Radio Navigator

How to calculate groundspeed using the HSI or DI

By Rory Bennett (ATPL, FI: CPL, IR, ME, UPRT) · Published 25 May 2026

Direct answer

Once you have the wind-corrected heading, look at the angle from the wing tip (heading ±90°) to the wind on the HSI or DI. 60° or above: take the full wind speed as head or tailwind. 0° to 60°: that angle as a fraction of 60, times total wind speed. Add a tailwind, subtract a headwind, from TAS[1].

HSI face on the wind-corrected heading. The angle from the wing tip (heading ±90°) to the wind is measured off the rose. Above 60° gives the full wind speed as head or tailwind; 0–60° gives the angle as a fraction of 60, multiplied by the wind speed.
HSI face on the wind-corrected heading. The angle from the wing tip (heading ±90°) to the wind is measured off the rose. Above 60° gives the full wind speed as head or tailwind; 0–60° gives the angle as a fraction of 60, multiplied by the wind speed.

The full technique in under a minute. Transcript is below the video.

Video transcript
Wind corrections: estimating groundspeed. In our last video, you learned how to calculate drift using the HSI. In certain situations, such as a VFR diversion or holding, you will also need to calculate your groundspeed. When calculating drift, you use the angle from the track to the wind. To calculate groundspeed, you must work from the wind-corrected heading. However, because you are now calculating on a different axis, you need to look at the angle from the wing tip to the wind, not the heading or track. Take the angle from the wing tip to the wind. If 60° or above, take the full wind component as headwind or tailwind. 0 to 60°: the angle from wing tip to wind as a fraction of 60 is the fraction of total wind that applies as a headwind or tailwind. Add or subtract to or from true airspeed and you have your groundspeed.

Why the wing tip?

Drift and groundspeed are resolved on different axes. Drift is a cross-track component, measured from the track-to-wind angle. Groundspeed is a head/tail component on the aircraft itself, measured from the wing-tip-to-wind angle - this allows us to use the same clock code.

The wing tip is heading ±90°. Once you have the wind-corrected heading from the wind correction step, the wing tips are just 90° either side of it on the rose - many DI or HSIs have handy lubber lines here.

You will use this on diversions, VFR navigation, ETA updates en route, hold timing corrections, and any time you need an in-flight groundspeed without GPS.

How to read the wing-tip angle

Take the angle from the wing tip to the wind:

  • 60° or above: take the full wind speed as head or tailwind.
  • 0° to 60°: the angle as a fraction of 60 is the fraction of total wind that applies as head or tailwind.

Use whichever wing tip is on the same side as the wind: the right wing tip if the wind is from the right, the left wing tip if the wind is from the left.

Headwind sits forward of the wing-tip line and subtracts from TAS. Tailwind sits aft of the wing-tip line and adds to TAS.

Worked example

Scenario: TAS 120 kt, wind 30 kt from 060°, wind-corrected heading 360°.

Wing tip: right wing tip is at 090°. Angle from 090° to the wind at 060° is 30°.

Clock code: 30 ÷ 60 = 0.5. Component = 0.5 × 30 kt = 15 kt.

Head or tail: the wind sits forward of the right wing tip, so it is a headwind. Groundspeed = 120 − 15 = 105 kt.

Common mistakes

  • Measuring from track instead of wing tip. Drift uses the track-to-wind angle. Groundspeed uses the wing-tip-to-wind angle, on the wind-corrected heading axis. Switching reference axis when you switch from drift to groundspeed is the whole point of the technique.
  • Forgetting to use the wind-corrected heading. The wing tip is heading ±90°, where heading is the wind-corrected heading you just worked out - not the track.
  • Applying a fraction of max drift instead of a fraction of the wind speed. The clock-code fraction for groundspeed multiplies the total wind speed, not max drift. Max drift is for cross-track drift; the wing-tip rule resolves head/tail wind component.

Practise this in the simulator

Reading the theory only goes so far. The simulator lets you fly the scenario in your browser with realistic instruments and wind.

Try it in the free simulator

Check your understanding

Read each question, work out the answer in your head, then reveal to check. Retrieval beats re-reading.

TAS 120 kt, wind 30 kt, wind-corrected heading 360°, wind from 060°. What groundspeed?
Show answer
Right wing tip is at 090°. Angle from 090° to 060° is 30°. 30 ÷ 60 = 0.5. Headwind component = 0.5 × 30 = 15 kt. Groundspeed = 120 − 15 = 105 kt.
TAS 120 kt, wind 30 kt from 180°, wind-corrected heading 360°. What groundspeed?
Show answer
The wind is dead astern, so use the wing tip opposite the wind: the right wing tip at 090° to the wind at 180° is 90°. 90° is ≥ 60°, so take the full wind component. The wind is from behind, so it is a tailwind. Groundspeed = 120 + 30 = 150 kt.
TAS 120 kt, wind 30 kt from 240°, wind-corrected heading 360°. What groundspeed?
Show answer
The wind is behind the left wing tip at 270°. Angle from 270° to 240° is 30°. 30 ÷ 60 = 0.5. Component = 0.5 × 30 = 15 kt. Because the wind sits behind the wing-tip line (further aft than 270°), it is a tailwind. Groundspeed = 120 + 15 = 135 kt.

Frequently asked questions

Why the wing tip and not the heading?

Groundspeed is resolved on the heading axis. The component of wind along the heading axis is what adds to or subtracts from TAS. The wing tip is the perpendicular to the heading axis, so the angle from wing tip to wind tells you how much of the wind sits along the heading axis as head or tail.

Does the wing-tip rule work without an HSI?

Yes, on any DI. Picture the heading at 12 o’clock and the wing tips at 9 and 3 on the compass card. Measure the angle from the nearer wing tip to the wind direction and apply the clock code.

Why a fraction of total wind, not max drift?

You are resolving the wind into a head or tail component on the aircraft axis, not into a cross-track drift component. Different axis, same clock-code shortcut, but the thing the fraction multiplies is the wind speed itself, not max drift.

How do I tell head from tail?

If the wind comes from in front of the wing-tip line (forward of 9-to-3), it is a headwind: subtract. If it comes from behind the wing-tip line (aft of 9-to-3), it is a tailwind: add.

Sources

  1. Good Flying Made Easy, How to calculate groundspeed using the HSI or DI. youtube.com/watch?v=bpZgORz0T80
  2. Good Flying Made Easy, Wind Corrections: How to calculate a wind corrected heading and groundspeed (long-form companion). youtube.com/watch?v=32Ty5XRZbe4

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This article does not constitute flight instruction. Always defer to the guidance of your qualified flight instructor and to current charts and procedures or regulations published by your country's aviation authority.