Which system does the APU support while the engines are not running?

The auxiliary power unit (APU) serves a crucial function when the main engines of the MH-60S helicopter are not operational. Primarily, the APU provides a source of electrical power, which is vital for various aircraft systems. This includes powering the electrical systems, which supply electricity to avionics, instruments, lights, and other critical components necessary for pre-flight checks and operation.

Additionally, the APU also supports the air conditioning systems, ensuring that the helicopter’s cabin remains climate-controlled for crew comfort during ground operations, maintenance, or when the engines are not running. The ability of the APU to deliver both electrical power and pneumatic pressure for air conditioning makes it indispensable in maintaining operational capability and comfort while the engines are off.

In contrast, the propulsion system relies on the main engines when operational. The fuel management system operates in conjunction with the engines and is not primarily managed by the APU. Likewise, while the navigation system is vital, it does not specifically require support from the APU when the engines are inactive, as it typically can operate on battery power or other available sources. Thus, the role of the APU in supporting the electrical and air conditioning systems stands out as the correct option.