ram air turbine

A ram air-driven turbine used in emergency situations to generate sufficient power to keep an aircraft flying long enough to land. Thus, the RAT performs the same function as an auxiliary power unit (APU). But, whereas an APU is essentially a small jet engine that burns regular jet fuel to generate this additional power, the RAT consists of a propeller that is spun by the high-speed air flowing past the aircraft. The spinning propeller powers a turbine that provides the emergency electricity needed to keep critical systems running, such as hydraulics, flight controls, and key avionics. In a normal flight, the entire assembly is folded up and stored in the aircraft fuselage or wing.