Corrosion.

[Tom-D]

Fortunately I didn't see any of those issues in the original post, in fact it looked as good as new after he cleaned it up. We are talking floor skin and seat rail here with superficial oxidation, not carry through spars with corrosion you depict above.

As scarry as it looked, it really wasn't that bad. all any one can do is clean it up treat it, and stop the continuation of the corrosion.
when it becomes bad enough for the A&P-IA to determine a replacement is required, it is a major undertaking, and considered a major repair, following the Cessna structural repair manual. and documented on a 337 to OKC.

 

[Dan Thomas]

Not true..Landing gear boxes are the skin forward of the floor skin.

Isn't the gear box immediately at the aft edge of that floor skin? #59 in the picture? It helps prevent twisting of the box, and, along with the bottom skin, keeps it square to the fuselage.

 

[Skyrys62]

Topics like this should help educate potential buyers of what to watch out for when looking at ancient airplanes

And students, and renters (like me) who fly these damn things.
Although I'm hopeful that FBO's will take better care of their planes than some unheeding owners, I cannot help but think about it.

 

[Tom-D]

Isn't the gear box immediately at the aft edge of that floor skin? #59 in the picture?

Where the rear of the seat track ends is where the gear structure starts. On the assembly line they go in as two different assembles.
True when all are assembled they all become one structure. But if you get picky, that would include the roof, door posts, boot cowl. - every thing.

 

[PaulS]

As scarry as it looked, it really wasn't that bad. all any one can do is clean it up treat it, and stop the continuation of the corrosion.
when it becomes bad enough for the A&P-IA to determine a replacement is required, it is a major undertaking, and considered a major repair, following the Cessna structural repair manual. and documented on a 337 to OKC.

Yeah, that's what I said, I thought it looked fine especially after you cleaned it up, nowhere near the corrosion in some of the other parts shown in later, but I'm not an aviation expert, just a degreed Mechanical engineer with material science as part of that curriculum and experience designing and testing mechanical parts out of aluminum for mil specs. Trust me, if the standard were to reject parts that looked like the ones in your first post, nothing would ever get built. I think you made a good decision, apparently other don't. But thanks for posting the pics, pretty interesting stuff.

 

[Paulie]

You want to see corrosion pull out a glley or lav in a airliner, it a wonder what was holding them in place. The limit oncorrision is 10% loss of thickness. I've also changed many belly skins on Boeings because in flight they're a little nose up in cruise. Everything ends up back there. End up splicing in hat sections which is primary structure, and the cost of that stuff is insane.
Years ago at AA a guy shipped hazmat 5 gallon cans that were not labeled as hazmat to save a buck. It broke open. The B-727 ended up getting a new aft belly skin as well as some frames. The shipper got to pay for it plus all the lost revenue will the airplane was down. Took at least a week to repair. Plus he got fined by the feds.