Aside from the obvious and necessary need for propulsion, an aircraft’s engines also provide power for various other systems such as the electrical, pneumatic, and air conditioning. But these systems are in use even in situations when running the main engines would be unsafe or inefficient in terms of fuel usage, such as when passengers are boarding and during pre-flight procedures. In these times, power can be provided by ground carts, but in situations where they are not available or are insufficient, the auxiliary power unit is utilized instead.
An auxiliary power unit, or APU, is a small gas turbine engine fitted to aircraft (typically installed at the base of the aircraft’s tail) to provide electrical power from shaft-driven generators, pneumatic duct pressure for air conditioning and engine starting purposes, and hydraulic power in some aircraft. APUs are primarily used on the ground, when an aircraft’s main engines are not running. On most modern aircraft, APUs are used to provide air conditioning during take-off and landing, or to back up the main engines in case of a generator or air system failure.
An APU consists of a gas turbine engine that functions identically to the engines used to create thrust: air is fed in via the intake, compressed by a series of fans, and then ignited alongside a mixture of fuel to expand and rotate a turbine, which in turn generates power. Despite operating on the same principles as the main engines however, an APU does not provide thrust to the aircraft.
While APUs are rated to run at the maximum cruising altitude of the aircraft they are fitted to, their ability to take on loads diminishes with altitude. An APU’s operating altitude is usually limited to 15,000 to 20,000 feet, and past that, the APU control unit can only provide electrical power. Even this can also be limited to less than the maximum cruise height. Most APUs give shaft priority, which means that if the air and electric generators are on, the generators have priority. Most aircraft use constant frequency generators, which means that their APUs run at a constant 100%, and do not require a constant speed drive unit to maintain constant output. If the airload becomes too high, the APU will reach its max engine gas temperature, causing the control system to back off and limit the fuel supply to prevent damage from occurring.
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