When the engine quits, the first two questions are how far you can go and how long you have. Both have simple answers that every pilot should be able to work out — and both are more optimistic in the POH than they are in real life. This guide covers the math behind glide distance, how wind and weight change it, and how to turn the numbers into a margin you can actually use.
Glide Ratio
An airplane's best glide ratio is the horizontal distance it travels for each unit of altitude it loses, flown at the best-glide speed (VG or VBG) in the configuration the POH specifies. It equals the airplane's maximum lift-to-drag ratio. Typical piston singles fall somewhere around 8:1 to 10:1. A 9:1 ratio means 9,000 ft of ground covered for every 1,000 ft of altitude lost — about 1.5 nautical miles per 1,000 ft.
6,076 ft = 1 nautical mile. Use height above the terrain you will land on, not altitude MSL.
Worked Example
An airplane with a 9:1 glide ratio and a best-glide speed of 68 knots loses its engine at 6,000 ft AGL.
- Still-air distance: 6,000 × 9 = 54,000 ft = 8.9 NM
- Descent rate: 68 kt ≈ 114.8 ft/s; ÷ 9 = 12.8 ft/s ≈ 765 fpm
- Time to the ground: 6,000 ÷ 765 ≈ 7.8 minutes
Wind Changes Distance, Not Time
At best-glide speed your descent rate through the air mass doesn't depend on the wind, so the time available is the same in any wind. What changes is how much ground you cover in that time. Scale the still-air distance by the ratio of your groundspeed to your airspeed:
Wind is positive for a tailwind and negative for a headwind.
In the example above:
- 20-knot headwind: 8.9 × (68 − 20) ÷ 68 ≈ 6.3 NM
- 20-knot tailwind: 8.9 × (68 + 20) ÷ 68 ≈ 11.5 NM
The circle of reachable fields is really an oval stretched downwind. When you choose a landing site, a field 7 miles upwind may be out of reach while one 10 miles downwind is not.
Speed to fly. The POH best-glide speed maximizes distance in still air. Into a strong headwind you'll get slightly farther by flying a little faster than VBG; with a strong tailwind, a little slower. The adjustment is small for typical light-aircraft winds, and holding a steady speed matters more than chasing the exact optimum.
What About Weight?
Maximum lift-to-drag ratio depends on the airplane's shape, not its weight, so the best glide ratio is essentially the same light or heavy. The best glide speed, however, decreases as weight decreases. Many POHs publish only one best-glide speed, for maximum gross weight; when you're well under gross, a few knots slower is closer to optimum. A heavier airplane flown at its correct speed glides just as far — it just gets there sooner.
Why Real Glides Come Up Short
The book glide assumes you are already stabilized at best-glide speed, in the right configuration, flying straight. In practice you lose distance to:
- The transition. Slowing from cruise to best-glide speed trades speed for some altitude gain at first, but hesitating at cruise speed or overshooting below VBG costs distance.
- Turns. Every turn increases the sink rate. Turning back toward a field you've passed, or maneuvering for a pattern, can cost several hundred feet.
- Propeller and configuration. A windmilling propeller usually creates more drag than a stopped one, and many POH figures assume a specific propeller condition and flaps up. Extended flaps or gear steepen the glide.
- Sink and turbulence. Downdrafts near ridges and on the lee side of terrain can overwhelm a light airplane's entire glide margin.
Treat the calculated distance as an outer limit and pick landing sites well inside it — many instructors suggest planning on roughly two-thirds of the book glide. Arrive over the chosen field high enough to fly a recognizable pattern; it is easy to lose altitude with a slip or flaps and impossible to get it back.
Using Glide Numbers in Flight Planning
Glide distance is a planning tool, not just an emergency calculation. Higher cruising altitudes widen the circle of reachable landing sites over hostile terrain or water, and a quick glide check can tell you whether a direct route across a mountain range or a lake keeps a field within reach. Pair it with density altitude planning on hot days, when climbing to that safer altitude takes longer.
Using the CockpitCalc Glide Calculator
- Enter height above ground in feet.
- Enter your best glide ratio from the POH (or convert a "NM per 1,000 ft" figure: ratio ≈ NM per 1,000 ft × 6.08).
- Enter best-glide speed and the wind component along your intended glide path (tailwind +, headwind −).
- Calculate to see still-air and wind-adjusted distance in NM, statute miles, and time to the ground.
Calculate Your Glide Range
Enter height above ground, glide ratio, best-glide speed, and wind to get still-air and wind-adjusted glide distance.
Open the Calculator →