Top of Descent & Descent Rate
Where to start down and how fast: descent distance from an angle, rate or gradient at your ground speed, the rate a 3° path needs, and the point before a fix.
The distance the descent takes, the vertical speed the path needs at your ground speed, the time, how far before a fix to begin, the altitude you would cross the fix at, and the 3-to-1 and 5-times-ground-speed rules beside them.
Example: 9,000 to 2,000 ft at 120 kt on a 3° path: 318 ft per nm, so 22.0 nm and 637 ft/min over 11 minutes — start 8 nm before a fix that is 30 nm away.
Feet per mile,
then miles.
Why a descent is a gradient in feet per nautical mile, how angle, rate and ground speed convert into one another, where the two rules of thumb come from, and what is left out.
Gradient first
A path angle is a gradient: tan(angle) × 6,076 ft per nautical mile, which for 3° is 318 ft/nm. A vertical speed at a ground speed is also a gradient: rate × 60 ÷ ground speed. Once the gradient is known, the distance to lose a height is height ÷ gradient and the rate the path needs is gradient × ground speed ÷ 60. The three ways of stating the descent are one fact seen from three sides.
The rules of thumb
A 3° path is close to 300 ft/nm, so three nautical miles per thousand feet is the “3 to 1” rule, and 318 × GS ÷ 60 is close to five times the ground speed in feet per minute. Both are shown beside the exact figures; the difference is the 6 % that 300 understates 318.
Fix and altitude
Given the distance to a fix where the target altitude is wanted, the page says how far before the fix to start and, if the descent began now, the altitude the aircraft would cross the fix at. A negative “start before” means the top of descent is already behind; the note gives the rate a straight descent to the fix would now need.
What the number does not settle
Ground speed is taken as constant; it changes with altitude, wind and the speed you fly. Deceleration segments, level-offs for airspace and ATC, and speed and configuration limits are yours. A planning aid, not a substitute for the pilot’s operating handbook, the aircraft’s limitations or the rules that apply to the flight. Inputs stay in your browser; the same anonymous usage counts as the rest of the site apply.