A 20 km/h Headwind Halves Your Glide
A glider's glide ratio is a property of the wing, not of the day. A 9.6:1 wing glides 9.6 metres forward for every metre it descends through the air, and that number does not change when the wind gets up.
What changes is how far that gets you over the ground — and the difference is much larger than most pilots' intuition allows for.
The same wing, five different days
Take a glider flying 38 km/h with a 1.1 m/s sink rate. The glide ratio calculator gives 9.6:1 in still air, and then works out what the ground actually sees:
| Wind | Groundspeed | Air glide | Ground glide |
|---|---|---|---|
| 20 km/h headwind | 18 km/h | 9.6:1 | 4.55:1 |
| 10 km/h headwind | 28 km/h | 9.6:1 | 7.07:1 |
| Still air | 38 km/h | 9.6:1 | 9.60:1 |
| 10 km/h tailwind | 48 km/h | 9.6:1 | 12.12:1 |
| 20 km/h tailwind | 58 km/h | 9.6:1 | 14.65:1 |
A 20 km/h headwind — not a dramatic wind — cuts the effective glide to less than half. The wing is performing exactly as it always does. The ground is simply not cooperating.
What that means in metres
Put it in terms of a real decision. From 500 metres above your landing field:
- Still air at 9.6:1 — you can cover about 4,800 m.
- 10 km/h headwind at 7.07:1 — about 3,535 m.
- 20 km/h headwind at 4.55:1 — about 2,275 m.
A pilot who has learned their glide in still air and applies the same figure into wind will plan to reach something more than twice as far away as they can actually get to. That error does not announce itself gradually; it arrives at the point where the options have run out.
The asymmetry that catches people out
Notice that the headwind penalty is larger than the tailwind bonus. Going into 20 km/h costs 5.05 points of glide ratio; running with 20 km/h gains 5.05 — but as a proportion, the loss is 53% and the gain is 53% of a smaller base. The downwind leg feels wonderful and the upwind leg is where the trouble is.
Which is why a flight plan that works beautifully on the way out can fail on the way back, and why "I got there easily" is not evidence that the return is available.
Where the numbers come from
You can get a glide ratio two ways. Directly from a flight — horizontal distance divided by height lost, so 8,000 m over 1,000 m of descent is 8:1. Or from the glider's polar — airspeed divided by sink rate, converted to consistent units, which is how 38 km/h at 1.1 m/s becomes 9.6:1.
The polar figure is the manufacturer's best case: a clean wing, smooth air, the right speed, an undisturbed pilot. Real flights rarely reproduce it. Measuring your own from logged flights gives you a number that includes your actual flying, and it is usually the more useful one for planning.
What the calculation deliberately does not do
This is arithmetic, and arithmetic does not fly the wing. A few things it cannot see:
- Sinking air. A glide through sink loses height faster than the polar says, and the ground glide degrades further. Air is not still just because the wind is steady.
- Wind gradient. Wind generally decreases near the ground, so the headwind you are fighting at height is not the one you will have on final.
- Gusts, rotor and terrain. None of which appear in a steady wind figure, and all of which matter more than the average.
- Speed to fly. Flying faster into a headwind can improve ground glide even though it worsens air glide. Getting that trade right is a real skill and it is not something a ratio tells you.
Most importantly: a number that says you can just reach a field is not a plan. Final-glide decisions are taught, practised and supervised for good reason, and this calculator is a way of understanding the shape of the problem on the ground, not a way of deciding in the air. If you take one thing from it, take the habit of assuming the headwind case rather than the still-air one — and keeping alternatives that do not depend on the arithmetic being right.
Flying faster into wind
There is a partial remedy, and it is worth understanding rather than attempting from a description. Because a headwind steals ground distance for every second you are airborne, spending less time airborne can improve the ground result even though it worsens the air glide.
Pushing speed bar increases airspeed and increases sink, so the air glide ratio drops — but groundspeed rises by more than the sink penalty costs, up to a point. That point depends on the wing's polar and on the wind, and finding it is the substance of speed-to-fly theory.
What the arithmetic here can show is why the idea works at all: at 38 km/h into 20 km/h of wind your groundspeed is 18 km/h, so more than half your airspeed is being cancelled. Recovering even a few km/h of groundspeed changes the ratio substantially. What it cannot show is how much bar is appropriate on your wing, in that air, on that day — which is a trained skill and belongs with an instructor.
Building the habit
Two practical numbers to carry. Your measured glide, taken conservatively from your own logs rather than from the polar. And the headwind case for the wind you actually fly in — for many sites, that means knowing what your glide looks like at 10 and 20 km/h without having to compute it.
Then plan to the worse figure and keep alternatives that do not depend on it. The purpose of understanding this arithmetic is not to fly closer to the edge with more confidence; it is to notice, earlier, when a plan has quietly stopped being available.