You are at FL300, ATC gives you a restriction of 2000 ft by 20 DME from a VOR you are navigating from. Using the 3-to-1 rule, approximately how many nautical miles from the fix would you begin descent to reach 2000 ft?

• A. 84 NM
• B. 93 NM
• C. 126 NM
• D. 70 NM

Answer: (B)

The short way to plan a descent is the 3-to-1 rule: for every 1,000 feet you need to lose, you descend over about 3 nautical miles. Here you must drop from FL300 (30,000 ft) to 2,000 ft, a change of 28,000 ft. That requires about 28 × 3 = 84 NM of horizontal distance.

You need to be at 2,000 ft by the time you are 20 DME from the VOR, so you would begin the descent about 84 NM before you reach that 20 DME point. In other words, roughly 84 NM from the fix is where descent would start to meet the 2,000 ft restriction at 20 DME.

Keep in mind this is a rule-of-thumb calculation; actual descent planning can be adjusted for speed, descent angle, and airspace constraints, but the 84-NM figure is the standard approximation using the 3-to-1 rule.