Crabwise · Library
How much time can be hidden by rounding every route leg?
Four legs of 17, 22, 13 and 9 NM take 34.36 minutes in total, yet adding their rounded times gives 33, hiding more than a minute of fuel.
Unrounded minutes in every leg
The time mode returns t = D ÷ GS × 60 with its full decimals, and the step list shows the exact value. A flight log usually shows whole minutes. Rounding each leg first and adding the rounded values afterwards lets small differences pile up, especially when several legs round in the same direction.
The safer order is to add the unrounded leg times, then round the total for display, and keep any deliberate planning margin as its own line.
Rounding to the nearest whole minute can shift a single leg by up to half a minute. Across four legs the rounded total can be off by up to two minutes, and the error only cancels when legs happen to round in opposite directions.
Worked example
The calculation retains approximately 5.454545 minutes. A displayed whole-minute value should not silently replace it in every later sum.
- Unrounded time for one example leg (min)
- 5.454545
Total a route correctly
- Run the time mode for each leg with its own distance and ground speed.
- Copy each result with its decimals into the log's working column.
- Add the unrounded times for the route total.
- Round only the displayed figures, and use the unrounded total for fuel.
Four short legs
17 NM at 110 kt takes about 9.272727 minutes, 22 NM at 115 kt about 11.478261, 13 NM at 95 kt about 8.210526, and 9 NM at 100 kt exactly 5.4. The unrounded total is about 34.361514 minutes. Rounded legs of 9, 11, 8 and 5 add up to 33.
- Unrounded route time (min)
- 34.361514
- Sum of rounded legs (min)
- 33
- Fuel at 9 per hour, unrounded
- 5.154227
- Fuel at 9 per hour, rounded legs
- 4.95
Every leg here rounds down, so the rounded log is short by more than a minute and about 0.2 units of fuel.
Common mistakes
- Rounding 5.4 minutes to 5 on every one of many identical legs.
- Using the rounded total as the input to the fuel screen.
- Hiding a planning allowance inside rounded leg times instead of listing it.
- Rounding the ground speed before the time calculation and then rounding the minutes again.
Scope
This is arithmetic precision, not an operational tolerance. The FAA navigation chapter below shows the same time computation per leg.
Educational calculation only. Check current aircraft limits, weather, performance and reserve requirements with approved information.