Crabwise · Library
Why can the same endurance produce different ranges?
Thirty units at 9 units per hour give 200 minutes of endurance, which becomes 316.67 NM at 95 kt or 433.33 NM at 130 kt over the ground.
Endurance comes from fuel, range from ground speed
Crabwise's endurance mode divides the fuel you enter by the hourly flow and returns minutes: t = F ÷ FF × 60. Fuel and flow can be in gallons, litres or pounds as long as both use the same unit. Range is a separate step: the time–speed–distance screen multiplies those minutes by a ground speed. Wind changes the ground speed and therefore the range while leaving the endurance unchanged.
The division uses exactly the quantity entered. It does not subtract a reserve or unusable fuel on its own.
Worked example
Twenty-four units at 8 units per hour gives 180 minutes. At constant ground speeds of 100 kt and 120 kt, that same three-hour period corresponds to 300 NM and 360 NM respectively; these are arithmetic cases, not approved range figures.
- Arithmetic endurance before separately specified deductions (min)
- 180
From fuel to two distances
- Enter the fuel quantity and the hourly flow in one unit and read the endurance in minutes.
- Decide how many minutes are held back as reserve and subtract that fuel first if needed.
- Run the distance mode once for each ground speed you want to compare.
- Label each distance with its ground-speed assumption.
One endurance, two ground speeds
With 30 units at 9 units per hour, endurance is 30 ÷ 9 × 60 = 200 minutes. At 95 kt that time covers 316.666667 NM; at 130 kt it covers 433.333333 NM. If 45 minutes are held as reserve, that reserve is 9 × 45 ÷ 60 = 6.75 units and the remaining 23.25 units last 155 minutes.
- Endurance from 30 units (min)
- 200
- Distance at 95 kt (NM)
- 316.666667
- Distance at 130 kt (NM)
- 433.333333
- Endurance after a 45 min reserve (min)
- 155
Common mistakes
- Multiplying endurance by true airspeed and calling the result range.
- Entering fuel in litres and flow in US gallons per hour.
- Calculating range from the full tank and then subtracting a reserve in minutes from the distance.
Scope
These are arithmetic cases with a constant flow and a constant ground speed. Climb, changing power and wind shifts are outside them, and the reserve is your entry, not a regulation. The FAA navigation chapter below explains the time and distance relations.
Educational calculation only. Check current aircraft limits, weather, performance and reserve requirements with approved information.