What action must you always take when closing and securing a panel door on the aircraft?

When closing and securing a panel door on the aircraft, the action that must be taken is to double tap. This procedure serves as a critical verification step to ensure that the door is properly closed and secured. The double tap refers to the practice of physically tapping the door to both confirm that it is shut and to check its integrity. This step is essential in aviation safety protocols, as it helps prevent any accidents or malfunctions that could arise from an improperly secured door during flight.

Implementing this action helps ensure that the door is not only closed but also latched securely, which is vital for maintaining the aircraft's structural integrity and ensuring passenger safety. By incorporating this simple yet effective action into the routine, it reinforces the importance of thoroughness and attentiveness in aircraft operations.