4

Boeing 737NG aircraft are currently being checked for cracking in the "pickle fork" assembly that attaches the fuselage to the wings. The affected joint has 4 fasteners that attach the fork to the wing. It's intended mostly to resist shear loads, as the fuselage twists relative to the wing. Boeing's inspection data is available at the FAA. That includes this image: Boeing inspection image

There is much discussion of this problem at the pilot's forum pprune.org Someone there states that the fasteners are meant to be an interference fit in holes that have been reamed and cold worked, and that the cracks have occurred because holes were drilled 6 thou oversize.

  • Why would this arrangement - 2 large fasteners and 2 small fasteners - have been chosen?

  • Boeing refer to the small fastener area as the "fail-safe strap". In what way might this be regarded as fail-safe?

  • Where is it appropriate to use interference fit fasteners in these specially prepared holes?

  • Why would drilling the holes oversize lead to the observed cracking?

emrys57
  • 165
  • 4
  • A large team of engineers worked on huge amounts of data to come up with the design - we don't have that data... – Solar Mike Nov 07 '19 at 12:18
  • @SolarMike Indeed, But you know there is always a guy who comes up with brilliant and surrealistic ideas in this community, I prepared some popcorn, you can join me and watch :) – Sam Farjamirad Nov 07 '19 at 13:49
  • In what material are the cracks occurring ? ( aluminum , titanium, steel ?). – blacksmith37 Nov 07 '19 at 16:22
  • 1
    I think another very important point is : How were the holes cold worked and what quality assurance is done? – blacksmith37 Nov 07 '19 at 16:24
  • I think the parts are aluminium alloy of some sort. Clearly, QA would have been inadequate, were the holes oversize. But, why would an interference fit, with cold worked holes, be chosen? – emrys57 Nov 07 '19 at 16:58
  • Probably cold work will leave a residual compressive stress on the surface of the holes which would significantly improve fatigue life. – blacksmith37 Nov 07 '19 at 20:04
  • If it is a Boeing, then it would be aluminum... – Solar Mike Nov 07 '19 at 20:37
  • I don't think that small fastener area is the fail-safe strap. If you look at the upper right picture, it appears that there's a strap running along the outboard edge of the pickle fork. – TimWescott Nov 08 '19 at 00:05
  • Those aren't the main mounting holes -- again, if you look at the upper right picture, there's a row of fasteners marching down the visible branch of the fork. A reason (which I can't vouch for) for having two sizes of fasteners there is that they're holding two different sizes of things on the backside. Although why and how they are load-bearing is beyond me. Perhaps they're holding up something other than the fuselage, which vibrates and starts a crack that could then propagate across the "working part" of the pickle-fork. – TimWescott Nov 08 '19 at 00:08
  • I recommend moving the question to the Aviation StackExchange. There are many people who are active directly in this field. – kamran Nov 08 '19 at 00:54
  • @SolarMike and the are the same large team of engineers who "designed" the battery fires on the 787 and the MCAS also on the 737. (And I used to work with them on a daily basis, so I'm not just calling them names with no first hand evidence). – alephzero Nov 08 '19 at 02:09
  • @alephzero but that may not be the people who did the work out in the field ie following the procedures correctly or not... – Solar Mike Nov 08 '19 at 05:13
  • The holes might be the correct size, but fabricated with the wrong process. That would be difficult for receiving inspection to catch. – Phil Sweet Nov 08 '19 at 22:31

0 Answers0