The C-40A "Clipper" flies the Navy Unique Fleet Essential Airlift (NUFEA) mission, providing high-priority cargo and passenger airlift for the U.S. Navy fleet worldwide. The C-40A is a Boeing 737-700 aircraft certified to operate in three configurations; all-passenger configuration (121 passengers); all-cargo configuration (up to eight pallets); and a combination, or "combi," configuration (up to three cargo pallets and 70 passengers). The Navy began replacing its aging fleet of 29 C-9B Skytrain aircraft in 1997 and has ordered nine C-40s to date. The U.S. Naval Reserve will operate and maintain the aircraft.