What is the primary purpose of the APU?

The primary purpose of the Auxiliary Power Unit (APU) is electrical power generation. The APU serves as a self-contained power source that can generate electricity to support various aircraft systems, particularly when the main engines are not running. This is crucial during pre-flight checks, ground operations, and in situations where the main power sources are unavailable.

While other functions are associated with the APU, such as providing pneumatic power for starting engines and environmental control for cabin conditioning, its main role focuses on supplying the necessary electrical energy. This capability allows for a wide range of operations to continue without dependence on external power sources, thereby enhancing safety and operational flexibility.

Other options, although important, represent secondary functions of the APU rather than its primary role.