While several types of aircraft provide aerial refueling services, the principal effort is currently carried out by the Air Force fleet of 414 KC-135 aircraft. Originally fielded in the 1950s, KC-135 aircraft are considered the mainstay of the tanker fleet, supporting combat air assets, deployment of airlift aircraft, and nuclear combat refueling missions. With an average age of nearly 51 years and more than 16,000 flight hours on each aircraft, the KC-135s will approach over 80 years of age when the fleet is retired as projected in the 2040 time frame.