Performance & Planning
Rule of Three Calculator
3:1 descent planning — top of descent distance and rate of descent.
Inputs
ft
kt
°
Result
Top of descent
31.4nm
Start down 31.4 nm before the target
Rate of descent
≈ 1,250 fpm by the 5×GS rule
1,327fpm
Descent gradient
3° path
318ft/nm
TOD 31.4 nm out, descend 1,327 fpm (3° path)
Formula
Ref- TOD distance = height ÷ (tan(angle) × 6076) [nm]
- Rule of thumb: distance (nm) ≈ 3 × height/1000
- Rate of descent (fpm) ≈ 5 × ground speed (kt)
Worked example
RefDescend from FL100 to sea level (10,000 ft) at 250 kt, 3° path.
- 1.3:1 rule: distance ≈ 3 × 10 = 30 nm before the target.
- 2.Gradient for 3° = tan(3°) × 6076 ≈ 318 ft/nm.
- 3.Rate of descent ≈ 5 × 250 = 1,250 fpm (exact ≈ 1,327 fpm).
Start down ~31 nm out, descend ≈ 1,300 fpm.
How it works
RefThe rule of three keeps a jet on a stabilised ~3° path: three track miles for every thousand feet to lose. Pair it with the 5×ground-speed rate-of-descent rule and you can plan a descent — and monitor it — without any automation.
DGCA student tip
ExamAdd a mile per 10 kt of tailwind and subtract for headwind, and add distance to slow down. Examiners expect the 3× distance and 5×GS rate as instant recall.
Common mistakes
Watch- Forgetting to add track miles to decelerate as well as descend.
- Applying 5×GS at very shallow or steep angles where it no longer holds.
- Using indicated altitude difference without accounting for the target's own elevation.