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Forces in cable fixed and not fixed one end

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  1. Jun 11, 2016 #1
    Hello all
    Apologies if this is in the wrong section.
    The below tv show about an air crash investigation makes a statement about forces on a cable that did not ring true to me. I wanted to check my understanding of forces.

    Simplified as I understood it the following seemed to be suggested: Cable A with a joint midway is fixed to a static point. Cable B with joint midway is pulled from both ends. Cable B is more likeley to break. In both cases one end can apply a max force that can not be exceeded (this would be the pilot end).

    A link to the video is below. I would appreciate any guidance on this matter.

    Thanks

    41 min 27 Sec
     
  2. jcsd
  3. Jun 11, 2016 #2

    Simon Bridge

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    Welcome to PF:
    It's a 41min vid - do you expect us to watch the whole thing to find the segment you are talking about?
    Please provide a timestamp and/or transcript.

    What is it that you feel does not "ring true" in the statement?
     
  4. Jun 11, 2016 #3
    The point I am referring to is at 41.27. The whole statement that the joint is more likeley to fail if pulled from both ends rather than one is the part that does not ring true.

    Thanks
     
  5. Jun 11, 2016 #4

    Simon Bridge

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    Ah so we would have had to watch just about the whole thing!

    Seems legit for this kind of program.
    Imagine you are the joint.
    It will probably hurt more if you are pulled by both arms in different directions than if you are only pulled by one right?
    What is happening is that the pilot is initially pulling (via the thrust lever) on the cable with some safe force, but the system starts pulling back ... so the pilot has to apply more force to keep the thrust high. The cable fails at the weakest point, which is the connector.
     
  6. Jun 11, 2016 #5
    I am not so sure they are saying that cable B would break before Cable A. I did not see any reference for comparison in the video to a Cable A.
    They did say though that the test cable broke by being "pulled" from both directions, so I guess that sets up a certain incorrect line of thinking for the viewers.

    In any event, if you attach a cable A to a fixed point and pull with X amount of force, or if you attach a cable B to a cow or a horse, and pull with 100 pounds of force, the other end fixed or to the cow or horse has a force of X amount. One can consider the cow or horse locking themselves in position, much the same as the co-pilot did with the lever.

    Three debatable points:
    Whether the co-pilot can stress the cable to the breaking point is debatable as the program does not show how it is attached to the lever. By locking your arm and holding in position, one can "withstand" or counter a larger force.
    Whether the connection of the actual cable from that unfortunate flight broke at the test force, the design force, or a lessor force will not be known. The investigators should have ( they may have - programs can relay only so much information ) tried to determine if the connection material was substandard. Perhaps the design limits on the male and female threads was, for this particular piece, at the maximum tolerance each giving a loose fit.
    Whether the failsafe can pull with enough force to break the cable. Apparently it did for this cable.

    Compressing 6 months or more of investigation into 40 minutes of interesting video is difficult for the story-tellers, and the end result does become a summary of what they feel is important video for an audience.
     
  7. Jun 11, 2016 #6
    Apologies the a and b cable were my attempt to consolidate my issue with the comment.

    The narrator states that the "automation was pulling while the officer was pushing" and that the "combined forces" we're too much for the cable.

    They earlier state that there was no way the officer could have applied enough force to break the cable.

    I wanted to check my understanding of forces and stress. I believe I am right in saying that the only explanation was that the officer did in fact apply enough force and that there was no addition of the forces?
    Fair point re condensing into 40 mins. Actual strength of material/ officer were probably worth commenting on.
    Yes the point about the test cable being pulled from "both directions" is exactly the point I want to clarify. Is it in fact correct that it would make no difference to the test if one end was fixed?

    Thanks

    Tom
     
    Last edited: Jun 11, 2016
  8. Jun 11, 2016 #7

    CWatters

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    Usually accident investigation reports are made public so probably best to read that.
     
  9. Jun 12, 2016 #8

    jack action

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    You are right, there is no addition of the forces. Both forces are equal and opposite. Whatever is applied at one end is necessarily felt at the other end. The phrasing used in the narration is misleading.

    What sounds more right would be: The safety system had to increase its pulling force on the throttle lever cable to also compensate for the pilot's applied force.
     
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