GPS True Airspeed Measurement (4-Leg Box Method)
Measure your aircraft's actual TAS with nothing but GPS ground speeds on four cardinal headings — and get the wind vector as a bonus.
Fly four steady legs by heading (not track) at constant altitude and power, noting stabilized GPS ground speed on each. Hold headings within ~5° and the result lands within a knot or two of truth — better than most ASIs are calibrated.
Formula
⚠️ For flight planning and education only — verify with your POH/AFM, certified instruments and official sources. Not for primary navigation or airworthiness decisions.
Measure your aircraft's actual TAS with nothing but GPS ground speeds on four cardinal headings — and get the wind vector as a bonus.
About GPS True Airspeed Measurement (4-Leg Box Method)
Is your airspeed indicator honest? Most pilots never find out, yet the test requires nothing but four steady GPS readings. Fly cardinal headings — north, east, south, west — at constant power and altitude, enter the stabilized ground speeds, and this calculator separates the wind vector from your true airspeed: the reciprocal pairs cancel the wind, the cross-pairs reveal it. It's a flight-test technique sized for a Saturday morning.
How to use GPS True Airspeed Measurement (4-Leg Box Method)
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula wind = half the reciprocal-pair GS differences; TAS = √(pair-average² − crosswind²), averaged substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use GPS True Airspeed Measurement (4-Leg Box Method)?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula wind = half the reciprocal-pair GS differences; TAS = √(pair-average² − crosswind²), averaged with sources cited on the page
- ✓Fly four steady legs by heading (not track) at constant altitude and power, noting stabilized GPS ground speed on each. Hold headings within ~5° and the result lands within a knot or two of truth — better than most ASIs are calibrated.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
Why fly headings rather than GPS tracks?+
The method's algebra assumes the air vector points where the nose points, with the wind added on top. Flying by track would let the wind contaminate the very vectors you're using to solve for it. Hold compass/HSI headings, let the airplane drift, and the mathematics cleanly separates the two unknowns.
How accurate is this compared with the POH TAS?+
With steady air, ±100 ft of altitude discipline, and headings held within 5°, scatter is typically under 2 knots — tighter than many factory ASI calibrations. Run it at two or three power settings and you've effectively re-derived your aircraft's cruise performance table, including the position-error correction the POH only states for one weight.
What conditions ruin the measurement?+
Changing wind: thermals, shear layers, or legs flown minutes apart in evolving weather violate the uniform-wind assumption that lets reciprocal pairs cancel. Fly the box compactly (one to two minutes per leg after stabilizing), in smooth air, away from terrain-driven gradients, and at the same indicated altitude throughout.
Can I do it with three legs instead of four?+
Yes — Doug Gray's three-leg solution solves the same unknowns with any three tracks via simultaneous equations, and several E6B apps implement it. The four-leg box trades one extra leg for arithmetic you can do on a kneeboard and an internal consistency check: if the two TAS estimates (N-S and E-W pairs) disagree by more than a couple of knots, the wind changed and the data is suspect.
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