Crabwise · Library
How do you calculate a route time when each leg has a different ground speed?
Three legs of 45 NM at 110 kt, 30 NM at 90 kt and 52 NM at 125 kt take 69.51 minutes, a route average of about 109.63 kt over 127 NM.
One time calculation per leg
The time mode of the time–speed–distance screen solves t = D ÷ GS × 60 for one distance and one ground speed. A route whose ground speed changes, because of wind shifts or a different cruise speed, is built from several such calculations. Add the leg distances and the leg minutes separately; a route average, if you need it, comes from the totals in the speed mode.
Each leg's ground speed should come from its own wind-triangle solution or observation.
Worked example
The first 60 NM at 120 kt takes 30 minutes.
- First-leg time (min)
- 30
Worked example
The next 90 NM at 90 kt takes 60 minutes. Together the route is 150 NM in 90 minutes, corresponding to a 100 kt route average.
- Second-leg time (min)
- 60
Build the route total
- List each leg with its distance in nautical miles and its ground speed in knots.
- Run the time mode for every leg and keep the unrounded minutes.
- Add distances and minutes in two separate columns.
- For an average, run the speed mode with total distance and total minutes.
A three-leg route
45 NM at 110 kt takes 45 ÷ 110 × 60 ≈ 24.545455 minutes. 30 NM at 90 kt takes 20 minutes. 52 NM at 125 kt takes 24.96 minutes. The route is 127 NM in about 69.505455 minutes, and 127 ÷ (69.505455 ÷ 60) ≈ 109.631684 kt.
- Leg 1 time (min)
- 24.545455
- Leg 2 time (min)
- 20
- Leg 3 time (min)
- 24.96
- Route average (kt)
- 109.631684
Using the last leg's 125 kt for all 127 NM would give 60.96 minutes and miss more than eight minutes.
For fuel, the route total feeds the fuel-burned mode once: at 9 units per hour, 69.505455 minutes uses about 10.425818 units. Running fuel per leg and adding the three results gives the same total.
Common mistakes
- Applying the most recent ground speed to the whole route.
- Averaging 110, 90 and 125 kt and dividing the route by that mean.
- Leaving climb and descent legs out of the list without saying so.
- Mixing nautical miles on one leg with statute miles on another in the same route list.
Scope
The total is only as good as the ground speeds entered for each leg. The FAA navigation chapter and the NIST unit table below cover the relations and units.
Educational calculation only. Check current aircraft limits, weather, performance and reserve requirements with approved information.