Airlines pilots choose to leave the cockpit door open during boarding procedures for various reasons:

(1) Facilitating the movement of individuals:

The cockpit door is kept open to accommodate the entry and exit of multiple individuals into the flight deck before departure. For example, the First Officer is required to temporarily leave the cockpit to conduct a preflight external walk-around before returning.

(2) Coordinating with flight attendants:

The flight attendants work in coordination with the pilots to manage the boarding process. If there are issues related to catering, aircraft cleanliness, or necessary equipment, the flight attendants can communicate with the pilots through the open cockpit door.

(3) Addressing security concerns:

While an armored and locked cockpit door helps reduce the risk of unauthorized interference from passengers, it can also create other security challenges. When the cockpit door is closed, the pilots may not be aware of potential security threats outside the cockpit.

(4) Meeting operational requirements:

Opening the flight deck door during flight is essential to cater to the pilots' biological needs and fulfill operational requirements related to safety.

Summary:

In conclusion, keeping the cockpit door open during boarding operations allows for the necessary movement of people and equipment, enables communication between flight attendants and pilots, and addresses security concerns.