Ram Air Turbine (RAT)

A ram air turbine (RAT) is a self contained turbine, or propeller, unit with a hydraulic pump attached, these are generally only used in emergency situations. The RAT is mounted on a shaft that provides swivel couplings for the pump supply line and pressure pipelines, when stowed it is held in position by a lock, aircraft skin contour is maintained by doors closing over the RAT stowage.RAT may be deployed into the airflow by selection of a switch on the flight deck, but is more usually deployed automatically when main system pressure drops below a predetermined value.

The RAT must be capable of operation over a broad speed range, maintaining normal hydraulic pressure, whether the aeroplane is at maximum speed and height or slowing for landing. To accomplish this the propeller is of a variable pitch design, controlled by a mechanical governor. An increase or decrease in RAT speed will change the centrifugal force on fly weights and blade pitch will be adjusted to maintain a constant RPM during operation .

A volumetric fuse or flow control valve is usually fitted to the RAT allowing the propeller to attain operating speed, before being loaded by the pump, this allows full system pressure to be achieved approximately 4-5 seconds after deployment into the air stream.

Image shows RAT deployed for maintenance.this RAT is stowed in the flap track fairing.the place of stowage can be different from aircraft to aircraft. generally it may be under the wings,under the belly, or at sideways of the aircraft.