Controlled substitution, though not officially sanctioned, has become a way of life. The drill goes like this: Troops take one helicopter out of service because of a failure, for example, in the nose gearbox. They don’t have an immediate replacement, so it sits in the hangar. In the meantime, a second aircraft suffers a rotor blade failure. That part, too, is not available. Rather than have two helicopters out of service, the unit commander tells the maintainers to take a blade off the first aircraft and install it on the second. Now, the second aircraft is ready to go, but the first aircraft is in even worse shape.