What is the secondary function of the APU?

The secondary function of the Auxiliary Power Unit (APU) is pneumatics. The APU is primarily known for generating electrical power and providing bleed air for various pneumatic needs, such as starting the main engines and supplying air conditioning to the aircraft while on the ground. Its ability to generate pneumatic pressure becomes crucial in maintaining system operations, especially during engine start-up and when main engines are not running. This secondary pneumatic function allows the aircraft to operate essential systems without the need for the main engine, making the APU a valuable component in aviation operations.