Still-air glide range calculator
With the engine stopped, how far can you go? The arithmetic is simple, but the answer is a theoretical best case, and the gap between it and reality is the important part of this page.
Calculator
This is an educational and planning aid. Use current official weather, your aircraft flight manual, and the rules that apply to your flight before you act on any figure here. See the aviation disclaimer.
What the result means
The distance is how far the aircraft would travel in perfectly still air, from the height you entered, at the glide ratio you entered, flown precisely at best glide speed from the moment the engine stopped.
None of those conditions hold in a real engine failure. Treat the figure as an upper bound that you will never quite reach, not as a distance you can count on.
When this is useful
- Pre-flight planning over inhospitable terrain, to work out what height keeps a landable field in reach.
- Understanding the glide performance figures in your aircraft flight manual.
- Training and study, where the relationship between height, glide ratio and distance needs to be second nature.
How it is calculated
Glide ratio is horizontal distance divided by height lost. Multiplying the height by the ratio gives the horizontal distance in feet, which is converted to nautical miles.
No glide ratio is filled in for you. It differs between aircraft types, and within one type it changes with weight, configuration and whether the propeller is windmilling or stopped.
glide distance = height above target x glide ratio
Worked example
- Height above the target
- 5,000 ft
- Glide ratio
- 9 to 1
The theoretical still-air glide is 7.4 NM, at a glide angle of 6.3 degrees below the horizon.
Check: 5,000 ft at 9 to 1 gives 45,000 ft of horizontal travel. Divided by 6,076 ft in a nautical mile that is 7.41 NM. The height lost per nautical mile works out at 675 ft, and 5,000 divided by 675 gives 7.4 NM, so the two forms agree.
Limitations
- Still air is the headline assumption and it is almost never true. A 20 kt headwind on a 60 kt glide costs a third of the distance. A tailwind extends it.
- It assumes you are already at best glide speed, wings level and trimmed. The seconds spent recognising the failure, lowering the nose and settling on speed all come out of this distance.
- It makes no allowance for manoeuvring at the far end. Arriving over a field with no height left is not a landing. Always plan to arrive with height in hand for a circuit.
- Glide ratio from the manual assumes a specific weight and a specific propeller condition. A windmilling propeller produces considerably more drag than a stopped one.
Common mistakes
- Using altitude above sea level rather than height above the landing site. Subtract the site elevation first.
- Treating the answer as achievable. It is a theoretical maximum in ideal conditions.
- Guessing a glide ratio. Use the figure from your own aircraft flight manual.
Sources
- Airplane Flying Handbook (FAA-H-8083-3), emergency approach and landing , US Federal Aviation Administration