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Cracked Pickle Fork

Discussion in 'AviatorChat.com' started by Bob Parks, Sep 30, 2019.

  1. Bob Parks

    Bob Parks F1 Veteran
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    Another 737 issue, Max or not. Mother said that there would be days like this but she never said that they would come in a bunch like bananas. Any structural wonkies out there that can figure out how to repair a bulkhead forging without dismantling the entire fuselage? I think back about the bulkhead ring forgings and bottle pin system on the KC-135 ,707, and 720. There has never been a problem with them...that I know of. Of course they were more expensive and heavier.
     
  2. Bob Parks

    Bob Parks F1 Veteran
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    I didn't mention that the 707 series and the KC-135 did not have a landing gear beam and all the loads were fed into the wing / center section / fuselage joint at the front and rear spars.
     
  3. jcurry

    jcurry F1 World Champ
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    Would be interesting to know if the observed cracking is what they predicted and based their supplemental (repetitive) inspection program on. The fact that cracks are appearing well below the threshold for the supplemental inspection program to begin may or may not be surprising.
     
  4. Bob Parks

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    From what I just read, these forgings were supposed to be good for 90,000 cycles and cracks are appearing in airplanes with way less than a third of that. Metal alloy problem?
     
  5. Gatorrari

    Gatorrari F1 World Champ
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    How far back in 737 history does this happen? Was there a fundamental redesign, or did the loads go up to the extent that a functional design was no longer functional?
     
  6. Korr

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  7. Bob Parks

    Bob Parks F1 Veteran
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    Thanks for posting those illustrations ! It shows what happens under load. On the 707 series the bending loads didn't feed into the center section and fuselage because there was a beam across the center section rear spar that connected the two terminal pins. The bulkhead ring was continuous through the entire circumference of the frame station and maintained some integrity in the assembly. I worked with the engineer that designed the rear spar terminal forging and he told me that it was the structural heart of the airplane. It took the flight loads, braking drag forces, in flight drag forces, landing loads, crosswind landing loads, and turning loads. There was never a problem with it or the entire wing/body/ center section joint installation. Many times I watched the mechanics wearing thick insulated gloves install the bottle pins in the fuselage forging and center section when the center section and fuselage were aligned . The pins had been soaked for 48 hours in liquid nitrogen and pulled out of the bucket and quickly slammed into place. Any delay in getting the pins bottomed out would be a huge problem and would require many hours of work to destroy the pins to remove them and start over. A fascinating process!
     
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  8. jcurry

    jcurry F1 World Champ
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    Nice pics. Whats not shown in the pics is the failsafe strap that runs along the outer chord, which ensures the shear load in the rear spar will transfer between wing and fuselage. What also is not shown is that the spar is not continuous across the fuselage boundary, and all bending loads are transferred via the cruciform fittings that run over the entire chord of the wing.

    Of course the stress models will show that its the inner chord that has tension loading and thus susceptible to fatigue cracking. But whether that is actually where the current issues lie is unknown.
     
  9. Tcar

    Tcar F1 Rookie

    So are there 4 pickle forks per plane? 2 each F & R spars?
     
  10. Bob Parks

    Bob Parks F1 Veteran
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    Yes, four.
     
  11. Bob Parks

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    "Criciform fitting". What we called the "plus chord"?
     
  12. jcurry

    jcurry F1 World Champ
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    yes, same thing
     
  13. Bob Parks

    Bob Parks F1 Veteran
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    Thanks. There was a lot left out of that illustration that would have explained things better.
     
  14. Tcar

    Tcar F1 Rookie

    The Pickle Fork issue has hit the mainstream media... more Boeing 'piling-on' the 737 ....

    The story pointed out that this is in ADDITION to the 737 MAX issue and includes other 737's.
     
  15. Gatorrari

    Gatorrari F1 World Champ
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    They're going to need Mulally to come out of retirement.....
     
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  16. Bob Parks

    Bob Parks F1 Veteran
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    Absolutely ! I was at Boeing when the "Douglas Types" showed up. I left a couple of years later and the damage was present. As far as I'm concerned, Boeing is still suffering from it.
     
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  17. Jaguar36

    Jaguar36 Formula Junior

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    Keep in mind however that this issue is on the Next Gens, which were designed under the reigns of Shrontz and Condit, before the merger.
     
  18. jcurry

    jcurry F1 World Champ
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    assuming it is an engineering issue and not a supply chain screwup
     
  19. NürScud

    NürScud F1 Veteran

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    Today i was watching this. Very strange that happened so early(!)..

     
  20. Bob Parks

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    This Pickle Fork thing reminded me of the large panels behind the cowling on the C-97's that were called "Elephant Ears".
     
  21. tazandjan

    tazandjan Three Time F1 World Champ
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    They finished the inspections and the number with cracks was actually pretty small, 38 out of 810 (4.7%). Popular press, who understand little about technical issues, will overblow the problem most likely.
     
  22. Gatorrari

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    The video indicated that this pickle-fork design did NOT carry over into the MAX and that the AD only involved the NG aircraft. Can anyone verify that?
     
  23. Bob Parks

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    Maybe so, Terry, but the cracks are there and they shouldn't be. They can start a fast run unexpectedly and there are more of them out there just waiting. Personally, even though I'm not a trained or experienced structures guy, I don't see this design as a well balanced robust system when the forgings can be spread and bent with no tie to constrain or react the movement.
     
  24. Tcar

    Tcar F1 Rookie

    I'd be interested to hear if those forks were all from the same "batch" if there is such a thing, or ???
    Are they made by Boeing or some outfit in China or????

    Planes with an inordinate number or severity of hard landings or ???
     
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