Crabwise · Library
How can a timed distance check be used to calculate observed ground speed?
Timing 18 NM in 8.5 minutes gives 127.06 kt, but a quarter-minute timing error moves it between 123.43 and 130.91 kt on such a short interval.
Ground speed from a timed check
The speed mode computes GS = D ÷ (t ÷ 60) from a distance in nautical miles and elapsed minutes. Between two identifiable points, that is the average ground speed over the interval. It is a different quantity from true airspeed; the difference between the two is what the find-wind mode turns into a wind estimate.
The shorter the interval, the more a small timing error changes the result, because the same error is a larger share of the minutes.
The result is in knots only when the distance is in nautical miles; the app does not convert units inside the speed mode.
Worked example
Covering a stated 24 NM interval in 12 minutes gives 120 kt. It does not establish that the aircraft travelled at exactly 120 kt at every moment.
- Interval-average ground speed (kt)
- 120
Time a distance check
- Pick two points with a known distance between them.
- Start and stop timing at the points, and note the uncertainty of your timing.
- Enter distance and minutes in the speed mode.
- Repeat with the timing shifted by its uncertainty to see the range.
Short and long intervals
18 NM in 8.5 minutes gives 18 ÷ (8.5 ÷ 60) ≈ 127.058824 kt. If the true time was 8.75 minutes the speed is about 123.428571 kt; at 8.25 minutes it is about 130.909091 kt. Over a 60 NM interval timed at 30 minutes, the same quarter-minute error only spans 119.008264 to 121.008403 kt.
- 18 NM in 8.5 min (kt)
- 127.058824
- Range with ±0.25 min (kt)
- 123.428571 – 130.909091
- 60 NM in 30 ± 0.25 min (kt)
- 119.008264 – 121.008403
The reason is proportion: a quarter minute is about 2.941176 percent of 8.5 minutes but only about 0.833333 percent of 30 minutes. The relative error in the speed follows the relative error in the time.
Common mistakes
- Treating a ground speed from a two-minute check as precise to the knot.
- Using statute miles from a road map in a knots calculation.
- Comparing an observed ground speed directly with TAS and calling the difference an error.
- Starting the clock late at the first point and on time at the second, which biases the speed upward.
Scope
The result is an interval average; it does not validate the distance source. The FAA navigation chapter and NIST table below cover the relations and units.
Educational calculation only. Check current aircraft limits, weather, performance and reserve requirements with approved information.