What is the primary function of an APU?

The primary function of an Auxiliary Power Unit (APU) in an aircraft, specifically the A320, is to provide electrical power and bleed air when the main engines are not running. The APU operates independently and is crucial for starting the main engines, as it supplies the necessary electrical power to the aircraft's systems and provides bleed air for air conditioning and engine start procedures.

When the aircraft is on the ground, especially before takeoff or after landing, the APU allows for functionality without the need to rely on external ground power. This capability enhances operational efficiency and flexibility, enabling the aircraft to maintain essential services and support systems while on the ground.

This choice accurately reflects the comprehensive roles of the APU in powering devices and systems that require electricity and specific pressurized air. Understanding this function is essential for effective ground handling and pre-flight procedures in an A320.