Planning a Flight From Numbers — and Where They Stop
Most of what decides a flight happens before you get to the hill. Here is what the arithmetic can settle in advance, and where it stops.
Before the season: know your kit
Weigh everything you fly with — harness, reserve, instruments, helmet, boots, clothing, water — and add it to your body weight for a true all-up figure. Run that through the wing loading calculator against your wing's area.
A 70 kg pilot with 15 kg of gear on a 25 m² wing sits at 3.60 kg/m². The same pilot on 28 m² sits at 3.21. An 85 kg pilot on the 25 is at 4.20. Those are meaningfully different aircraft, and the number should be checked against the manufacturer's certified range, not against the descriptive band.
Do the same for the harness with the size calculator, and note that height and weight can point at different sizes — a 175 cm pilot at 70 kg gets an M from height and falls below its weight range.
Before the flight: know your glide
Use a glide ratio measured from your own logs rather than the polar. Then apply the wind, because the ground glide is what determines whether a field is reachable.
From 500 m above the landing area, a 9.6:1 wing covers about 4,800 m in still air, 3,535 m into 10 km/h, and about 2,275 m into 20 km/h. Knowing those three numbers before launch is worth more than computing them under pressure.
On the hill: the numbers stop
Split the wind into components if it helps you describe the day — 15 km/h at 30° off is 12.99 head and 7.50 cross — but understand what that has and has not told you.
It has not told you about the cycle, the gust factor, whether the wind is switching, what the terrain upwind is doing to the air, or whether today is the day. Those are read in person, by people who know the site, and the decision belongs to the pilot on the hill and the instructors and guides there — not to a figure computed the night before.
After the flight: build the picture
This is where the tools earn most of their value, and it is the part most often skipped.
Log the climbs and the sink. +200 m in 120 s is 1.67 m/s; +150 m in 180 s is 0.83. Over a season those figures describe your site — when it switches on, how strong it gets, how long the usable band lasts — in a way no general advice can.
Compute the glide you actually achieved rather than the one you planned, and keep the conservative end of the spread as your planning figure. Recompute wing loading whenever the kit changes.
The order that matters
Arithmetic on the ground, judgement in the air, and instruction underneath both. The numbers are useful precisely because they are settled in advance, which leaves attention free for the things that cannot be settled in advance.
Nothing here substitutes for training with a certified school, and no calculation on this site should be read as permission to fly. What it can do is make sure that when you are standing on the hill, the questions you are weighing are the ones that genuinely need a person to answer them.
A pre-season checklist worth running once a year
- Weigh the whole kit. Not estimated — on a scale, assembled, including water and the clothing you actually fly in.
- Recompute wing loading against the manufacturer's certified range for your specific glider, not against a descriptive band.
- Recheck the harness against both height and weight, and remember they can disagree.
- Re-derive your glide from last season's logs rather than carrying forward a figure you no longer remember the source of.
- Book the service items — line check, porosity, trim, reserve repack — because every one of them affects the numbers above.
None of this takes long, and all of it is easier at a desk in the off-season than in a car park on a good day when everyone else is already launching.
What changes when conditions change
The figures worth recomputing are the ones with wind in them. A site you know in light conditions is a different site in 20 km/h, and the ground glide is the number that changes most.
It is worth knowing your reachable distance from your usual heights at a few wind speeds, so the comparison is available without arithmetic when it matters. From 500 m: roughly 4,800 m still, 3,535 m into 10 km/h, 2,275 m into 20. Those three numbers cover most of the planning most pilots need.
And the part that is not arithmetic
Everything above is preparation. The decision to launch, the decision to leave a thermal, the decision to commit to a glide and the decision to turn back are made in the air by a trained pilot reading what is actually there.
That training comes from a certified school and supervised practice, and it is what all of this preparation exists to support rather than replace. A well-prepared pilot arrives at the hill with fewer things to work out, which leaves more attention for the things that cannot be worked out in advance — and that, rather than any single figure, is what the numbers are for.