What is the first step in the procedure to open the FD door during flight?

The first step in the procedure to open the FD (Flight Deck) door during flight is to switch on the fasten seat belt sign. This action is crucial as it signals the cabin crew and passengers to be seated and prepared for any transitions happening within the aircraft. By ensuring passengers are securely buckled in, the crew effectively enhances safety during the process of door access, which may involve entering or exiting the flight deck.

The fasten seat belt sign is a standard procedure in aviation safety protocols, ensuring that all personnel are aware and ready for any operational changes. In the context of opening the FD door, it is an important precautionary measure before proceeding with any further actions. This step prioritizes passenger safety and aligns with the structured procedures that govern airline operations during flight.