Porosity can be measured, excuses cannot!

There’s only one thing worse than weak porosity values: a manufacturer’s resistance to legitimate warranty claims. No one deliberately chooses bad material. But quality control is a duty. When internal mistakes go unnoticed, it becomes risky for the pilots who use the product.

A concrete case from my workshop: a brand-new tandem wing in purple. I planned to add branding for a tandem company and measured the fabric beforehand. I set up a JDC Mark 2, wrote a protocol, and saw an average of about 300 s on a factory-new canopy. My recommendation: return it. After some back and forth, a replacement arrived with better numbers — around 650 s on average — plus a promise that the fabric would remain stable for a long time. Sounds nice, but paper accepts anything.

Exactly ten months later, after commercial use and a timely annual inspection, the new readings averaged about 30 s. Not even 200 flights. You expect such averages when a six-year-old tandem is on its third life somewhere, not in Europe and not this soon.

Today another wing with the same purple panel came in for a check. Same base cloth: Dominico TEX 30 DMF / N20 DMF, again purple. Average porosity around 40 s. On small black spots, by contrast, 850 s. That does not align with hot-car storage in midsummer or packing it wet and drying later. The pattern is consistent: a material issue linked to color or batch is more likely than user error.

What’s actually in the fabric

Paraglider canopies are usually nylon 6.6 ripstop in 20–30 denier. Airtightness is achieved by calendering and coatings. Polyurethane (PU) coatings are common, sometimes combined with silicone. At Dominico, “DMF” indicates a PU coating applied using the solvent dimethylformamide. The goal is an even, thin layer that slows air passage and resists UV and moisture. If recipe, solvent residues, drying, dyeing, or base-fabric batch don’t align perfectly, values can drop faster than expected.

How porosity is measured

In the field we often use the JDC Mark 2 porosimeter. It measures the time in seconds for a defined air volume to pass through a defined area at constant vacuum. In production, lab instruments such as the TexTest FX 3300 are used, operating at a constant pressure differential with a larger test area. Reproducibility matters: avoid re-measuring the same spot, sample multiple zones, average the results, keep a written protocol.

What a pilot should do

• Measure right after purchase, before the first flight. Record values, take photos.

• Insist on an exchange if porosity deviates significantly from expectations.

• During inspections, ask for details, not just a line on the sheet. Have the points and spread explained.

• Watch color variants. If one color keeps being an outlier, look closer.

• Avoid UV and heat, store dry. Good care cannot fix bad chemistry.

My conclusion from two real cases

I don’t see a conspiracy. I see a quality issue, likely tied to color or batch, which with Dominico 30 DMF / N20 DMF in purple led to weak initial values and fast degradation. These are observations from my workshop, not statistics. That’s why transparency matters. Measure, talk, resolve.

Want your wing’s porosity tested? Contact me: paraglidingservice.com/kontakt

Quick explainer: porosity numbers

The higher the seconds in the JDC test, the tighter the cloth. A drop from 650 s to 30 s in ten months under moderate commercial use is not normal and should be investigated.