I get your point but how does MIL-R-47196A stack up against AC 43.13-1B? That document states 2D for edge distance.
Thats certainly a valid question, but if you look at the intent, AC 43.13-1B is geared toward repair and the guidance is more of a one size fits all generic approach.
Quote:
"This advisory circular (AC) contains methods, techniques, and practices acceptable to the Administrator for the inspection and repair of nonpressurized areas of civil aircraft,
only when there are no manufacturer repair or maintenance instructions."
The section of I believe you're referencing is section 4-57 If so, it says in part;
"Unless structural deficiencies are suspected, the rivet spacing and edge distance should duplicate those of the original aircraft structure. If structural deficiencies are suspected, the fol*lowing may be used in determining minimum edge distance and rivet spacing.
(1) For single row rivets, the edge dis*tance should not be less than 2 times the di* ameter of the rivet and spacing should not be less than 3 times the diameter of the rivet."
So the question in play is simply how do you determine if structural deficiencies are suspected?
If Vans spells out a dimension, it's pretty cut and dried that they can specify bigger, smaller, whatever. They have done the engineering to back it up, and you as the manufacturer/assembler also have the engineering to back it up in the form of the assembly print.
If there's no dimension, we're now expected to use industry accepted design practices, right? Everybody points to the 2d callout in AC43.13. There's certainly nothing wrong with that, but what I'm saying is that in addition to AC43.13, MIL-R-47196A is approved data that it isn't specifically geared toward repairs or structural deficiencies.
It's also referenced by Vans in section 5 of their build manual as justification for why other areas of AC43.13 aren't written in stone. Specifically when they talk about the size of driven shop heads.
I've spent quite a bit of time on this reply because I'm genuinely trying to help, but my fear is that I've made it as clear as mud, so I guess I'll just end my manifesto by saying if you run into a situation where you've got less than 2d and if you want to know FOR SURE what you're dealing with, call Builder Support