What signifies the end of the sterile flight deck?

The end of the sterile flight deck is indicated by two low chimes. The purpose of this auditory signal is to communicate that the flight crew can now engage in non-essential conversations, as the critical phases of flight requiring strict communication protocols and minimal distraction have concluded. This practice is part of standard operating procedures to enhance safety and ensure that the crew maintains focus during crucial periods such as takeoff and landing.

Other options represent different types of signals that may not have specific meanings related to the sterile flight deck protocol, thus reinforcing the importance of understanding the specific auditory cues used in aviation communication.