DEFINITION:

APU are subsystems that deliver power, either mechanical, electrical, hydraulic or pneumatic, for specific periods of the aircraft mission, when the main power source (driven by the engines) is no longer available or insufficient , on ground or in flight: on ground before and during engine start, in flight shut downs, special power needs for payload or armaments, more generally every power needs in emergency circumstances. APU include a turbine , and an electric or hydraulic generator to convert mechanical power, and a system for derive compressed air for delivery to other systems such as engine starter or environmental control system (ECS). The generated power is in the range of a few kW up to several 100kW.

(Source: ACARE Domain 308)

SUBDOMAINS:

The evolutions have the same goals as those of propulsion turbines: increasing power per kilogram, decreasing fuel consumption, reducing costs through various ways (reduction of part counts, more efficient manufacturing process ,..)

Another trend is the integration of APU in the broader platform system optimisation including the main power generation, and the main power consumers: in this scope some effective concepts have evolved such as the Integral Power Unit which is a machine integrating on a single shaft the former APU , the electrical generator and the ECS compressor and turbine . Such an IPU has a much longer operating time, and a broader operating domain that may open design options to variable cycle concepts.

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