Which rule is used to calculate the top of descent given your level and a crossing restriction?

• A. 3 to 1 Rule
• B. 2 to 1 Rule
• C. 4 to 1 Rule
• D. 5 to 1 Rule

Answer: (A)

Top-of-descent planning uses a practical rule of thumb that ties how much altitude you must lose to how far you must travel horizontally. For every 1,000 feet of altitude you need to lose, descend over about 3 nautical miles. This 3-to-1 rule helps you start your descent so you hit the crossing restriction at the fix.

For example, if you’re level at 14,000 feet and must cross the fix at 6,000 feet, you need to lose 8,000 feet. That’s eight increments of 1,000, so you’d plan to begin the descent about 8 × 3 = 24 nautical miles before the fix. If the distance to the fix is shorter than that, you’d be too high to meet the crossing altitude with a simple descent plan, and you’d need to adjust.

Other ratios would give different timing—2-to-1 would mean a different horizontal distance for the same altitude loss, and 4-to-1 or 5-to-1 would push the top of descent even farther out. The standard method used for meeting a crossing restriction aligns with a typical descent path near 3 degrees, which is why this rule is the best fit.