Ram Air Turbines
Please describe the propeller located on the back of the wings on the
Airbus A330 used during the miraculous flight recently.
- question from Ruth Treloar
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)
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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