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Lion Air 737 Crash

  1. Nov 14, 2018 #1

    russ_watters

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    I've been following the Lion Air 737 crash investigation with interest. As is often the case, the media's extremely poor understanding of technical matters makes it a little difficult to follow. One article I raid proclaimed that "the black box" had been found, but the author apparently didn't understand that there are in fact two black boxes and didn't specify which was found or if both were (my understanding is only the flight data recorder was found). So let's try to sort through the chaff here.

    Currently, the investigation seems to be focused on:
    1. An airspeed indicator failure.
    2. An angle of attack indicator failure.
    3. A flight control logic problem related to the two issues above.

    Early reports were that the plane's airspeed indicator had been reported by pilots to have malfunctioned on 3 previous flights. One previous flight even showed the plane flying erratically in a manner similar to the crash profile (several dives when it should have been climbing steadily.

    In addition, the angle of attack sensor was replaced the day before the crash. Boeing has issued a safety bulletin regarding AOA indicator failiure.

    Things that aren't clear to me
    1. Was the plane was in autopilot or manual control or a hybrid (caused by a sensor failure)?
    2. If in manual, were automated stall prevention measures by the flight control system in effect?

    My line of reasoning/question is this: Air France 447 happened because a pilot didn't handle a faulty airspeed reading properly, stalled the aircraft, and it fell out of the sky. I'm wondering if this crash happened because the aircraft misunderstood a faulty airspeed/AOA indication and crashed itself -- even, possibly, against the pilot's efforts to avoid the crash.

    https://www.nbcnews.com/news/world/boeing-warns-pilots-over-737-max-sensors-after-lion-air-n933376
    https://theaircurrent.com/aviation-...bulletin-on-aoa-warning-after-lion-air-crash/
     
    Last edited: Nov 14, 2018
  2. jcsd
  3. Nov 14, 2018 #2

    fresh_42

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    The news here say that they've only found the data flight recorder so far.

    Yes, but the air speed indicator failure of AF 447 was because it had frozen. AFAIK, water ran into the tubes during a cleaning on ground, which froze in air. This scenario can be ruled out for Lion Air. Also I cannot really imagine they flew on autopilot while still on the climb, so they should have known (?) which AOA settings to choose.

    The question for me is, whether Boeing changed cockpit installations at all with this relatively new design. It would surprise me, to be honest.

    But the following is strange:
    https://www.welt.de/wirtschaft/arti...len-im-Notfall-eine-Handkurbel-verwenden.html

    At least it points to a problem with the trim. However, there are more possibilities for a wrong setting than a technical failure - first of all the cargo!
     
  4. Nov 14, 2018 #3

    russ_watters

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    Understood; it was almost certainly a different failure mode.

    One noteworthy feature of AF 447 though was that modern aircraft have multiple flight control modes with multiple layers of automation. The airspeed indicator failure caused AF 447 to drop a level or two of automation so that whereas the plane would normally not allow the pilot to stall it, the more manual flight control mode did. Similarly:
    I don't think this is known yet, but airplanes will pretty much fly themselves from takeoff to touchdown if desired, so it isn't out of the realm of possibility that it was already in autopilot. One issue created by modern flight control systems though is that autopilot has become very specific and selective: There are something like half a dozen different autopilot modes that the plane can be in, and it is not always apparent when the plane hands partial control back to the pilot. E.G., a Russian airliner once crashed because the pilot allowed his kid to sit behind the controls while it was in autopilot. The pilot assumed the kid couldn't do anything, but the kid leaned on the ailerons and after 30 seconds, the plane assumed he wanted control of the ailerons -- and only the ailerons. Since the autopilot was still trying to hold altitude, it went into a steeper and steeper turn until it stalled.
    Evidently, Boeing did change the behavior of the flight control system - without notifying the pilots - but it isn't clear to me exactly what the change was:
    https://www.seattletimes.com/busine...atic-systems-change-linked-to-lion-air-crash/
    The Boeing bulletin discusses this issue without specifying how it has changed. What it says is that the elevator trim is used to avoid stalling. The pilot will be pulling back on the yoke with steady pressure, but the plane will automatically trim itself down, requiring more back-pressure to maintain the climb.
     
  5. Nov 14, 2018 #4

    fresh_42

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    They have had only one side frozen, so the pilots got contradicting information. Unfortunately, they've chosen the wrong one to rely on.
    Strange. I would have changed the least as possible on that particular system, as it was definitely mature.

    Do they know whether it stalled nose down or tail down? I think it is still a possibility that the cargo wasn't properly fixed, resp. wrong data of weight distribution were reported.
     
  6. Nov 14, 2018 #5
    I thought that this was a good preliminary analysis (speculation really).
     
  7. Nov 14, 2018 #6

    russ_watters

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    That isn't clear; the first officer flying the plane pitched up immediately after the autopilot disengaged and the crew never discussed their situation in a way that made sense, so it isn't possible to know what decisions the crew (in particular, the first officer) was making.
    Perhaps in response to AF 447?
    It isn't clear that it stalled at all. It appears to me that the leading theory is a bad indication caused the plane to pitch itself down in response to a stall that wasn't happening.
     
  8. Nov 22, 2018 #7

    CWatters

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    My understanding is that the trim on a modern airliner is very powerful. The whole tailplane moves not just a small tab. In fact it's so powerful that full trim can over power the elevator. In other words if something demands full down trim the pilot may not be able to over ride that by applying full up elevator.

    The automatic systems also fly the aircraft using the trim. So potentially if the automatics go wrong they can override inputs by the pilot making control difficult or impossible unless thecautomatics are turned off.

    I believe this is the background to the directive that was issued. it's also played a part in previous accidents on other aircraft.

    Edit: On some aircraft if you hold in say up elevator for long enough the auto trim winds in up trim so you no longer have to do so. So if for some reason you hold in up elevator (perhaps because you think you are going too fast) you can end up with the automatics applying loads of up trim which doesn't go away immediately if you apply down elevator. So tentative application of down elevator to see if that gets rid of stall warnings may not improve the situation enough. I believe that played a part in AF447.
     
    Last edited: Nov 22, 2018
  9. Nov 22, 2018 #8

    CWatters

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    There is a long thread running on the Professional Pilots Forum called PPrune.
     
  10. Nov 22, 2018 #9

    phyzguy

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    Have you seen this article? It sounds like what you just said - that the automatic anti-stall system pitched the nose down to correct a stall that faulty instruments said was happening
     
  11. Nov 24, 2018 #10

    russ_watters

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    I hadn't, thanks -- that's a much more detailed description/analysis than I had seen.
     
  12. Nov 28, 2018 #11
    https://www.cnn.com/2018/11/28/asia/lion-air-preliminary-report-intl/index.html

    This past summer I flew on two Lion Air flights in Indonesia. I knew about their bad record but just played the odds. It's well known they have plane supply issues so they run them to the max.
     
  13. Nov 28, 2018 #12
    The pitot tubes frozen up because the plane flew through storm clouds.
     
  14. Nov 28, 2018 #13
    In a lot of these crashes where a sensor causes a screwy reading, I don't understand why the pilots don't go into manual mode and fly the plane until the issue gets resolved.
     
  15. Nov 28, 2018 #14

    anorlunda

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    That is the official procedure. In today's news, Boeing says that procedure was not followed.
     
  16. Nov 28, 2018 #15

    CWatters

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    Because even in "manual" mode the aircraft monitors its sensors and will try to stop the pilot doing something it thinks is stupid...



    https://en.m.wikipedia.org/wiki/Air_France_Flight_296

    This wasn't the only or even the main cause of the crash but...

     
  17. Nov 28, 2018 #16

    Klystron

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    Turning on pitot valve heaters to prevent sensor freezing is also a procedure; manual on older model 737's, if memory serves.
     
  18. Nov 28, 2018 #17
    If you already have a freezing issue, how long would it take to clear up after you turn on the heater?
     
  19. Nov 28, 2018 #18

    Klystron

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    This member of an airline pilot forum describes the procedure but without time data:
    https://www.airliners.net/forum/viewtopic.php?t=752075

    Google found this Wikipedia article. Seems grounded (no pun intended):
    https://en.wikipedia.org/wiki/Pitot-static_system

    NASA has conducted numerous ice mitigation experiments in wind tunnels. NASA Ames hosts several full-motion flight simulators. Human factor studies attempt to answer the question, "How long does it take for the crew to react to an icing situation?".

    One improvement in airline cockpits was to digitize and display emergency checklists to reduce time spent finding relevant hardcopies.
     
    Last edited: Nov 28, 2018
  20. Nov 28, 2018 #19
    Is the sensor heater status included in the data recorder's log?
     
  21. Nov 28, 2018 #20

    OCR

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    These videos, although a bit old, still seem relevant... they are worth a watch, anyway. . :oldsmile:

    Children of Magenta


    Below is the main channel, look for anything presented by Captain Warren VanderBurgh ... . :frown:

    Flight Crew Guide


    Or, just go here...

    .
     
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