Cracked Pickle Fork

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.

 

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.

 

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?

 

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!

 

Yes, four.

 

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

 

Thanks. There was a lot left out of that illustration that would have explained things better.

 

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.

 

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

 

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.