If you are at 21,000 feet and need to descend to 11,000 feet, and you are traveling at 300 knots, how far in advance should you begin your descent?

To determine how far in advance to begin a descent from 21,000 feet to 11,000 feet while traveling at 300 knots, it's essential to consider both the altitude change and the descent rate.

Firstly, the change in altitude is 10,000 feet (from 21,000 to 11,000). A common rule of thumb for descents is that for every 1,000 feet of altitude loss, you should plan to descend for approximately 3 nautical miles. Therefore, for a descent of 10,000 feet, you would multiply 10 (the number of thousands to descend) by 3 nautical miles, resulting in 30 miles.

In terms of speed, traveling at 300 knots means you cover a significant distance quickly. Since you need enough time to manage the descent safely and comfortably, starting your descent 30 miles before reaching 11,000 feet allows for a smooth transition.

Thus, the answer indicating that you should begin your descent 30 miles in advance accounts for both the necessary altitude change and the speed, ensuring an appropriate rate of descent that aligns with standard aviation practices.