Ram air turbine

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Ram air turbine on F-105 Thunderchief fighter-bomber

A ram air turbine (RAT) is a small propeller and connected electrical 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 turbine engine. The RAT generates power from the airstream due to the speed of the aircraft, and 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 more likely. Fewer commercial aircraft are fitted with them, although most airliners are (since the 1960s on the Vickers VC-10). The Airbus A380 has the largest RAT propeller in the world at 1.63 m in diameter, but around 80 cm is more common. A typical large RAT on a commercial aircraft can be capable of producing, depending on the generator, from 5 to 70 kW. Propellers started as two-bladed or four-bladed models but military (and increasingly commercial) models now 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.