Which system does the APU help to operate when the main engines are not running?

The Auxiliary Power Unit (APU) is a critical component on the C-130J that provides power when the main engines are not operational. It primarily supplies electrical energy needed to run various aircraft systems while on the ground or during specific scenarios like an engine-out situation. This capability is essential for maintaining the functionality of crucial electrical systems, which include lighting, cabin pressurization, and other electronic equipment.

While other systems like flight control, navigation, and communication systems may interact with the APU indirectly—particularly if they rely on electrical power—the APU's main role is directly related to supplying electrical power. This ensures that vital aircraft operations continue smoothly without requiring the main engines to be running, thus allowing for safer ground operations and preparations for flight.