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A ram air turbine (RAT) is a small engine that provides emergency power to aircraft, typically used in the event of engine failure. They can generate up to 50 kW of power and are standard on larger commercial and military planes. RATs can also be used to power auxiliary equipment, such as chemical spray pumps on crop spraying planes. Modern RATs have multi-blade propellers with closed ducts for increased efficiency.
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 sprayers, powers auxiliary equipment. These turbines are stowed behind locked panels during normal flight, and are generally only deployed in the event of a total loss of power due to multiple engine failures. RATs are standard parts of redundancy backup systems on larger commercial, military, and crop spraying aircraft. The average ram air turbine in an airliner can generate more than 50 kilowatts (kW) of power and have a propeller several feet in diameter. Older RAT designs featured twin-helix propellers, although newer examples generally have multi-propeller and ducted fans.
Most of the larger aircraft generate the power needed to run all of their systems with generators driven by auxiliary gearboxes on the engines or by an auxiliary power unit (APU) while the engines are not running. If a failure occurs during flight where neither of these power sources is available, a propeller-driven ram air turbine can be deployed to generate power from the air flowing past the aircraft. These small generators are located behind bays or panels in the fuselage that are operated remotely from the flight deck. If the need arises, pilots will deploy the RAT and the bay will open, thus swinging the turbine propeller into the airflow past the plane.
In some cases, such as aircraft used in crop spraying operations, a ram air turbine is used during normal operation to drive chemical spray pumps. This reduces demand on the aircraft engine and allows the pump to be located as low in the airframe as possible, making the spray system much simpler and more efficient. The ram air turbine used on these planes usually has a fairly small propeller and generates between 500 watts and 5 kilowatts. RATs on commercial airliners are obviously much larger with the Airbus A380 ram air turbine measuring five and a quarter feet (1.63m) wide. The average RAT is slightly smaller, at about 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 aircraft systems. This will reduce as the speed of the aircraft decreases and is a factor pilots must deal with during emergency procedures. Older RATs had standard two- or four-bladed propellers of similar design to those used to drive propeller-driven aircraft. Modern variants have multi-blade propellers with shorter blades, with closed ducts, greatly increasing efficiency.
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