What this calculator does
A glide ratio of nine to one means nine units forward for every one down, which from 5,000 feet above the ground gives 7.4 nautical miles in still air. That is the headline number, and the useful one is what the wind does to it.
Wind changes the distance without changing the descent rate at all. The aeroplane still takes six minutes and twenty-one seconds to reach the ground from 5,000 feet; it simply covers less ground doing it. A 20 knot headwind on a 70 knot glide cuts the range from 7.4 miles to 5.3 and the effective ratio from nine to 6.4.
The formula
Still-air distance is height × glide ratio ÷ 6076.12. The descent rate follows from the airspeed and the ratio, and the time to the ground follows from the height and that rate. Distance over the ground is then groundspeed times that time, where groundspeed is airspeed less the headwind component. The effective glide ratio is the book ratio scaled by groundspeed over airspeed.
| Term | Meaning |
|---|---|
| Glide ratio | Forward distance per unit of height lost, at the best glide speed in the configuration the manual specifies. |
| Best glide speed | The airspeed that achieves the book ratio. Flying faster or slower than it costs distance. |
| Effective ratio | The book ratio adjusted for wind. What you actually get on the day. |
| Height above ground | Not altitude above sea level. The difference matters over terrain. |
The inputs explained
| Field | What to enter |
|---|---|
| Height above the ground (ft) | Height above the ground you are gliding over, not altitude above sea level. |
| Best glide ratio (:1) | Best glide ratio from the flight manual. Typical light singles sit between 7 and 10 to 1. |
| Best glide airspeed (kt) | Best glide airspeed from the manual. The ratio only applies at this speed. |
| Headwind component (kt) | Headwind component along the glide. Enter a negative figure for a tailwind. |
| Distance to the field (nm) | Distance to the field you are considering, used for the height needed and the margin. |
When to use it
Working out what is within reach
Enter your height and the distance to a candidate field. The height needed and the spare height are the two numbers that answer whether it is reachable, and the margin should be generous rather than exact.
Understanding why into-wind is expensive
A 20 knot headwind on a 70 knot glide removes nearly 30 per cent of the range, because it removes 30 per cent of the groundspeed while the aircraft sinks at the same rate. Downwind options are worth considerably more than they look.
Checking a published glide ratio
Set the wind to zero and compare the still-air distance against the rule of thumb you were taught. Many are conservative, which is the right direction, but knowing by how much is useful.
Worked examples
Every figure in the tables below is produced by this page’s own calculator at build time, so the numbers and the tool always agree. Select any row to load that scenario.
How far can you glide from each height?
Only the height above the ground changes.
| Height above ground | Glide distance | Time to the ground | Height you have spare |
|---|---|---|---|
| 1,000 ft | 1.48 nm | 1 min 16 s | -2,376 ft |
| 2,000 ft | 2.96 nm | 2 min 32 s | -1,376 ft |
| 3,000 ft | 4.44 nm | 3 min 49 s | -376 ft |
| 5,000 ft | 7.41 nm | 6 min 21 s | 1,624 ft |
| 8,000 ft | 11.85 nm | 10 min 9 s | 4,624 ft |
| 10,000 ft | 14.81 nm | 12 min 42 s | 6,624 ft |
What does wind do to the glide?
The height and the aircraft are identical. Only the wind along the glide changes.
| Headwind | Glide distance | Effective glide ratio | Height needed to reach the field |
|---|---|---|---|
| 20 kt tail | 9.52 nm | 11.57:1 | 2,625 ft |
| 10 kt tail | 8.46 nm | 10.29:1 | 2,954 ft |
| 0 kt head | 7.41 nm | 9.00:1 | 3,376 ft |
| 10 kt head | 6.35 nm | 7.71:1 | 3,938 ft |
| 20 kt head | 5.29 nm | 6.43:1 | 4,726 ft |
| 30 kt head | 4.23 nm | 5.14:1 | 5,907 ft |
Questions
Why does wind change the distance but not the descent rate?
Because the aircraft descends through the air at a rate set by its airspeed and configuration, and the air is moving over the ground independently. The time to the ground is unchanged; only the ground covered in that time moves.
Does a windmilling propeller matter?
A great deal. Book glide ratios usually assume the propeller in a specific condition, and a windmilling propeller on a failed engine creates substantial drag that can cut the ratio noticeably. This calculation uses whatever ratio you enter.
Should I use height above ground or altitude?
Height above the ground you intend to glide over. Using altitude above sea level over high terrain will overstate the distance by exactly the terrain elevation times the glide ratio.
Is flying faster than best glide ever better?
Into a strong headwind, slightly, because spending less time in the wind can beat the lost ratio. The effect is small and the speed increase modest, and this page does not model it, so treat the book speed as the default.
How much margin should I leave?
More than the arithmetic suggests. The figures here assume best glide speed held precisely from the moment of failure, a clean aircraft and an accurate wind, and none of those is reliable in the first thirty seconds of a real engine failure.
For the descent planning version of the same geometry, see top of descent. Wind along the glide can be resolved with wind correction angle, and fuel burn endurance and range covers the case where the engine is still running.