The problem with air gaps is someone inevitably comes along and joins the two together to solve his/her problem.
When I was working for guvmint many moons ago we used to talk about using two different network technologies that cannot be joined together by "chance". But the people at the top never understood it plus they couldn't justify spending more in a climate of spending less on administrative costs. That said, the real big leaks in that place came from a manager with the assistance of a HR person, who would lend his secretary to other senior managers to data mine their computers when their secretary called in sick. Fortunately it all came unstuck for him in a most tabloid kind of way.
More PPRuNE discussion here
http://www.pprune.org/military-aircr...h-america.html