According to Army regulations, any sane person would naturally not want to fly combat missions because they are so dangerous. By requesting permission not to fly combat missions, on the grounds of insanity, the bombardier demonstrates that he is in fact sane and therefore is fit to fly. Conversely, any flyer who wished to fly on combat runs implicitly demonstrated that he was insane and was unfit to fly and ought to be excused, and to be excused this person would only need to submit a request. Naturally, such flyers never submitted such requests. Of course, if they did, the "Catch" would assert itself, short-circuiting any such attempt to escape from combat duty.