What is the function of the APU on the CRJ?

The function of the Auxiliary Power Unit (APU) on the CRJ is primarily to provide power and air for starting the engines and to meet in-flight electrical needs. This small turbine engine is essential for supplying electrical power to the aircraft when the engines are not running, and it also supplies bleed air, which is used for starting the main engines and for environmental control systems such as cabin heating and cooling.

When the APU is operating, it allows the aircraft to be self-sufficient on the ground, eliminating the need for external power sources and enabling operations in remote locations where ground services might not be available. This is particularly important during pre-flight preparations and when the aircraft is on the ground for extended periods. Therefore, the correct answer accurately captures the multifaceted role of the APU in both ground operations and during flight.

Other options do not align with the fundamental role of the APU. Enhancing fuel efficiency during cruise flight, serving as a backup for navigation systems, or increasing passenger capacity are functions not associated with the APU's design and operational purpose.