What type of door must be installed between the passenger and pilot compartments?

The requirement for a lockable door between the passenger and pilot compartments is primarily focused on safety and security. The door serves to prevent unauthorized access from the passenger area to the cockpit, ensuring that the flight crew can operate the aircraft without interference or distraction. This measure is particularly important in maintaining control over the aircraft and safeguarding against potential threats.

In the context of FAA regulations, a lockable door provides an essential barrier, allowing the flight crew to secure the cockpit during flight. Other door types, such as emergency exits or sliding doors, do not fulfill the same purpose of securing the pilot area from the passenger section in a manner consistent with current safety protocols. A double door, while it may provide additional structural integrity or soundproofing, does not inherently offer the same level of security that a lockable door does.

Therefore, the emphasis on a lockable door is crucial for both the operational integrity of the flight crew and passenger safety, aligning with regulatory requirements that prioritize safety in aviation operations.