Why is the length of paragliding lines changing?

Brand new gliders are coming out of the factory with +/- 6-7 mm. The factory tolerance for the lines for a brand new glider is +/- 10 mm. The difference of +/- 10 mm or more offers a decent amount of room for improvement through the Line Trim Tuning Service. To do the Line Trim Tuning Service at least once after first 40 hours is the best investment you can do for your glider. The lines will shrink and stretch at different rates based on conditions, line material, line diameter, line loading and flying style of each pilot, so two identical gliders will need different adjustments. Another consideration is an asymmetry between left and right-hand side of your glider as most pilots have a preferred side. Preferred side to turn often also shows on a different length of brake lines as one line is usually being pulled more often with more tension than the other side.

Once you get a nice clean trim within +/- 10 mm the angle of attack is the next critical consideration to make sure that the angle of attack is as intended by the manufacturer. The angle of attack will often be higher on a glider that has not been re-trimmed than the one on brand new glider so the glider that has not been trimmed will fly slower and hang back more than a brand new glider. All these changes will happen within first 20-40 flying hours. After that lines settle and the changes in line length are minimal over the next 100 hours.