Glide Ratio Explained: How Far Can You Really Go From That Altitude
"Can I make that ridge from here?" is one of the oldest questions in free flight, and the honest answer always starts with glide ratio — how far your wing travels forward for every unit of height it gives up. It's a simple idea with a few real subtleties that matter the moment wind gets involved.
The basic idea
A glide ratio of 9:1 means 9 metres of forward travel for every 1 metre of altitude lost, in still air. You can measure it two equivalent ways. If you already know how far you travelled and how much height that cost — from a flight log, a GPS track, or a simple before-and-after altimeter reading — the ratio is direct division: distance over height. If instead you know your glider's airspeed and its sink rate at that speed (from a polar chart, a manufacturer spec sheet, or a vario reading in steady flight), the same ratio falls out of dividing one by the other, after converting to matching units.
Every wing has a best glide speed — the airspeed at which its ratio peaks — and that speed is usually a little faster than the minimum-sink speed most pilots fly at when they just want to stay up. Best glide and minimum sink are different goals: one gets you the furthest, the other keeps you up the longest, and they rarely happen at exactly the same speed.
Where wind changes everything
Here's the subtlety that catches pilots out: wind does not touch your still-air glide ratio at all. Your wing doesn't know or care what the air mass it's flying in is doing relative to the ground. What wind changes is how far you get over the ground for that same height loss — because your ground speed is your airspeed plus (or minus) the wind component along your track.
A headwind eats directly into your ground distance. Flying into a 15 km/h headwind at a 38 km/h airspeed only gets you 23 km/h of ground speed — you're still sinking at the same rate through the air, so the same height loss now buys you noticeably less distance over the terrain below. A tailwind does the reverse: your ground speed climbs to 53 km/h at the same airspeed and sink rate, stretching the same height budget considerably further over the ground.
This is why "can I make that ridge" genuinely depends on which direction you're asking it in. A glide that comfortably reaches a landmark downwind might fall well short of the same landmark upwind, even though your wing's actual glide performance through the air hasn't changed by a single degree.
The trap of a strong headwind
Push the headwind example further and something important shows up: if the headwind component is stronger than your airspeed, your ground speed goes negative — you are moving backwards relative to the ground even though you are still flying forward relative to the air around you. This isn't a hypothetical edge case; it happens in real conditions when a pilot commits to penetrating into wind that's stronger than expected, or when wind increases faster than anticipated during a glide. Recognising the possibility before you commit to a glide over unlandable terrain against the wind is a genuinely important piece of judgment, not just an arithmetic curiosity.
Building the margin in
Working glides needs margin for reasons beyond wind alone: sink between you and the target, a headwind that builds partway through the glide, or simply misjudging the distance from altitude. Experienced pilots typically plan glides with a real safety buffer over the bare calculated number, and treat any calculated glide ratio — still-air or ground-adjusted — as a planning estimate to check against what the vario and the ground are actually telling you as the glide unfolds, not a number to fly right up to the edge of.
Final-glide planning in real conditions, including how to build in margin properly and what to do when a glide isn't working out as expected, is exactly the kind of judgment a certified instructor teaches through supervised practice — it's worth treating any calculator or rule of thumb as a starting point for that learning, not a replacement for it.