When should you change to the local altimeter setting prior to descent within the NAS?

The correct choice reflects the point at which pilots should transition to the local altimeter setting prior to descending within the National Airspace System (NAS). This transition typically occurs at or above the Flight Level 180 (FL180). It is important because below this altitude, aircraft operate under the altimeter setting of the local area, which ensures accurate altitude readings and compliance with air traffic control instructions.

At FL180, pilots transition from using standard pressure (29.92 inHg) to the local altimeter setting in effect for the area they are flying in. This practice enhances safety by ensuring all aircraft are accurately calibrated to the local atmospheric pressures, thereby maintaining proper vertical separation from other aircraft.

After this transition point, pilots receive updates to the altimeter setting from air traffic control as they approach their destination or as weather conditions change. By adhering to this standard practice, pilots help maintain the safety and efficiency of flight operations within the NAS.