When approaching 20,000 feet from 35,000 feet at 300 knots, how many miles out should you begin your descent?

To determine the appropriate distance to begin a descent from 35,000 feet to 20,000 feet while traveling at 300 knots, a common rule of thumb in aviation is to descend at a rate of approximately 3 nautical miles for every 1,000 feet of altitude loss.

In this scenario, the altitude that needs to be lost is 15,000 feet (from 35,000 feet to 20,000 feet). To calculate the distance to begin the descent, you multiply the altitude loss by the descent distance per altitude:

15,000 feet ÷ 1,000 feet = 15 (units of 1,000 feet)

15 x 3 nautical miles = 45 nautical miles

Therefore, the descent should begin approximately 45 nautical miles from the point where you plan to reach 20,000 feet. This rule provides a safe margin which allows the aircraft to descend smoothly and maintain a comfortable rate of descent in preparation for approach and landing.

This calculation reinforces why beginning the descent at 45 miles out is the correct choice, allowing for an efficient descent profile while maintaining safety standards.