When should you change the altimeter setting to local settings before descent?

The correct timing to change the altimeter setting to local settings before descent is before 18,000 feet, which is the transition altitude in the United States. The industry standard requires pilots to switch from standard pressure (29.92 inHg) to local altimeter settings in order to ensure accurate altitude readings relative to the terrain and airport elevation as the aircraft descends.

Changing the altimeter setting before reaching FL 185 allows for the adjustment to be made well in advance of descent, ensuring a smooth transition and facilitating compliance with air traffic control instructions while managing altitude in relation to other traffic and obstacles. Adhering to this practice helps maintain safe vertical separation from other aircraft and supports better situational awareness.

The other altitude options would not align with the regulatory requirements and best practices, as they occur above the standard transition altitude, potentially leading to improper altitude awareness during descent.