Ram Air Turbine (RAT) | How it works

Ram Air Turbine- A320

The Ram Air Turbine (RAT) is deployed automatically if all AC electrical power sources are lost at speeds greater
than 100 kts, or manually by using either one of the guarded P/Bs.

RAT supplies hydraulic power in less than 4.5 seconds from the time it gets the electrical command to extend the RAT. The hydraulic power available from the RAT is sufficient to supply the flight controls and the CSMG for electrical power. The RAT is available and will operate through the entire flight envelope. The RAT has a stall protection device which prevents a RAT stall below the governed speed range. The stall protection device limits the maximum power the pump can take from the turbine.

The RAT has a de-icing device which is installed in the nose of the turbine hub. The de-icing device produces heat which prevents icing conditions at the turbine hub when the RAT is in operation. The heat is produced by the eddy current heating device. The heating device has an electrically conductive aluminium plate and an even number of samarium cobalt magnets. The magnets are mounted on a stationary magnetic plate. When relative rotation take place, the flux field, when it passes through the aluminium plate, produces a current. As aluminium has an electrical resistance, power is generated which is than dissipated in the form of heat.

Mechanism

The turbine is a constant-speed variable-pitch unit with two blades. The mechanical governor and the pitch-change mechanism are installed in the hub.
-At LOW speed the governor spring holds the in FINE position (HIGH POWER EXTRACTION)
-At HIGH speed the governor spring holds the in COARSE position (LOW POWER EXTRACTION)
* Refer Diagram

The hydraulic pump has a stall protection which makes sure that the power demand from the pump does not exceed the power available from the turbine. The RAT actuator is attached to the top end of the leg assembly and the RAT mounting frame. The RAT actuator extends the RAT into the airflow with the force of a spring. Therefore no hydraulic power is necessary to extend the RAT. Hydraulic power is required to retract the RAT. You can retract the RAT only when the aircraft is on the ground. Two solenoids and one proximity switch are installed on the top of the RAT actuator. The two solenoids release a lock and the RAT extends. The RAT extends when both or one solenoid is energized. An extend lock keeps the RAT in the extended position. A stow proximity switch gives a signal to the ECAM system if the RAT actuator is extended.