What is the additional function of an auxiliary power unit (APU) in an aircraft?

The additional function of an auxiliary power unit (APU) in an aircraft is to provide hydraulic power. An APU is a small engine that generates energy for various functions when the main engines are not operating or when additional power is needed. One of its key roles is supplying hydraulic power for systems such as landing gear operation, flaps, and other flight control surfaces, as well as driving various auxiliary systems.

While APUs are primarily known for providing electrical power and starting main engines, their capability to ensure a supply of hydraulic power is essential, particularly during ground operations or in situations where the main engines are not running. This allows the aircraft to perform critical functions that support safe operations without having to rely solely on the main engines for all power needs.