Yellowstone is a Boeing Commercial Airplanes project to replace its entire civil aircraft portfolio with advanced technology aircraft. New technologies to be introduced include composite aerostructures, more electrical systems (reduction of hydraulic systems), and more fuel-efficient turbofan engines (such as the Pratt & Whitney PW1000G Geared Turbofan, General Electric GEnx, the CFM International CFM56, and the Rolls-Royce Trent 1000). The term "Yellowstone" refers to the technologies, while "Y1" through "Y3" refer to the actual aircraft.