What is operable in Pax Off and Crew Off?

The correct answer is that the main pack is operative, while the respective cold and hot valves are closed during the Pax Off and Crew Off configurations. This means that the system is designed to maintain certain operational functions even when the passenger and crew cabin conditions are not actively heated or cooled.

In the Pax Off and Crew Off scenarios, the primary goal is to ensure that the aircraft can maintain a safe environment without unnecessary energy consumption or environmental control when there are no passengers or crew aboard. By keeping the main pack operative but closing the valves for heating and cooling, the aircraft can still monitor or manage other air systems while conserving resources.

This functionality allows for operational readiness and safety checks, as the system can be activated quickly should it be needed. Other options suggest that the main pack is either inactive or entirely non-functional, which does not align with the operational capabilities of the aircraft when there are no occupants.