A ram air turbine (RAT) is a small propeller and connected generator used as an emergency power source for aircraft. In case of the loss of both primary and auxiliary power sources the RAT will power vital systems (flight controls, linked hydraulics and also flight-critical instrumentation).

Modern aircraft generate power through the main engines or additional fuel-burning turbines, auxiliary power units, often a small tail-mounted jet engine. The RAT generates power from the airstream due to the speed of the aircraft, if aircraft speeds are low the RAT will produce less power. In normal conditions the RAT is retracted into the fuselage (or wing), deploying automatically in emergency power loss. In the time between power loss and RAT deployment batteries are used.

RATs are common on military aircraft, where sudden and complete loss of power is most likely. Fewer commercial aircraft are fitted with them, although they are becoming standard on modern designs. A commercial RAT propellor can be as large as 1.5 m (around 80 cm is more common) and capable of producing, depending on the generator, from 5 to 70 kW. Propellors started as two- or four-bladed models, military (and increasingly commercial) models use ducted multi-blade fans. Smaller, low airspeed models may generate as little as 400 watts.

In another military use, pod-fitted units such as the M61A1 Vulcan or electronic systems (e.g. the AN/ALQ-99 TJS) can be powered by a RAT in standard operation.

Hamilton Sundstrand is the main US supplier of large RAT systems.

See also

• Gimli Glider