When is the APU primarily used?

The Auxiliary Power Unit (APU) is primarily utilized when the aircraft is parked at the gate. The APU provides essential functions such as supplying electrical power and bleed air for environmental control systems, allowing the aircraft's systems to operate while the engines are off. This capability is especially important on the ground, as it enables the crew and passengers to maintain comfort and necessary operations without relying on the main engines or external power sources.

During takeoff or in-flight, the aircraft relies on its engines for power, and while the APU can have utility during certain emergency scenarios, its primary and most common role occurs while parked, ensuring that everything from lighting to climate control remains functional until the engines are started and engines take over these functions.