What is the primary use of bleed air from the APU?

The primary use of bleed air from the Auxiliary Power Unit (APU) is for engine start and cabin air. During engine start, the APU provides the necessary bleed air to help start the main engines by assisting in the ignition and combustion process. This is particularly important on the ground when external power is not available or during specific operational conditions.

In addition to engine start, the bleed air from the APU is also employed to condition the air in the cabin, providing heating or cooling depending on the requirements of the flight. This dual function of supporting engine operations and maintaining cabin comfort highlights the versatility and importance of the APU in aircraft systems.

While bleed air can be used for other functions, such as wing anti-icing, hydraulic pressure, and landing gear operation, these are not the primary roles of bleed air from the APU. Instead, those systems typically have dedicated sources of bleed air or hydraulic pressure that may differ from what is supplied by the APU, reinforcing the primary functions of engine start and cabin air control.