Ram air turbine
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A ram air turbine (commonly known by the acronym RAT) is a small turbine that is connected to a hydraulic pump, or electrical generator, installed in an aircraft and used as a power source. The RAT generates power from the airstream by ram pressure due to the speed of the aircraft.
With the exception of crop dusters (see below), modern aircraft only use RATs in an emergency — 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). 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 or fuel pump.
Modern aircraft generate power in the main engines or an additional fuel-burning turbine engine called an auxiliary 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. Many modern types of commercial airliners are equipped with RATs. In the 1960s the Vickers VC10 was one of the first types of airliner equipped with a RAT. 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. Smaller, low airspeed models may generate as little as 400 watts.
A possible "pioneering" use of such a device could be said to have been the standardized installation on the Me 163B Komet rocket fighter in 1942-43. The unit was installed in the extreme nose of the aircraft to provide an auxiliary source of electrical power while the aircraft was in flight.
In other military uses, pod-fitted systems such as the M61A1 Vulcan or electronic systems (e.g. the AN/ALQ-99 TJS) can be powered by a RAT in standard operation. Also, some free-fall nuclear weapons, such as the British Yellow Sun and Blue Danube, used RATs to power radar altimeters and firing circuits; these were a more reliable alternative to batteries.
In non-military use, RATs have been used to power centrifugal pumps to pressurize the spray systems on aircraft that are used as crop dusters to deliver liquid agents to cropland. The major reason for choosing a RAT is safety; using a RAT allows, in the case of the US, the FAA-certified engine and power systems on the aircraft to remain unmodified. There is no need to use an engine power 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.
This is a list of aviation incidents involving a ram air turbine:
- Air Canada Flight 143; better known as the Gimli Glider incident
- Air Transat Flight 236
- The hijacking of Ethiopian Airlines Flight 961
- Pinnacle Airlines Flight 3701
- US Airways Flight 1549
- Wald, Matthew L. (2009-01-17). "Investigators Offer Details of Flight’s Few Minutes". Manhattan (NYC): NYTimes.com. Retrieved 2009-01-20.
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