I dont know , but I'll bet it's not even 040. Aircraft grade aluminum is stronger though, so I guess it can be thinner than you would think. I put graphics on a helicopter once and was surprised how thin the surface felt. There was a former NTSB guy on the news last night talking about it, and he did say the "structure" of these planes is very strong, but i got the impression he wasnt talking about the skin. he also said that a tear would always stop at the next line of rivets, so there is no chance the hole would have ever been any larger than it was on that southwest plane.