The computer promises arrival, not a runway.
Your flight computer's arrival altitude is geometry: distance, polar, wind, MacCready. It is an excellent number and you should trust it for what it is — a statement about height over a point, computed for the air the forecast promised. It says nothing about whether that point has a surface you can land on, and it cannot know about the sink street between here and there.
So the working rule of cross-country soaring is double bookkeeping: the computer carries the arithmetic of the glide, and you carry a continuously refreshed answer to one question — if this stops working in the next five minutes, where exactly do I land? The rest of this article is the discipline that keeps that answer honest.
Heights are decisions.
The classical ladder gives every altitude band one job. Around 600 m above terrain, while still soaring normally, you start shortlisting: which two or three fields ahead look plausible. By 450 m you choose one and position yourself to keep it in reach. At 300 m you are committed — any further climbing attempts happen over the field, in position, never by leaving it. From 250 m you fly a circuit.
Different schools teach the metres differently, and mountain terrain moves every number up. What no school varies is the sequence, because each step needs the time the previous one protected. The ladder is not about altitude — it is about refusing to do two jobs at once.
The minutes between soaring and committed.
The red line is your altitude if nothing works. At 1.5 m/s of ordinary between-thermal sink, the whole journey from starting the shortlist at 600 m to committed at 300 lasts about three minutes — which is why the shortlist has to exist before the sink does. The reach numbers say which fields are even candidates: into a 20 km/h wind the circle is noticeably lopsided — and they assume neutral air on the glide, so sink en route shrinks them further.
Reading a field from 500 metres.
Wind first — everything else is negotiable, landing direction is not. Then length: 200 m of usable surface is a comfortable minimum for most gliders, more with obstacles on the approach. Slope next, because it is nearly invisible from above: if you can see it at all, it is significant, and you land uphill even with some tailwind.
Surface is read by colour and season. Harvested stubble and mown grass are gifts; dark, tall, uniform green means crops that swallow a wingtip; freshly ploughed earth is soft enough to stop you in metres. Livestock disqualifies a field outright. And wires: you will not see the wire, you will see the poles — assume a wire runs between any pole and anything else, and plan the approach over the middle of the field's open side.
Fly the circuit you know.
The last 250 metres are deliberately boring: the same circuit you fly at home, compressed but complete — downwind with the field inspection done, a proper base, a final at normal approach speed plus half the gust factor. Gear down early, checks done high, and no experiments. The statistics of outlanding accidents are blunt about where it goes wrong: low saves attempted below the commit gate, and stalls out of skidding turns onto short finals. Both are cured by the ladder, not by skill on the day.
In your cockpit.
Use the instrument for what it is superb at: the nearest-landable page, wind-corrected arrival altitudes to every field in the database, and the live wind that decides your landing direction. Then hold the one thing it cannot compute — that the number on the screen refers to a real rectangle of ground you have personally inspected from the air and would bet your fuselage on.
The arithmetic this article deliberately refuses to fully trust — distance, wind, margin, arrival — has its own article: the final glide.