What must be ensured when one crew member leaves the flight deck?

When one crew member leaves the flight deck, it is crucial to ensure that another crew member is present in the flight deck. This measure is in place to maintain safety and security during flight operations. The absence of a crew member can lead to vulnerabilities, particularly in circumstances where decision-making and immediate action may be required for emergency situations. Having another crew member ensures that the cockpit remains staffed, allowing for continual monitoring of flight operations, communication with air traffic control, and the ability to address any issues that may arise.

Monitoring the flight from outside the cockpit or automating the flight may not provide the same level of immediate response capability needed. Additionally, locking the door is an important security procedure but does not directly address the operational readiness and continuous monitoring necessary for safe flight operations when one member is absent.