Crabwise · Library
Why is the direction uninformative when the calculated wind is zero?
When air and ground vectors match, Crabwise reports 0 kt and a direction of 0 as a placeholder; at 1.45 kt a 1° track change swings it by 93°.
A placeholder, not a direction
The find-wind mode subtracts the air vector from the ground vector. If the two are identical, the difference has length zero and no direction. In that case Crabwise sets the FROM direction to 0 rather than computing an angle from a zero vector. That 0 is a display value, not a north wind.
Near zero the direction is still computed, but it becomes very sensitive: a fraction of a degree or a knot in the observations can move it a long way.
Worked example
Identical 120 kt eastbound air and ground vectors produce zero wind. The direction field’s zero placeholder does not establish wind from 000°.
- Reconstructed wind magnitude (kt)
- 0
Read magnitude first
- Read the wind speed before the direction.
- If the speed is 0, ignore the direction field.
- If the speed is only a knot or two, repeat the calculation with each observation shifted by its likely error.
- Report a small, uncertain wind as calm or light rather than giving it a precise bearing.
How unstable a light wind is
TAS 120 kt on heading 090° and ground speed 120 kt on track 090° give a wind of 0 kt, shown with direction 0. Change the observations to 121 kt on track 090.5° and the wind is about 1.451122 kt from about 316.689865°. With the track at 089.5° instead, the speed is the same 1.451122 kt but the direction is about 223.310135°. A ground speed of 119 kt on track 090° gives 1 kt from 090°.
- Wind with identical vectors (kt)
- 0
- Track 090.5°, GS 121 kt: wind from (°)
- 316.689865
- Track 089.5°, GS 121 kt: wind from (°)
- 223.310135
- Speed in both shifted cases (kt)
- 1.451122
Common mistakes
- Logging the placeholder 0 as wind from north.
- Quoting a direction to a tenth of a degree for a wind of one knot.
- Mixing a magnetic heading with a true track, which alone can create a false small wind.
- Treating a direction that flips between two runs as a wind shift instead of observation noise.
Scope
The mode cannot be more precise than its observations. The FAA navigation chapter below explains the vectors involved.
Educational calculation only. Check current aircraft limits, weather, performance and reserve requirements with approved information.