What is the primary function of the Auxiliary Power Unit (APU) on the MH-60S?

The primary function of the Auxiliary Power Unit (APU) on the MH-60S is to provide power for the aircraft’s electrical systems and air conditioning. The APU is a small turbine engine that generates electrical power when the main engines are not running, which is crucial for powering avionics, lighting, and other electrical equipment necessary for pre-flight checks and operations on the ground. Additionally, the APU supplies bleed air, which is used for air conditioning, particularly in hot environments or during maintenance activities when the engines are off. This capability ensures that the crew can maintain comfort and operational readiness without relying solely on the main engines.

The other functions mentioned, such as powering the main rotor, serving solely as a backup engine, or assisting with fuel management, do not accurately describe the APU’s primary roles. The main rotor is operated by the helicopter's main engines, and while the APU can provide additional backup support, its main purpose revolves around electrical power and environmental control systems. Thus, the chosen answer effectively captures the APU's vital contributions to the operation of the MH-60S.