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Thread: Winshield crack

  1. #1

    Winshield crack

    Well is was a sad day at the airport today when I saw a small crack
    about 1/2 inch long extending up from the bottom center screw. I
    just had this winshield put in 1 year ago when I put in all new glass
    when the new interior went in. Is there any repairs that can be done
    like on glass to fix this? I hate to have to stop drill a new
    windshield.

    Jeff
    N2008T





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  2. #2

    Winshield crack

    >I saw a small crack about 1/2 inch long extending up from the bottom
    >center screw. I just had this winshield put in 1 year ago. Is there any
    >repairs that can be done like on glass to fix this? I hate to have to stop
    >drill a new windshield.

    Stop drilling is the START of the repair. Perhaps an examination of the
    CAUSE of the crack is in order, so that you might investigate preventing
    new cracks at other locations.

    All screw holes in plexi need to be significantly larger than the actual
    screw size. Too often I see plexi with holes only slightly larger than the
    screw shank, and cracks radiating out from the hole. Because Plexi has a
    MUCH higher coefficient of expansion than aluminum or steel, it grows and
    shrinks much faster with temperature than the aircraft structure. This
    means that the hole may move relative to the screw and push the side of the
    hole up against the shank of the screw, and if its an "all thread" screw,
    that is so much the worse with those sharp edges of the threads biting into
    the soft plexi.

    A good rule of thumb is a minimum of 50% over size for the screw holes, and
    where edge distance and the trim around the plastic allow, even 100% over
    size is not out of the question. We want to CLAMP the plexi, but never
    bear against the edge of the hole. Therefore a 3/16" screw should have a
    9/32" clearance hole at a minimum, and as much as 3/8" is good if you have
    room.

    As for repairing the crack, stop drilling, or better yet, stop MELTING the
    end of the crack with a fine pointed tool heated enough to melt the hole at
    the end of the crack, would be the starting point. Some Methyl Chloride to
    soften the edges of the crack and allow them to fuse together can be a good
    follow up step.

    Any acrylic glazing shop will have MC in a small quantity. Avoid CA or any
    other "cement". We are NOT gluing the crack, we are softening the base
    material and letting the edges flow back together. Once the MC has fully
    evaporated (over night) the cloudy area can be polished to restore the optics.

    For an explanation of what the approved methods of repairing the plastic
    windows, consult the AC 43.13-1B "Acceptable Methods, Techniques and
    Practices" Chapter 3, section 3 for repairs to Transparent Plastics.

    http://www.airweb.faa.gov/Regulatory...E?OpenDocument

    Sorry for the nasty wrapping URL, but that is the FAA for you!

    Here is another link to the same page that has been "fixed" with the "URLizer".

    http://urlizer.com/00/1877/

    Obviously some of the repairs are worse than the initial crack. Drilling
    holes and lacing with safety wire is one example that one would only use
    to ferry a plane to get it really fixed. Gluing a slab of plexi on the
    inside or outside is an ugly but very effective long term fix.

    Bob Steward, A&P IA
    Birmingham, AL



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