A ram air turbine (RAT) is a small airflow driven engine that supplies emergency electrical or hydraulic power to aircraft or, in the case of crop dusters, powers ancillary equipment. These turbines are stowed away behind closed panels during normal flight and are generally only deployed in the case of total loss of power due to multiple engine failures. RATs are standard parts of the redundancy backup systems on larger commercial, military, and many crop dusting aircraft. The average ram air turbine on a passenger aircraft can generate over 50 kilowatts (kW) of power and have a propeller several feet in diameter. Older RAT designs featured twin bladed propellers although newer examples generally have multi-bladed, ducted fans.
Most larger aircraft generate the power needed to run all their systems with generators driven by ancillary gearboxes on the engines or via an auxiliary power unit (APU) while the engines are not running. Should a failure occur during flight where none of these power sources are available, a propeller driven ram air turbine may be deployed to generate power from the air flowing past the aircraft. These small generators are situated behind bays or panels on the fuselage which are remotely operated from the flight deck. Should the need arise, the pilots will deploy the RAT and the bay will open, thus swinging the turbine propeller into the airflow past the aircraft.
In some cases, such as aircraft used in crop dusting operations a ram air turbine is used during normal operation to power chemical spray pumps. This lowers the demand on the aircraft's engine and allows the pump to be located as low on the airframe as possible, thereby making the spray system far simpler and more efficient. The ram air turbine used on these aircraft usually has a fairly small propeller and generates between 500 watts and 5 kilowatts. The RATs on commercial aircraft are obviously far larger with the Airbus A380 ram air turbine measuring five and a quarter feet (1.63 m) across. The average RAT is a little smaller, at approximately 2.6 feet (80 cm) in diameter.
These large commercial ram air turbines are capable of delivering between 25 and 70 kW of peak power to the aircraft systems. This will reduce when the aircraft's speed drops and is a factor which pilots have to contend with during emergency procedures. Older RATs had standard two or four bladed propellers similar in design to those used to drive propeller aircraft. Modern variants have multiblade propellers with shorter, duct enclosed blades which greatly increase efficiency.