What is the primary function of the APU engine in relation to airflow?

The primary function of the Auxiliary Power Unit (APU) engine is to provide supplementary air and power to the aircraft systems when the main engines are not operating or are insufficient. In particular, the APU engine is designed to take in a large quantity of air and perform tasks that support the operation of the aircraft, such as providing bleed air for starting the main engines, air conditioning, and electrical power. This ability to intake and manage significant airflow is critical for maintaining the necessary operations of the aircraft systems and ensuring overall performance, particularly on the ground.

While the APU does compress air as part of its operation, which is essential for specific functions like engine start, the primary focus is on the volume of air it can handle. The APU's role in adding heat energy or converting heat energy to horsepower is secondary to its primary function of managing airflow. Consequently, the ability to intake a large quantity of air is fundamental to its operation and support for the aircraft systems.