The C-9 is the only transport aircraft in the history of the United States Air Force designed specifically for medical airlift missions, with a dedicated stretcher and patient-moving device on it. To replace the C-9, medical airlift missions have been added to a variety of other types of USAF transports. The USAF installed a new type of aeromedical equipment called patient support pallets on these transports, which allows these "non-specialized" transports to perform medical airlift missions.
In August 1967, the USAF selected McDonnell Douglas' successful civilian twin-engine jetliner, the DC-9-30, to satisfy a solicitation for a "no customization required" aircraft that could perform emergency air medical transport missions.
In August 1968, the first 21 aircraft, designated C-9A Nightingale, entered service with the 375th Emergency Medical Transport Wing.
C-9A Nightingale
McDonnell Douglas decided in the early 1960s to develop the short- and medium-range DC-9 airliner. Design work on the aircraft began in 1962, test flights began on February 25, 1965, and it received its FAA type certificate on November 23 of the same year and entered service on December 8th. The use of the DC-9 to convert a medevac aircraft offers many advantages that other aircraft do not have. First of all, time is life, cruising speed of more than 900 km / h jet airliner compared to helicopters and other means of transportation, not only to transport more casualties, the speed is also much faster, can significantly reduce the transport time. Airliners with turbofan engines also have many advantages in attempting medical evacuations, whereas turboprops are noisy and piston planes have high mechanical vibration, both of which are less suitable for medical aircraft. To replace the C-9s, medical airlift missions have been added to a variety of other USAF transport aircraft. The USAF has installed a new type of aeromedical equipment called patient support pallets on these transports, which allows these "non-specialized" transports to perform medical airlift missions.
The McDonnell Douglas C-9 is not only extremely reliable, it is also extremely efficient, flying one mission and being ready for the next. It takes only 20 to 30 minutes after landing to take off again for a new mission, whereas other airplanes usually take an hour or more.
As you can see from this, the two designations correspond to one type of airplane