If all outflow valves close during cruise flight, what happens to pressure differential?

When all outflow valves close during cruise flight, the pressure differential increases because no air can escape from the cabin. The outflow valves are responsible for regulating cabin pressure by allowing excess air to leave as the aircraft climbs and ensuring that the cabin pressure is maintained at a safe and comfortable level for passengers and crew.

If the outflow valves are closed, the cabin air can continue to be pressurized by the aircraft's environmental control system, but there is no mechanism to release any excess pressure. As a result, the pressure inside the cabin can rise significantly above the pressure outside the aircraft, leading to an increased pressure differential.

This is critical because maintaining an optimal pressure differential is essential for the structural integrity of the aircraft. If it were to increase beyond safe limits, it could lead to a potential failure of the fuselage structure, as the cabin is designed to handle certain pressure loads.

Thus, the correct answer points to the fact that the pressure differential would indeed reach its maximum value under these circumstances.