Ram Air Turbines

The device you speak of is known as a ram air turbine, or RAT. The RAT is used in emergency situations to generate sufficient power to keep an aircraft flying long enough to land. Thus, the RAT performs the same function as an auxiliary power unit (APU). But while an APU is essentially a small jet engine that burns regular jet fuel to generate this additional power, the RAT consists of a propeller that is spun by the high-speed air flowing past the aircraft. The spinning propeller powers a turbine that provides the emergency electricity needed to keep critical systems running, such as hydraulics, flight controls, and key avionics. In normal flight, the entire assembly is folded up and stored in the aircraft fuselage or wing. A typical ram air turbine produced by Hamilton Sundstrand is shown below.

Ram Air Turbine (RAT)
Ram Air Turbine (RAT)

Although the Airbus A330 is equipped with a ram air turbine, as evidenced by the recent emergency landing you speak of, most commercial aircraft tend to use more traditional APUs instead. The RAT is primarily used on military fighters and transports while some advanced designs employ emergency power units (EPU) that burn hydrazine to generate emergency power. However, APUs and RATs are designed for extended use while the EPU only functions for a brief period that allows the pilot to safely eject.
- answer by Joe Yoon, 16 September 2001

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