What must remain closed and locked at all times when not in use?

The flight deck door must remain closed and locked at all times when not in use to enhance security and ensure the safety of the flight crew and passengers. This limitation is critical as it prevents unauthorized access to the flight deck, thereby safeguarding the operating crew from potential threats and ensuring that cockpit operations are conducted securely. This practice is part of standard protocols in aviation to maintain operational safety and security while the aircraft is in flight or on the ground.

While the passenger cabin door and emergency exit doors also have safety protocols around them, they do not share the same level of security risk as the flight deck door. The baggage compartment, though it must be secure, does not directly impact the immediate safety of flight operations to the same extent the flight deck does.