For what purpose is APU bleed air primarily used?

The primary purpose of APU bleed air is for engine starting and environmental control. The Auxiliary Power Unit (APU) generates bleed air that is vital during the engine start sequence, providing the necessary air pressure to start the engines efficiently. During this process, the APU supplies bleed air to start the main engines, which helps to turn the high-pressure turbine and initiate combustion.

Additionally, the APU bleed air is integral to maintaining environmental control in the cabin, as it feeds into the air conditioning system. It provides the necessary pressurized air that helps regulate temperature and ensures passenger comfort before and after engines are started.

This dual utility – facilitating engine starts and aiding in cabin climate control – highlights the significance of APU bleed air in the aircraft's overall operation.