Paraglider check explained: porosity, line strength and trim — what the numbers mean

TL;DR

• A check measures three things: cloth porosity, line strength and length, and the trim of the whole wing. The report should give numbers, not just "OK".
• Most manufacturers require a check every 24 months or 100 hours, whichever comes first. Some use different intervals; the manual for your model decides.
• When you buy used, the report matters more than the hours. No report means you assume the worst or pay for a new check.

Why a check exists

A paraglider is fabric and string. Both change with use: UV and folding wear the coating off the cloth, and lines shrink, stretch and lose strength. None of this shows on a quick look. The check turns condition into numbers so you can compare them to the limits the manufacturer sets for that model.

Porosity

What it is

Paraglider cloth is coated so air does not pass through. Over time the coating cracks and wears and the cloth becomes more permeable. A porous wing loses internal pressure, inflates less crisply, glides worse and, in the worst case, collapses more easily.

How it is measured

The two common instruments are the JDC air permeability tester and a porosimeter. Both pass air through a small area of cloth and measure the time in seconds. A longer time means a tighter cloth.

• New cloth: typically well above 100 seconds on these devices, with many readings in the hundreds.
• Worn cloth: many manufacturers define a limit in the range of roughly 20-30 seconds on the top surface, below which the wing needs action. Some set the limit higher for leading-edge areas.
• Different instruments and different manufacturers give different scales. Never compare a JDC value with a porosimeter value. Compare the reading to the limit for that model in the manual.

Readings are taken at several points on the top and bottom surfaces, on the leading edge and near the trailing edge. The top surface near the leading edge ages fastest, because it takes the most sun and the most abrasion in ground handling.

What to do with the result

• Well above the limit: nothing.
• Close to the limit: the wing is near the end of safe use or needs a manufacturer-approved coating process, if the manufacturer offers one. Do not accept a seller's promise it is "fine for another season".
• Below the limit: do not fly it, or fly it only after the manufacturer's recommended action.

For buyers: ask for the lowest reading and where it was taken. One good reading in the middle of the wing says little.

Lines

Strength

Individual lines are tested in a machine that pulls them until they reach a defined load or break. Manufacturers define a minimum strength per line group as a fraction of the wing's design loads. Most recreational wings are designed with large safety margins, so a line that has lost a third of its strength may still be above the minimum, but that depends entirely on the model. A wing with one weak line is a problem even if the average is fine.

Causes of strength loss: UV, repeated kinking, abrasion over sharp edges, chemical contact (salt, sunscreen, cleaning agents) and heat.

Length and shrink

Line length changes over time, and it does not change equally on every line. The behaviour depends on the line material:

• Unsheathed Dyneema (HMPE): very little stretch or shrink, but the thin lines are sensitive to wear and abrasion.
• Aramid (Kevlar) lines: known to shrink, particularly the sheathed ones, and need re-trimming.
• Sheathed lines: the sheath protects the core, but the sheath and core can shrink at different rates.

A shrink of a few centimetres on the A lines over the life of a wing is not unusual. What matters is not the absolute change but the difference between lines that should be equal, because that changes the wing's angle of attack, its profile and its collapse behaviour. Manufacturers give tolerances; they are usually in millimetres. Check yours in the manual.

What to look for as a buyer

• A report with measured line lengths per row, compared to the original values.
• Statement of which lines were replaced and when.
• A visual check of the sheath at the maillons, the cascade knots and where lines pass the pulleys.

Trim

Trim is the geometry of the whole wing as flown: line lengths, riser lengths, brake lines and the way the wing sits above the pilot. After the lines are measured, a technician compares them with the original line plan and adjusts the lengths to specification. Some manufacturers also check the wing in the air, to confirm it flies straight and the brake travel is right.

Signs of poor trim in flight:

• The wing turns without input
• The brakes feel different left and right
• Pitch behaviour is more nervous than it should be
• The wing is harder to launch

If a seller says "it pulls to one side a little", that is a trim problem and a re-trim, not a quirk.

Other parts of the check

• Cell structure: ribs, diagonals and internal reinforcements, for tears and stitching failures.
• Risers and maillons: webbing wear, the condition of the speed system and the maillon seating.
• Brake lines and handles: these wear fastest.
• Visual check of the whole canopy: burns, cuts, repairs, mould.
• Documentation: the report should list the date, the technician or centre, the instruments used and the measured values.

How to read a report as a buyer

1. Check the date. A report older than the interval in the manual is expired.
2. Check the instrument and compare porosity values to the manufacturer limit, not to a number from a forum.
3. Look at the lowest porosity reading, not the average.
4. Look for line replacement notes.
5. Check the serial number on the report matches the wing.

When you buy a used paraglider, use these as a checklist, and subtract the cost of any needed work from the price. The wing value tool shows market range; the report decides where in that range a wing sits. See also how condition affects price.

What a check costs

Prices vary by country and by what is included. A basic check is usually in the low hundreds of euros; a full line set replacement costs more. Ask for a quote before you commit. I do not quote fixed prices here because they depend on the model and the service centre.

Paramarket lists used wings; ask for the inspection data on any of them. If you want a notification for a certain model, set a gear alert. If you are selling and your wing is near its limit, the sell page explains how outright purchase works.

FAQ

How often should a paraglider be checked? Most manufacturers require a check every 24 months or every 100 hours, whichever comes first. Some use other intervals, and some countries or schools require more frequent checks for commercial use. Read your manual.

What is a good JDC porosity value? There is no single value. New cloth reads far above 100 seconds; many manufacturers set the action limit around 20-30 seconds on the top surface. The limit in your manual for your model is the one that counts.

Can I check porosity at home? There is a simple "fabric test" some pilots do with a hand held tool, but it does not replace a calibrated JDC or porosimeter measurement. For a purchase decision use a proper test.

Do line lengths change the way the wing flies? Yes. A few millimetres of difference between lines that should be equal can change the angle of attack and the collapse behaviour. That is why trim is part of the check.

Can a wing with low porosity be saved? Some manufacturers offer a recoating service for certain cloths. Others do not. Ask the manufacturer before you buy a wing that is below the limit.