What is the primary purpose of an Auxiliary Power Unit (APU)?

The primary purpose of an Auxiliary Power Unit (APU) is to provide ground electrical and pneumatic power when the main engines are not running. This is essential for powering various systems while the aircraft is on the ground, such as lighting, air conditioning, avionics, and starting the main engines. The APU ensures that these systems have the necessary power without relying on the aircraft's main engines or external power sources.

When an aircraft is on the ground, especially during pre-flight checks or when parked, the main engines are typically not running. The APU serves as a crucial resource for maintaining cabin comfort and operational systems until departure, allowing for safe and efficient ground operations.

The other options focus on aspects not directly related to the APU's core function. Increasing fuel efficiency relates more to the performance of the main engines rather than the auxiliary unit. Enhancing maneuverability pertains to flight controls and aircraft handling, which is outside the APU's scope. Similarly, assisting in the navigation system does not fall under the APU's primary function, as that responsibility lies with different onboard avionic systems.