A ram air turbine (RAT) is a
small wind 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. It may be called an air driven generator (ADG) on some aircraft.[1]
Operation
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 or
fuel pump. Some
constant-speed propellers, such as those of the
Argus As 410 engines used in the
Focke-Wulf Fw 189, used a propeller turbine on the
spinner to power a self-contained pitch governor controlling this constant speed.
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.
Military use
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 the
M61A1 Vulcan cannon. Some free-fall nuclear weapons, such as the British
Yellow Sun and
Red Beard, used RATs to power
radar altimeters and firing circuits; these were a more reliable alternative to batteries.
Argus As 410 engine and ram air turbine used to power the actuator of the variable-pitch propeller, which is visible in front of the engine and behind the ram air turbine
A rocket propelled
Messerschmitt Me 163B Komet with its nose-mount ram-air turbine as its only source of electrical power
Wing mount
High-powered electronics such as the
AN/ALQ-99 jamming system can be self powered by a RAT in standard operation. This allows their installation on a standard
hardpoint, 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 a
Boeing EA-18G Growler, with two under each wing and one under the
fuselage of the aircraft. Each AN/ALQ-99 contains two transmitters, each with its own directional antenna. They are used by the
Electronic Attack Squadron 134 (VAQ-134). They are not retracted, staying deployed continuously during flight.[4][5]
Growler aircraft with ram air turbines mounted under the wings and body
An EA-18G Growler, carrying under each wing an external fuel tank, a AN/ALQ-99 system with a ram air turbine, a jamming pod, and an
AGM-88C HARM anti-radiation missile, with an additional AN/ALQ-99 system centered under the fuselage of the aircraft
US Navy An E-A-18G Growler from Electronic Attack Squadron (VAQ) 129 N40562-21 carrying similar equipment. The spinning blades on the ram air turbines appear blurred.
Interview with U.S. Secretary of Defense
A wing-mount AN/ALQ-99 jamming system powered by a ram air turbine was chosen as a backdrop for an interview with Defense Secretary
Mark Esper by
Christiane Amanpour.
Cropped view of the ram air turbine—the turbine has four blades, which spin continuously to power the jamming system when the aircraft is in flight.
Civilian use
Many modern types of commercial airliners, from the
Vickers VC10 of the 1960s,[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]
The
Airbus 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 producing 5 to 70 kW, depending on the generator. Smaller, low airspeed models may generate as little as 400 watts.
RATs have also 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 in the United States allows an
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.
Civilian incidents involving RAT deployment
The following aviation incidents involved the deployment of a ram air turbine:
^Steve Ginter (2003). Naval Fighters Number Sixty-Four North American A-5A, RA-5C Vigilante—UTILITY HYDRAULIC SYSTEM: RAM-AIR TURBINE. Steve Ginter. p. 27.
ISBN0-942612-64-7.