What systems are powered directly by the APU?

The APU (Auxiliary Power Unit) primarily serves to provide electrical and pneumatic power to various aircraft systems while on the ground and during certain phases of flight. The systems that are powered directly by the APU include avionics, which are essential for navigation and communication; environmental control systems, which manage cabin temperature and air quality; and cabin lighting, providing crucial illumination for passenger comfort and safety.

Understanding the function of the APU in supporting these systems helps to ensure that the aircraft can operate efficiently, especially when the main engines are not running. This is vital during pre-flight checks, ground operations, and situations where engine power is unavailable. By supplying power to these systems, the APU plays a critical role in maintaining operational readiness and enhancing passenger experience.

The other options refer to different aircraft systems. Engine thrust systems and navigation primarily rely on the main engines and other critical avionics systems once airborne. Emergency power and fuel systems are designed to function independently of the APU in most scenarios, activating other sources below certain thresholds. Catering and in-flight entertainment systems, while they may utilize power, are not the primary functions of the APU, which is focused on core operational systems.