The trim switches (both of them) are not inhibited by MCAS. In this incident, either the captain or first officer could have continued to toggle for nose up trim. After a few seconds (10?), MCAS would again try to trim down, and it could be countermanded by either crew position. Indefinitely. Sure, it is irksome, but it can be stopped anytime with the electric trim cutoff switches (not the same thing as the column cutout switch) just like the runaway trim checklist says.

    No doubt MCAS will be made better. But I don't fault Boeing engineers for failure to anticipate that there would be two crew members on the flight deck who would both choose to stop trimming the airplane with the electric trim AND elect to not disable the electric trim.
     
    Last edited: Oct 29, 2019
  2. Oct 29, 2019 #382

    BBerson

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    My take of the description on page 49 and 50 is that the "column cutout switch" has the purpose of: to allow the pilot to take over control in the event of incorrect autopilot trimming. It cuts out the autopilot. All 737 have them. By simply pulling the wheel back, these pilots would expect the column cutout switches to function and stop any automated trimming.

    From page 50 - "When actuated, the column cutout switching modules interrupt the electrical signals to the stabilizer trim motor that are in opposition to the elevator command."
     
  3. Oct 29, 2019 #383

    Vigilant1

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    Yes, the column cutout switch functions differently when MCAS is operating. But the trim switches still work, and can be used to counter MCAS (as the captain did repeatedly). If anything, the fact that the trim doesn't stop when the column is bumped should make it look >more< like runaway trim, and was probably expected to make a crew execute that checklist.
    If the plane can be trimmed using the switches, would you stop doing that and just let the pressures become excessive? If you believe the autopilot or unknown Gremlins are causing the trim to go wacky, would you not turn it off and use the manual wheel?
    As David pointed out, I think they got task saturated by the IAS unreliable issue, the stick shaker, the trim issue, and the growing control pressures. Too many things for this crew to handle, at least in the way they handled them.
     
    Last edited: Oct 29, 2019
  4. Oct 29, 2019 #384

    BBerson

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    But the trim did stop. So what does runaway trim look like? What causes runaway trim?
    I don't think runaway is the correct term here. This was incorrect, but never the less purposeful MCAS trim that put the nose down. The MCAS Flight Control Computer was not able to do it's job correctly and unknowingly in the background.
    And the column cutout switch didn't cut off the flight officers MCAS because it was " inhibited" from cutout. (Inhibit: to prohibit from doing something)
     
    Last edited: Oct 29, 2019
  5. Oct 29, 2019 #385

    Vigilant1

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    "Runaway trim" can be caused by a faulty trim switch, faulty limit switches in the system, or a more complex source in a fly-by-wire system. The most common runaway trim situation is uncommanded trim to a full stop (nose up or nose down), but it can also manifest as repeated cycling from stop to stop. It can also be just intermittent uncommanded trim (if, say, a trim switch had an intermittent short). It is a real attention getter in the sim, and all who fly planes with electric trim (and certainly all ATPs) should know all about it. If I'd never heard of MCAS but my stab trim kept experiencing uncommanded nose down cycling, runaway trim is what I would think is happening. Executing the checklist for runaway trim addresses every MCAS problem this crew had.
     
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  6. Oct 29, 2019 #386

    BBerson

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    That's true in hindsight. Is that a memory checklist?
    But I don't know if any of these pilots called runaway trim or even told the mechanics.
    I think the jumpseat pilot may have had a better view of the trim wheel or something. He told the captain they were diving and apparently the captain called for them to switch off the trim switch and they continued with manual trim.
    Is it that hard to notice a moving trim in a 737?
     
  7. Oct 29, 2019 #387

    davidb

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    The column cutout function (not switches in the sense that the pilots can’t see them or “flip” them) are on all 737s. You seem to understand their normal function so I won’t repeat that.

    The Max’s MCAS trimming at actual high AoA and high power was deemed necessary for flight characteristics in that extreme region of flight. You seem to understand that too.

    The normal column cutout function would by design stop MCAS from doing what MCAS was designed to do in that extreme region; it would stop nose down trimming if the pilot was pulling up. So, they engineered a way to inhibit the column cutout function during MCAS activation.

    Everything worked as designed. The column cutout being inhibited was normal in the scenario.

    The problem is one of design, not operation. It operated as designed.

    The design problem is that this MCAS trim function could become active based solely on one faulty AoA input.

    I figured you understood all of the above long before this report came out. We’ve been discussing these facts and issues for months. The report didn’t introduce any new aspects of MCAS that we didn’t already know before the final report.

    I’m still not certain what is the point of your new concern. That the MCAS was designed poorly is old news. That a malfunction of MCAS can’t be overcome is still false.
     
  8. Oct 29, 2019 #388

    davidb

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    For variety, let me ask some questions on stuff we didn’t know before the final report. Does anyone think it’s okay to fly as a pilot while suffering flu symptoms?
     
  9. Oct 29, 2019 #389

    BBerson

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    I am not concerned, rather it's a mystery to me why all these pilots didn't shut off the trim if it was so easy. After looking through the report again, the captain didn't recognize "runaway trim" or ask the pilot monitoring to find the runaway trim checklist.
    I think this will be the biggest story of 2019. Just interesting to follow. No need to respond further.
     
  10. Oct 29, 2019 #390

    Himat

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    As a passenger I am not sure if I would fly with a pilot suffering from flu symptoms. The trouble starts the moment the flu reduces the pilots work capacity and situation awareness.
     
  11. Oct 29, 2019 #391

    Himat

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    I fly airliners in the passenger seat and have a small preference for the all automated way. I’m an engineer myself and I do not think they make less errors than pilots. But with a proper organizing of the work, testing and certification most errors will be filtered out. Then, again with a proper system faults will be rectified when found and not repeated. On the other side, there is always a chance that another crew will repeat a known error.

    What kind of crew to be expected is part of this. The western ex air force pilots and pilots of comparable standard may not be common in the future. With crews like here reported, I would as a passenger rather prefer a limp home mode in the flight control computer.

    Statistical I guess todays data is a draw on what is safest. With the bean counters meddling in both technical and training decisions another guess is that we head for automation. Less educated, less trained and cheaper pilots can then be utilized. Safety will be no better as the less educated, less experienced and cheaper engineers will be used too.:(
     
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  12. Oct 29, 2019 #392

    Speedboat100

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  13. Oct 29, 2019 #393

    Vigilant1

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    Nope, I wouldn't say it is okay. I'm surprised the report didn't go into that more. Without a toxicology report, we don't know what he might have been taking for that cough and other symptoms, either.
    Subsequent work will be required to identify the reasons this crew performed as it did. And, of course, to improve MCAS.
    I have almost no experience with emergency procedure traIning as it exists in the civilian/commercial world, esp in other countries. I do know about USAF EP training. It is one thing to know memory procedures and to have thorough systems knowledge (that is expected), but there's no substitute for no-holds-barred EP simulator training. I left every EP sim wet with sweat and mentally wrung out. The system issues start small, but they grow in severity and complexity as the student shows what he/she can do. This, combined with normal flight requirements, navigation demands, communication, etc makes for a very full plate. When the student finally lands or crashes, he is reset in another scenario and the "fun" repeats. This results, eventually, in the ability to recognize personal limits and to prioritize accordingly. It is a very different experience than the assembly-line pilot factory described by Langewishe, where one student is in the seat of the sim and several more are standing behind him and also logging time. And where virtually everyone who starts the course graduates.
     
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  14. Nov 17, 2019 #394

    davidb

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  15. Nov 17, 2019 #395

    BBerson

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    My concern was letting the pilot take control with the yoke, they fixed that:
    " The pilot will always be able to counter it by pulling back on the control column."
     
  16. Nov 17, 2019 #396

    Himat

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  17. Nov 17, 2019 #397

    Vigilant1

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    Nobody should expect a rubber stamp. Neither should anyone be surprised if certain entities take this opportunity to extract a competitive advantage by delaying the return of this plane to the marketplace. In the present trade environment these tactics will have an unfortunate tendency to escalate.
     
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  18. Nov 18, 2019 #398

    davidb

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    Yeah, that part is interesting. Boeing had maintained the column cutout feature had to be disabled. That might still be the case. Hard to be sure just from a news article.
     
  19. Nov 18, 2019 #399

    BBerson

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    The other thing that was interesting: I think it said the Ethiopia pilots did turn off the electric trim switch as trained, but still crashed.
    I think the Ethiopia final report will be interesting.
     
  20. Nov 18, 2019 #400

    Vigilant1

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    IIRC, they turned the electric trim cutout switches off, trimmed using the trim wheels for some time, then re-engaged the electric trim, followed some time later by the final dive. I think the area of uncertainty was why they turned the switches back on-- it might be because the pressures became too great to allow them to be trimmed off using the wheels. We'll see.
     

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