Since the late 1960s, the U.S. Air Force has been using modified DC-9s as medical transports. These were put in service during the Vietnam war when there were lots of battlefield casualties to move around. The air force currently has twenty of the C-9s (as they are called.) Because of the age of the aircraft, they are expensive to maintain. Each one racks up over two million dollars a year in maintenance costs. Patients (most of whom are dependents, not troops) are flown around on other aircraft as well, including KC-135 tankers (which frequently carry passengers), C-17s and C-135s. But the C-9 "Nightingales" are customized to move patients (special equipment, seats and berths) and, being smaller aircraft, have a "private jet" feeling about them. But the age of the aircraft and the expense of maintaining them has become a major issue. There's no money for a replacement jet, so it looks like patients will have to get used to moving around on larger, and less "patient friendly" aircraft in the future.