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. Lines will shrink unevenly due to the fact that the pilot loads the line groups (A’s, B’s, C’s & sometimes D’s) unevenly in flight. In flight, the A and B lines bear more of a pilot’s weight than the C’s and D’s. This usually results in the C and D lines shrinking more than the A’s and B’s.
As your paraglider falls out of trim, certain performance characteristics important for safe flight are diminished. As the C and D lines shrink relative to the A’s and B’s, 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 maillons 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.