Top of Descent Calculator
Where to start down from cruise for a stabilised path, and the rate to fly.
Inputs
Result
Start descent
100.5nm
100.5 nm before the target — losing 32,000 ft
Rate of descent
≈ 1,400 fpm by the 5×GS rule
1,486fpm
Descent gradient
3° path
318ft/nm
Top of descent 100.5 nm out, descend 1,486 fpm (32,000 ft at 3°)
Formula
Ref- Height to lose = cruise − target
- TOD distance = height ÷ (tan(angle) × 6076) [nm]
- Rate of descent (fpm) ≈ 5 × ground speed (kt)
Worked example
RefCruise FL350 (35,000 ft) to 3,000 ft, 280 kt, 3° path.
- 1.Height to lose = 35,000 − 3,000 = 32,000 ft.
- 2.Distance ≈ 3 × 32 = 96 nm (3:1 rule).
- 3.Rate ≈ 5 × 280 = 1,400 fpm.
Start down ≈ 100 nm out, descend ≈ 1,400 fpm.
How it works
RefTop of descent is where you leave the cruise to arrive at a lower level on a stabilised path. The 3:1 rule gives three track miles per thousand feet to lose; the 5×ground-speed rule gives the rate to fly. Together they let you plan and monitor a descent without automation.
DGCA student tip
ExamAdd a mile per 10 kt of tailwind and add track miles to decelerate. Examiners expect instant recall of the 3× distance and 5×GS rate for a 3° path.
Common mistakes
Watch- Forgetting to add track miles to slow down as well as descend.
- Using indicated cruise altitude without subtracting the target's elevation.
- Applying 5×GS at very shallow or steep angles where it breaks down.