At an altitude of 24,000 ft, when should you start your descent to reach 18,000 ft?

To determine the appropriate distance at which to start the descent from 24,000 ft to 18,000 ft, the general rule of thumb in aviation often suggests a descent rate and distance that correlates with altitude. A common guideline is to start descending for every 1,000 feet of altitude, you should begin your descent approximately 3 nautical miles before your intended waypoint or altitude.

In this scenario, you are descending from 24,000 ft to 18,000 ft, which represents a 6,000 ft descent. Using the 3 nautical mile per 1,000 ft rule, you would multiply 6 (the number of thousands of feet) by 3, resulting in a need to start the descent 18 nautical miles prior to reaching the intended altitude of 18,000 ft.

This calculation aligns with the answer that indicates beginning the descent 18 nautical miles before the waypoint, allowing adequate time and distance to safely reduce altitude while complying with airspace regulations and procedures.