What does “self-contained power unit” imply in the context of an APU?

The term “self-contained power unit” in the context of an APU signifies that it is capable of generating power independently. This means that the Auxiliary Power Unit (APU) can operate autonomously without needing to connect to an external power source, which is particularly useful during ground operations or when the main engines are not running.

This independence allows the APU to provide electrical power for onboard systems and assist with engine starting, as well as supply compressed air for environmental control and other systems. The ability to function independently enhances operational flexibility and reduces the reliance on ground support equipment, making the APU a crucial component in maintaining aircraft readiness and operational efficiency.