A Ram Air Turbine (RAT) is a small turbine that is installed in an aircraft and used as an alternate or emergency hydraulic or electrical power source. The RAT generates power from the airstream based on the speed of the aircraft and is connected to an electrical generator or to a hydraulic pump.
In general, modern aircraft only utilize the RAT in emergency: either in the event of loss of hydraulic systems or following the loss of primary electrical generation. In these cases the RAT will power vital systems such as flight controls or flight-critical instrumentation, navigation and communication equipment. Some RATs produce only hydraulic power which in turn is used to power an electrical generator.
Under normal conditions, the RAT is stowed in a compartment in the fuselage or wing. It can be deployed manually when required or, in some installations, will deploy automatically following a complete loss of AC power. In the interval between power loss and RAT deployment, aircraft batteries are used to power essential instrumentation.