Glide Distance from Altitude

A glide is a triangle. Height on one leg, ground distance on the other, and the ratio between them is the lift-to-drag ratio — so distance is simply height multiplied by $L/D$. From 3,000 m at a glide ratio of 15, that is 45 km. It is the arithmetic every pilot does silently the moment the engine stops, and it is worth having ready rather than deriving under pressure.

The reason $L/D$ doubles as a glide ratio is a small piece of statics. In a steady glide the aircraft is unpowered, so the weight is balanced by the resultant of lift and drag, and the flight path angle satisfies $\tan\gamma = D/L$. Shallow angles make the tangent almost the angle itself, and the ratio of the two legs of the triangle is simply $L/D$. Nothing about weight, size or altitude enters — which is why a heavy aircraft glides exactly as far as a light one, just faster and for less time.

Now the corrections, all of which take distance away. Wind is the biggest: a 20-knot headwind against a 65-knot glide removes roughly a third of the range, and the correct response is to fly FASTER than best-glide into a headwind and slower downwind, which trades a little efficiency for a better ratio over the ground. Height must be measured above the intended landing point rather than above sea level, and over rising terrain that difference is decisive. A windmilling propeller adds significant drag that a stopped or feathered one does not. And the turn toward the field costs both height and distance, especially if it starts as a turn away.

The one that surprises people is the impossible turn — the attempted return to the runway after an engine failure on climb-out. The arithmetic looks fine on paper because the glide ratio is generous, but the manoeuvre requires a turn of well over 180° to line up with a runway that is now offset, flown at low altitude in a steep bank where the stall speed has risen by 30% or more, from a nose-high climb attitude that must first be traded for glide speed. The height lost is far greater than the glide ratio alone suggests, and the published minimum altitudes for it are typically several hundred feet.

Treat the number this page returns as a theoretical maximum in still air, and plan to arrive high. Height above the field can always be given away with a sideslip or a circuit; height that is not there cannot be recovered.