Aram air turbine (RAT) is asmall wind turbine that is connected to ahydraulic pump, orelectrical generator, installed in anaircraft and used as a power source. The RAT generates power from the airstream byram pressure due to the speed of the aircraft. It may be called anair driven generator (ADG) on some aircraft.[1]
Modern aircraft generally use RATs only in an emergency.[2] 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).[3] Some RATs produce only hydraulic power, which is in turn used to power electrical generators.
In some early aircraft (including airships), small RATs were permanently mounted and operated a small electrical generator orfuel pump. Someconstant-speed propellers, such as those of theArgus As 410 engines used in theFocke-Wulf Fw 189, used a propeller turbine on thespinner to power a self-contained pitch governor controlling this constant speed.
Modern aircraft generate power in the main engines or an additional fuel-burningturbine engine called anauxiliary power unit, which is often mounted in the rear of the fuselage or in the main-wheel well. 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), and is deployed manually or automatically following complete loss of power. In the time between power loss and RAT deployment, batteries are used.
RATs are common in military aircraft, which must be capable of surviving sudden and complete loss of power.
They also power pod-fitted systems such as theM61A1 Vulcan cannon. Some free-fall nuclear weapons, such as the BritishYellow Sun andRed Beard, used RATs to powerradar altimeters and firing circuits; these were a more reliable alternative to batteries.
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High-powered electronics such as theAN/ALQ-99 jamming system can be self powered by a RAT in standard operation. This allows their installation on a standardhardpoint, without requiring a pod-specific power supply. As many as five AN/ALQ-99 systems with built in ram air turbines can be mounted on aBoeing EA-18G Growler, with two under each wing and one under thefuselage of the aircraft. Each AN/ALQ-99 contains two transmitters, each with its own directional antenna. They are used by theElectronic Attack Squadron 134 (VAQ-134). They are not retracted, staying deployed continuously during flight.[4][5]
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Many modern types of commercial airliners, from theVickers VC10 of the 1960s onwards,[6] are equipped with RATs. A ram air turbine driving an electrical generator was chosen for the VC10 because of its use of "packaged" hydraulically powered flying controls, rather than a centralised hydraulic system. The individual package units of the VC10 were each powered electrically and so emergency redundancy for the VC10 relied on quadruple generators and a backup RAT generator at a time when most RATs drove hydraulic pumps.[7]
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TheAirbus A380 has the largest RAT in the world at 1.63 metres (64 in) in diameter, but around 80 centimetres (31 in) is more common. A typical large RAT on a commercial aircraft can be capable of producing5 to 70 kW, depending on the generator. Smaller, low airspeed models may generate as little as 400 watts.
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RATs have also been used to powercentrifugal pumps to pressurize the spray systems on aircraft that are used ascrop dusters to deliver liquid agents to cropland. The major reason for choosing a RAT is safety; using a RAT in the United States allows anFAA-certified engine and power systems on the aircraft to remain unmodified. There is no need to use an enginepower takeoff to drive the pump, as the pump can be placed low or below the exterior of the airframe, greatly simplifying plumbing. Being the lowest point in the plumbing, it will have gravity feed from the spray tanks and never need to be primed. In the event of a pump failure that could result in seizure, there is no effect on the flying ability of the aircraft or its systems apart from the fact that the spray systems are non-functional.
The following aviation incidents involved the deployment of a ram air turbine:
