WHICH COULD SAVE YOUR LIMB AND LIFE
The line types most regularly used on paraglider suspension lines are Kevlar (Aramide), Polyester (Tergal or Terylene), Superaram, and Dyneema (Polyethylene). The most used in South Africa are the Kevlar and Dyneema lines, and also Technora, a kevlar type of line.
There are some misunderstandings regarding the characteristics of lines used on paragliders. It must be understood that there are always pros and cons with the use of either Dyneema or Kevlar lines (or any of the other line types).
For those who do not know the differences, here are short summaries on basic Kevlar and Dyneema lines:
- Has best resistance to heat of all the line types
- Does not stretch or shrink much
- Can become brittle
- Can be damaged through kinks, twists, etc
- Damage is usually difficult to see
- Has least resistance to UV rays
A manufacturer has to play off the pro's and cons of the one line type to the other. Sometimes one will find that all the lines on a glider are made from Kevlar. In other cases all the lines are Dyneema, and in some cases the top lines are Dyneema and the
bottom lines are Kevlar, or vice versa. There are arguable points for one choice or the other all the time.
- Can shrink, especially when moist
- Can regain its original length after shrinkage
- Is robust (does not weaken extensively from kinks, etc)
- Best resistance to UV
- Gets damaged sooner than Kevlar from heat
- Is about 1.5 times stronger than Kevlar
- Can over-stretch with extreme loads (really extreme)
Kevlar is used more in Europe with its smooth grassy slopes and low UV exposure. Its main advantage is that there will be very little canopy performance change due to stretching or shrinking of the lines, although it has been proven that even Kevlar has some shrinkage on the C and D lines. The main disadvantage is that the lines can get damaged through kinks and twists without being noticed. Line breakages can then occur very suddenly and without warning, although it would normally be one line only. When the lines are "old", i.e. usually well in excess of a 100 hours, one line break under extreme manoeuvres may lead to additional lines breaking due to the increased loads on the remaining lines. The sheath usually shows no damage, and the only indication is a stretch in the line, which could mean that damage has occurred inside the sheath and the line has parted. Extreme manoeuvres are spirals, wing overs and other aerobatic actions, as well as severe collapses and turbulence induced flight extremes.
Dyneema is popular with South African manufacturers because of its high UV resistance, and its strength. It is felt that the rough areas where we fly most of the time require a robust and strong line which is not going to break or damage (internally)
through catching or hooking on stones or rough vegetation. Its main disadvantage is that some shrinkage does occur with time and moisture, and some performance can be lost if the glider is not checked regularly.
A pilot whose glider has Dyneema lines should send it back to the manufacturer on a regular basis (every six months), and not leave the glider in a hot car in the bag, especially if it has become wet through late afternoon dew on the grass, etc. If you suspect that the lines on your glider has shrunk, the glider could be returned to the factory, or you could stretch the lines yourself. Occasionally a line could have stretched further than the original length through overloading, usually from catching on
something during pull up. Stretched lines (i.e. longer than normal) should be replaced. Replacing a line oneself is extremely simple, provided one makes sure that the lines are routed correctly.
Checking Dyneema and/or Kevlar lines
To check and stretch lines oneself is quite easy. SAHPA has a self-maintenance policy, and therefore there is nothing to stop one from doing it oneself. There is also nothing to stop one from sending it to a dealer who has the experience and knowledge to do proper glider checks.
Hook the risers or the links on the risers to something static (i.e. a peg, or gate, for example).
First check all lines for symmetry, right to left.
Sort out the same line on the left and right sides to compare. Hold (for example) the right line in your right hand and the left line in your left hand. Compare the lines for difference in length.
One can also measure the lines to determine whether there is a significant difference to the line chart, where such a chart is available.
When this is done, then pull with one arm only (either left or right) approx 30-40 kg for 10 seconds, or hook a weight of that much on to the line over a pulley. Compare the two lines again - if there is a difference, then the lines have shrunk and one should proceed to stretch all the lines in a similar fashion.
If a line breaks during this exercise, and it did not have an obvious weak point from prior damage, it is prudent to replace all similar lines, i.e. all long A-lines, etc.
Keep on comparing the lines. In general, one will find that Dyneema D-lines will stretch around 5 cm, the C-lines somewhat less, and usually very little if any on the A and B lines. South African Dyneema has more stretch than imported Dyneema lines. Gliders which have been left unchecked for very long, might have had more shrinkage. The A and B lines are not stretched, although strength tests on those are required. Two or three spot checks for strength on a group of lines (e.g. long A-lines) should show up a general weakness of the lines. Weakening of lines leading to line breakage at less than half line strength is very common in old(er) gliders that utilise Kevlar lines.
Gliders with Dyneema lines may show shrinkage and stretching on the lines, even when fairly new. This may become more noticeable with winching with a glider on which the C and D lines have shrunk, but the A and B lines through extra tension introduced
during the winching, have stretched back to their original lengths. The glider would appear to be flying slower than would be expected. Such gliders could go parachutal or may stall easier.
Should one line suddenly "give" during stretching, and afterwards one finds that the lines are no longer equal in length (right to left) after stretching both, regard yourself lucky to have found damage before having a line break during flying, and replace at least that line. Should the glider be on the older side and not have had a line change, it would be prudent to replace all the lines on the glider. Recent checks on an older Apco Prima showed line breakages at approx 25 kg tension on the long A-lines.
What is the cost of new lines compared with one's life?
When to replace lines
Any broken or damaged lines should be replaced as soon as possible.
Temporary repairs are possible to keep one flying over the weekend.
When the outer sheath on a line is damaged, push the sheath back over the core, and tape it up. This will prevent the core from weakening due to the direct exposure to UV, and being further damaged from contact with hard surfaces.
This method works well for all lines, and one can leave the C, D and brake lines like this as they do not carry the bulk of the pilot's weight.
With A and B lines (which are the main weight bearers on a paraglider) one can additionally add another line of equal length to help carry the strain (i.e. re-inforce the strength of the damaged line), so that one can still fly until the replacement line is received. Do not remove the damaged line in the case of the A and B line when the temporary line is installed. A little extra drag for the interim is not the end of the world, as long as one can still fly!
Use a line that is longer than required if one of the right length is not available. It should have at least one stitched loop, if possible. Loop this in at one end with the damaged line (use same attachment point). Then take other end and follow the damaged line through the other attachment point. Make a knot where it comes back so that it cannot slip out. Check against the damaged line for length as well as the line on the opposite side for symmetry. Tape up any loose ends.
If a line was broken, replace it with two temporary lines, in the case of A and B lines. C and D lines and brake lines would require only one temporary line to replace a broken line.
A tip to remember is to replace lines according to the manufacturer's recommendations, which are usually approx 100 hours flying time, or every two years, whichever comes first. If one cannot afford to replace all the lines, then at least replace the long A and B lines, which is most likely to weaken or break. Competition lines should usually be replaced every 100 hours or on manufacturer's recommendations. If you are flying a paramotor with the glider, the lines should be replaced more often, for two reasons: there is friction on the long lines along the cage during the pull up sequence, and the lines are subject to vibration and stress from the motor.
The moral of the story is that all gliders and lines should be checked regularly. Very small differences in line lengths do not matter, but larger variations should be investigated.
Annual glider checks should include line strength tests as well as line length checks.
As far as possible, avoid making knots in permanent lines as knots are not only the weakest point in a line, but also could render the glider certification worthless if they are used to shorten lines on a glider.
The weakest link in a paraglider is its owner, if he does not ensure that the lines are replaced or tested timeously. Look after your glider, and you will enjoy your flying for a long, long time.
Original publication: 1998
Re-edited: August 2005