Can You Restring A Paraglider If A Line Breaks?

Vale wrote:

...... could you tie them on yourself? Or, if one is broken, can you fix it yourself?

A bit of ’structural engineering’ style physics, it is generally stated that the best knots ‘only’ weaken a line/rope by about 40%, for those that are unsure of what this means you need to start with the (tension) load bearing rating (strength) of the basic rope:

In the example (NB values in ’spec sheets’ start with when the line is NEW) of a line with a ‘working strength’ of 100kg this line will support a STATIC load of 100kg (in normal use). The line can also be stated to handle a ’shock load’ of say 800kg but this (a good example is a safety ‘fall arrestor’ for when working high) may have a limit on the number of cycles this certification is limited to... (fall arrestors normally being limited to a SINGLE fall) so as an example of a ‘multi shock’ certified system a (skydiving) reserve parachute may be certified for a certain number of openings/deployments (say 20) while a ‘main chute’ could be approved for say 500 jumps with a further 200 jumps IF ‘factory inspected’ after the first 500...

Ok... I hope the example info above will make the following paragliding specific answer clear regarding line repairs....

When you look at the method used to join lines, both to the higher set of lines and to the wing its self the pilot will observe instead of knots which will ‘weaken’ (lower) the strength of the line locally (at the knot, mainly due to the tight radius of the bends forming the knot) the line end LOOP is formed by stitching the line back on its self, this helps avoid the worst of the ’sharpness’ of bending a line AND spreads the load of the join over a larger length of line (it’s also easier to be more consistent (strength), loop to loop, over hand tied knots)

A last and for paragliding where we have LOTs of attachments using lots of component lines.... with ’stitched loops’ (unlike with a hand tied knot ant both ends!) it is very easy to manufacture both new and replacement lines to exact lengths, clearly an important factor in the trim of the aircraft!

So when it comes to a pilot replacing a damaged or full broken line a simple freeing the nut on the Maillon Rapide permits the pilot to ‘un-loop’ lines up to the damaged line and reconnect everything in a ‘robust’ (inherent safety) manner without requiring significant engineering skills (or knot tying ability!)

In the case of the system that makes up a paraglider (example wing with a max in flight weight of 100kg) the ‘working load’ (static) is 100kg, it is approved say to be flown with a angle of bank up to 60* (giving an approx 2G load) so the maker expects to see in normal use loads of a couple of hundred kilos, further when it was tested the testing (standard in PG) was to EIGHT times gravity AND further tested to SIX G on its ’shock load test’.....

So while a (individual line, broken and tied back together, so additionally effecting trim/handling) ‘100kg line’ with a knot in the middle weakening THIS line by 50% may ’survive’ a nice gliding flight.... when the INDIVIDUAL line is only being loaded to say 5-10kg cyclic load if you get a collapse and 5G reopening load (on that line set) i.e. well under the (new wing) safety tests it is highly likely the older AND damaged line could fail... the failure on one line set of course increases the ’shared load’ on the other (remaining) line sets and there have been more than a few incidents (including deaths) of ‘cascade failure’ when many lines end up snapping in rapid succession!

Yours Murray Hay

Image By de:User:Flyout, SVG by en:User:Mysid [CC BY-SA 3.0 or GFDL], via Wikimedia Commons

So... what do you think? Please leave me a comment.

2 Comments:

  • Vale: So if one line is not is good condition and snaps during a flight, it can lead to other lines snapping and to the paraglider ultimately collapsing?

Add Your Own Comment:

By clicking 'Submit' you agree to the Site Terms
By entering this site you declare you read and agreed to its Terms, Rules & Privacy and you understand that your use of the site's content is made at your own risk and responsibility. Copyright © 2006 - 2018 My Paragliding Buddies