> a separate but related function of the hole: to release moisture from the air gap and stop (most) fog or frost from forming on the window. You could get arou

> a separate but related function of the hole: to release moisture from the air gap and stop (most) fog or frost from forming on the window. You could get around that by having the panes airtight and produced in a moisture free environment, but that would be more complicated. Think about failure rates in double paned windows which don't have to undergo the stresses on an aircraft. I'd assume even the outer panes aren't completely airtight to keep things simple: http://aviation.stackexchange.com/a/953 http://aviation.stackexchange.com/a/953