A ram air turbine (RAT) is a small propeller and connected hydraulic pump, or 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). Some RATs produce only hydraulic power, that is then used to power electrical generators. Modern aircraft generate power through the main engines or an additional fuel-burning turbine called an auxiliary power unit, which is 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 civilian aircraft are fitted with them, although most commercial 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. 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 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, and the pump can be placed low or below the exterior of the airframe greatly simplifying plumbing, and 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 obvious fact that the spray systems are non functional. Honeywell and Hamilton Sundstrand are the main US suppliers of RAT systems.
Aviation incidents
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
