How is cooling air supplied to the cabin if the fresh air valve is open?

When the fresh air valve is open, cooling air is supplied to the cabin primarily from the ram air through the NACA inlet. This method utilizes aerodynamic principles, where air is directed into the aircraft’s cabin from the exterior as the aircraft moves forward, effectively harnessing the natural flow of air to cool the interior. The NACA (National Advisory Committee for Aeronautics) inlet is designed to minimize drag while allowing for a steady stream of fresh air to enter the cabin, contributing to ventilation and cooling.

This mechanism is particularly effective at higher speeds, as the ram air flow increases with the aircraft's velocity, enhancing the cooling effect. In contrast, other systems like the heat exchanger or air conditioning unit typically deliver cooled air through different processes that may not rely on the aircraft's forward motion, making them less relevant in the context of an open fresh air valve. The emergency ventilation system is used for specific scenarios and does not serve as the primary source of cooling under normal operations.