Many commercial transport aircraft are fitted with an air-driven turbine that drives an emergency alternator, for use in the event of failure of all the engine-driven alternators.
A typical unit comprises a single stage turbine with direct drive to the alternator, which is usually of lower output capacity to the engine driven alternators and capable of meeting essential electrical requirements.
The unit is normally stowed in a compartment closed by a hinged door.
When activated, the door opens and the ram air turbine is deployed into the air stream, where it is rotated by the airflow passing through the turbine blades.
Variable incidence inlet guide vanes control the airflow into the turbine wheel. Constant rotary speed is maintained by variation of the inlet guide vanes under the influence of a flyweight type governor.
In some instances a variable pitch two-bladed propeller is used as the driving unit and certain aircraft have a ram air turbine driving an emergency hydraulic pump.
Typically, the unit is located in the underside of the wing root fairing and is deployed mechanically by spring action when a release catch is activated from the flight deck.