When the engine quits, the single most useful number is how far you can glide. This calculator turns your height above a landing site and the aircraft best glide ratio into a still-air range, a wind-corrected ground range, and the minutes you have before touchdown — the numbers behind a calm engine-failure decision.
What the calculator computes
The tool takes four inputs and returns three key figures:
| Input | What to enter |
|---|---|
| Height AGL | Your altitude above the intended landing site — not MSL altitude |
| Glide ratio | The aircraft’s best glide ratio from the POH (e.g. 9 means 9 ft forward per 1 ft down) |
| Sink rate at best glide | Feet per minute descent at best-glide speed — used to calculate time aloft |
| Wind component | Head or tail wind component along your glide path in knots |
| Output | What it means |
|---|---|
| Still-air range | Maximum horizontal distance in no-wind conditions |
| Wind-corrected ground range | Actual ground distance accounting for head or tailwind |
| Time aloft | Minutes available before touchdown at best-glide sink rate |
The maths
In still air, glide range is straightforward geometry:
still-air range (ft) = height AGL (ft) × glide ratio
still-air range (nm) = range (ft) ÷ 6,076.12Time aloft is the height divided by the sink rate:
time (minutes) = height AGL (ft) ÷ sink rate (fpm)The wind correction uses the fact that the whole air mass moves with the wind throughout the glide:
ground range = still-air range − (headwind kt × time minutes ÷ 60 × 6,076.12 ft/nm)A tailwind adds rather than subtracts. The key insight is that wind affects ground range even though it does not affect the aircraft’s performance through the air — which is why a strong headwind can make an apparently reachable field unreachable.
Worked example
For illustration: 5,000 ft AGL, 9:1 glide ratio, 700 fpm sink rate at best glide, 15 kt headwind.
- Still-air range: 5,000 × 9 = 45,000 ft ÷ 6,076.12 = 7.4 nm
- Time aloft: 5,000 ÷ 700 = 7.1 minutes
- Wind correction: 15 kt × 7.1 min ÷ 60 = 1.78 nm lost
- Wind-corrected ground range: approximately 5.6 nm
The difference of 1.8 nm between still-air and wind-corrected range can decide which field is reachable. Always run the wind-corrected figure, especially into a stiff headwind.
What the real glide falls short of
The calculation assumes a constant best-glide speed flown perfectly, in a straight line, with no configuration changes. Every deviation reduces the actual range:
- Speed too fast or slow — the glide ratio degrades on either side of best-glide speed. Flying 10 kt fast can cost a quarter or more of available range.
- Turns — each turn costs height without forward progress. The steeper the bank and the longer the turn, the more range you lose. Limit turns to the minimum necessary to line up with the field.
- Flaps — extending flaps early dramatically increases drag and sink rate. Keep the aircraft clean until the field is assured.
- Engine windmilling vs stopped — a windmilling propeller creates more drag than a stopped one. If the POH lists a procedure to stop or feather the propeller after engine failure, follow it.
Practical training use
This tool is useful for:
- Pre-flight planning — identifying reachable fields from cruise altitude along your route
- Training preparation — calculating the range you should achieve from practice forced-landing entry altitude before the lesson
- Cross-checking the POH — verifying that the glide range figure in your POH matches the formula at the stated glide ratio
Always plan to arrive over your chosen field with height in reserve — high enough to conduct a proper circuit and approach, not just to clear the fence.