What does a closed aircraft door need to be?

A closed aircraft door needs to be flush with the fuselage to ensure a proper seal and structural integrity of the aircraft. When the door is flush, it aligns perfectly with the surrounding surface, preventing any gaps that could compromise the safety and aerodynamics of the aircraft during flight. This alignment is crucial for maintaining cabin pressure and ensuring that external conditions do not affect the interior of the aircraft.

Options that suggest the door is slightly ajar or round at the edges do not provide the necessary sealing functionality required during operation. Additionally, while "under pressure" may imply some operational considerations related to door functionality, it does not specifically indicate the correct positioning of the door within the fuselage structure. Therefore, flush alignment is a critical safety feature that ensures the aircraft operates effectively and securely.