Definition
A simple mental calculation pilots use to estimate when to begin descent and at what rate, based on altitude to lose, distance available, and groundspeed. The most common version is the '3-to-1 rule': allow 3 nautical miles of distance for every 1,000 feet of altitude to lose. A companion calculation for vertical speed is groundspeed (in knots) divided by 2, then multiplied by 10, which gives a target rate of descent in feet per minute for a 3-degree path.
Plain English
A quick math shortcut for working out where to start coming down and how fast to descend, without needing a calculator or chart.
Context Anchor
Used during instrument descent planning, especially when preparing to leave cruise altitude and arrive at an assigned altitude or approach altitude in a controlled, predictable way.
Derivation
A 'rule of thumb' is an old expression meaning a rough but reliable practical guide, used when precise measurement isn't needed or available. In aviation, it survives because pilots often need fast answers in the cockpit while flying.
Why Pilots Care
Supports safe descent management, compliance with ATC clearances, and avoidance of excessive descent rates.
Grounding Statement
If you need to lose altitude smoothly, the descent rule of thumb gives you a quick first estimate for where to start down and how fast to descend.
Intuition Check
Do not treat a descent rule of thumb as a required procedure or a guarantee. It is only a planning estimate; the actual descent must still meet all published restrictions and instructions.
Example Sentence 1
Cruising at 36,000 feet and needing to be at 6,000 feet for the arrival, the captain applied the descent rule of thumb and began descent about 90 miles out.
Example Sentence 2
Applying the descent rule of thumb let the crew meet the approach altitude without exceeding a comfortable descent rate.