What action does the No.1 press after passengers have boarded to ensure safety?

The action that the No.1 takes after passengers have boarded to ensure safety is to inform the flight deck that the cabin is secured. This step is essential because it confirms that all passengers are in their seats, seatbelts are fastened, and any necessary safety protocols have been followed. By communicating this information to the flight deck, the No.1 ensures that the pilots are aware that the cabin is ready for takeoff, which is critical for overall flight safety.

The safety of the cabin is a top priority in aviation, and this communication allows for a smooth transition into the next phases of the flight preparation process. If the cabin is not secured, it can lead to various safety risks during takeoff, such as passengers not being properly restrained.

During this procedure, other actions like initiating the takeoff sequence, verifying passenger count, or notifying ground crew may occur, but none are focused specifically on the immediate safety status of the cabin in the same way that confirming to the flight deck does. Thus, the emphasis on cabin security makes informing the flight deck the most critical action.