In what situation must a challenge and response verification be used regarding the flight deck door?

The requirement for challenge and response verification is critical in maintaining security on the flight deck. This protocol is specifically implemented when the flight deck door is opened in flight. In-flight, the safety and security of the flight deck are paramount due to the unique risks posed by unauthorized access or potential threats. Using a challenge and response system ensures that only authorized personnel can enter the flight deck, thus providing an additional layer of security.

In contrast, keeping the flight deck door open at all times, or during situations where one flight crew member leaves the flight deck, does not necessitate this protocol since these scenarios do not involve the same level of risk as opening the door during flight. Additionally, while emergency access is important and can require action, it typically follows predefined procedures that may not directly involve challenge and response unless specific protocols for unauthorized entry situations are enacted. Therefore, the requirement for challenge and response verification is specifically aligned with the heightened security measures needed when the flight deck door is opened during flight.