Every paraglider is manufactured with a prescribed set of line lengths. With time and use, these lines have a tendency to shrink due to the nature of the line material, typically kevlar or dyneema. Some lines will shrink while others will actually stretch due to the fact that the pilot loads the line groups (A’s, B’s, C’s & sometimes D’s) unevenly in flight. The A and B lines normally bear most of the pilot’s weight, which prevents them from shrinking a significant amount and often results in them stretching beyond their original lengths. Meanwhile, the C and D lines are likely to shrink over time as they remain less loaded. All this to say that eventually your paraglider is going to need to be brought back into ‘trim’ in line with the original length specifications.
As your paraglider falls out of trim, certain performance characteristics important for safe flight are diminished. As the A’s & B’s stretch and the C’s & D’s shrink, your glider takes on a higher angle of attack. If you can picture how your speed bar decreases the AoA of your wing to increase its airspeed, an out of trim glider causes essentially the exact opposite effect. So the glider won’t inflate or kite like it should. It will also have diminshed performance in the air as it flies at a slower airspeed closer to the stall point. Bringing a paraglider back into trim allows it to fly as it was intended to.
It’s recommended to get a paraglider trimmed after 100 hrs of flight time or two years, whichever comes first.
Changes of a couple centimeters can affect the trim of a paraglider. Therefore, it’s crucial to be able to measure lines with accuracy down to the millimeter.
All of the lines on your paraglider are measured using a laser device on a rail system with a 5 kg load for the most precise measurements possible. After measuring all of the lines, the results are analyzed in a spreadsheet. Any necessary adjustments are then made using the quicklinks on the risers or in some cases, by sewing new lines. The lines are then remeasured for quality assurance and a post-trim report is created. Paragliders are trimmed to within 10 mm of factory specifications with a +/- 50 mm offset considered acceptable.
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