What is the Auxiliary Power Unit (APU)?

• A. A piston engine providing electrical power only
• B. A centrifugal flow gas turbine engine supplying air for ECS operation and MES and GMM through AMADs.
• C. A small turbine used for fuel conditioning
• D. A hydraulic pump unit for fuel transfer

Answer: (B)

An Auxiliary Power Unit is a small turbine that provides power and bleed air to operate aircraft systems when the main engines aren’t running or during startup. In this Growler, the APU is a centrifugal-flow gas turbine that supplies air for the environmental control system and also feeds the mission electronics and related equipment through the Airframe Mounted Accessory Drives. This setup lets essential systems operate and enables engine start without relying on the main engines. It isn’t a piston engine, it isn’t just for electrical power, it isn’t primarily for fuel conditioning, and it isn’t simply a hydraulic pump for fuel transfer.