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

To determine the distance required for a descent from 35,000 feet to 20,000 feet at a speed of 300 knots, you can apply a common rule of thumb used in aviation for calculating descent profiles. A standard descent rate is often cited as about 3,000 feet per minute. If a pilot maintains this rate while descending from 35,000 feet to 20,000 feet, they need to lose 15,000 feet of altitude.

Dividing 15,000 feet by a descent rate of 3,000 feet per minute gives you 5 minutes of descent time. At a ground speed of 300 knots, you can convert the speed to miles per minute to find the coverage during the descent. Knowing that there are 60 minutes in an hour, 300 knots translates to 5 miles per minute (300 knots ÷ 60 minutes = 5 miles per minute).

Multiplying the descent time of 5 minutes by the speed of 5 miles per minute indicates that you need to start your descent approximately 25 miles out. However, to account for the time it takes to transition from descent to level flight stable at the new altitude, it's common practice to extend this distance. This leads to