Catch 22: If you are insane, you do not have to fly in the war, but you must request to be grounded. But anyone who doesn't want to fly is obviously sane. So therefore if you request to be grounded, you are forced to fly. Anyone who wants to fly is obviously insane, but they can't be grounded until they request to be. And when they do that...
It seems that the only way to get out of this is to be a a bada*s mofo named Yossarian.