Crabwise · Library
How can a completed leg be compared with its original time and fuel assumptions?
A 42 NM leg planned at 115 kt takes 21.91 minutes; flown in 25 minutes on 4.1 units it averaged 100.8 kt and 9.84 units per hour.
Two records side by side
A comparison uses the same Crabwise formulas twice. The plan comes from the time mode, t = D ÷ GS × 60, and fuel burned, FF × t ÷ 60. The observation comes from the speed mode, GS = D ÷ (t ÷ 60), and the flow mode, FF = F ÷ (t ÷ 60). Keeping both sets of inputs lets you see whether the difference came from ground speed, fuel flow or both.
Overwriting the plan with the observed numbers erases the evidence of what changed.
Worked example
The observed 24 NM in 15 minutes gives 96 kt. This average is not a replacement for TAS and does not reconstruct changing conditions within the interval.
- Average observed ground speed for the stated interval (kt)
- 96
Compare a flown leg
- Keep the planned distance, ground speed and flow with their results.
- Record the actual start and end points, minutes and fuel used.
- Run the speed and flow modes on the observed values.
- Write the differences next to each other without editing the plan.
A slower and thirstier leg
Planned: 42 NM at 115 kt takes 42 ÷ 115 × 60 ≈ 21.913043 minutes, and at 9 units per hour uses about 3.286957 units. Observed: the leg took 25 minutes and 4.1 units. Ground speed was 42 ÷ (25 ÷ 60) = 100.8 kt and flow 4.1 ÷ (25 ÷ 60) = 9.84 units per hour.
- Planned time (min)
- 21.913043
- Planned fuel
- 3.286957
- Observed ground speed (kt)
- 100.8
- Observed flow (per hour)
- 9.84
Of the 0.813043 extra units, 0.463043 come from flying 25 minutes at the planned 9 per hour and 0.35 from the higher flow.
The observed leg took about 3.086957 minutes longer than planned, which matches the drop in ground speed from 115 to 100.8 kt.
Common mistakes
- Comparing a planned leg with an observed interval that starts at a different point.
- Attributing the whole extra fuel to wind when the flow also rose.
- Replacing the plan's ground speed with the observed one and losing the original.
- Rounding the observed minutes before computing the ground speed and flow.
Scope
Averages over 25 minutes do not show how speed and flow varied inside the leg. The FAA navigation chapter below describes the planning relations used.
Educational calculation only. Check current aircraft limits, weather, performance and reserve requirements with approved information.