What is RAM AIR TURBINE (RAT)?
The Ram Air Turbine (RAT) is the core part of an aircraft power system especially used in emergency situation. It is used when an extremely rare case happens on aircraft i.e. aircraft loses power and Auxiliary Power Unit (APU) also runs out. In this case, the Ram Air Turbine evolves out from the aircraft’s wing or fuselage and rotates to produce sufficient power to provide electric power to an aircraft.
RAT is a small turbine connected to the hydraulic pump and it generates power from the rotating turbine resulted by the airspeed of the aircraft. Each engine of an aircraft has a generator for producing AC (Alternating Current) power to supply the electrical power. In the case of engine failure, there is a backup called APU which can power the entire system in the flight or in the ground. After APU, there are batteries which provide DC (Direct Current) power, further inverted to AC for necessary power supply, however; they operate for minimum 20 minutes in flight.
In the case of failure of engine driven generators and APU, batteries will take over but it will last only for 20 minutes. After all critical circumstances the aircraft will encounter emergency electrical configurations and this is where RAT is desperately required. RAT immediately drops out of aircraft’s fuselage and the blades start rotating due to the airspeed which then powers the hydraulic pump but the power supply of RAT is 1/18 of the engine driven generators (RAT: 5kVA Engine: 90kVA). Due to low output from RAT, lots of electrical systems are shut down such as passenger entertainment system, coffee makers and also the most of the cockpit instruments.
RAT requires a minimum speed of 140 knots to operate normally. After landing, the RAT stalls and then batteries takeover. If RAT does not deploy automatically then it can be deployed manually through the push button located on the overhead panel.