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Thread: Super Cub Structural Failure

  1. #121
    Hardtailjohn's Avatar
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    Quote Originally Posted by mvivion View Post
    YIf you want a scary picture, look at a photo of the Taylorcraft “strut failure” that brought about the latest AD. That airplane was not even close to airworthy.
    MTV
    And that had nothing to do with a strut. Total longeron corrosion failure. But yes, for sure scary! Now, back to the thread....
    John
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  2. #122
    BC12D-4-85's Avatar
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    The pilot Dave Wiley maintained the Taylorcraft. The strut fitting was covered with fabric and the normal lower drain hole was.....well decide for yourself. Rust never sleeps and can fail to hold hands. Some edits shown. His and passenger's life ending story.

    Gary
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    Last edited by BC12D-4-85; 08-06-2020 at 01:14 AM.
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  3. #123
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    Steve Pierce,
    I have the NTSB Materials Laboratory Factual Report back from Washington D.C.. The rear spar fractured about 4 inches inboard the rear strut attach point, and also a foot and a half inside of that fracture point. The inboard spar fracture started with a hairline crack that started with a stainless steel #4 sheet metal screw that was screwed into the top forward spar cap to fasten the flap bay skin to the rear spar. It cracked all the way through the spar cap and only had to go 1/2 inch past the top spar cap to get to the lighting hole that was drilled out of the spar to save weight. I counted 22 lighting holes drilled out each spar that the original builder said he saved a total of 7 pounds.
    Last edited by mountainflier; 04-15-2021 at 10:27 PM.
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  4. #124
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    Quote Originally Posted by mountainflier View Post
    Steve Pierce,
    I have the NTSB Materials Laboratory Factual Report back from Washington D.C.. The rear spar fractured about 4 inches inboard the rear strut attach point, and also a foot and a half inside of that fracture point. The inboard spar fracture started with a hairline crack that started with a stainless steel #4 sheet metal screw that was screwed into the top forward spar cap to fasten the flap bay skin to the rear spar. It cracked all the way through the spar cap and only had to go 1/2 inch past the top spar cap to get to the lighting hole that was drilled out of the spar to save weight. I counted 22 lighting holes drilled out each spar that the original builder said he saved a total of 7 pounds.
    Thanks for the insight. Useful information. Light is good but a little engineering to go with it is important. Hope you two are ok without any prolong issues from this.
    Steve Pierce

    Everybody is ignorant, only on different subjects.
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  5. #125

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    Quote Originally Posted by mountainflier View Post
    Steve Pierce,
    I have the NTSB Materials Laboratory Factual Report back from Washington D.C.. The rear spar fractured about 4 inches inboard the rear strut attach point, and also a foot and a half inside of that fracture point. The inboard spar fracture started with a hairline crack that started with a stainless steel #4 sheet metal screw that was screwed into the top forward spar cap to fasten the flap bay skin to the rear spar. It cracked all the way through the spar cap and only had to go 1/2 inch past the top spar cap to get to the lighting hole that was drilled out of the spar to save weight. I counted 22 lighting holes drilled out each spar that the original builder said he saved a total of 7 pounds.
    Perhaps another reason not to use stainless PK screws as well. They don’t specify if the crack propagation started from corrosion, but stainless is farther from aluminum on the galvanic chart than carbon steel. Corrosion will start faster with stainless and aluminum than carbon steel and aluminum. That corrosion could cause the crack propagation to accelerate.


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  6. #126
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    dgapilot,
    No corrosion associated with the #4 sheet metal screw fracturing the rear spar on the inbound fracture. On the outboard fracture of the rear wing spar near the rear wing strut attach point the NTSB Materials Lab Report described the fracture under the microscope as mixed intergranular separation and dimple rupture.

  7. #127
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    Some pictures of the rear spar fractures. The first picture is the inboard rear spar fracture, and the second one is the outboard rear spar fracture.

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  8. #128
    behindpropellers's Avatar
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    Interesting. Looks like those lightening holes were punched.

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