Report about the investigation of a broken mainline in July 2022



In July 2022 we received the information, that a group of experienced highliners have had a mainline failure during a highline session on a 270m setup in France.

The ruptured slackline was one of our webbings from a batch dating production in 2018 (Joker) and equipped with t-loops made in a non-automated machine process, as well from 2018. The incident immediately alerted us to learn more and to collect every possible piece of information to analyze and understand this rare type of webbing failure, appearing in the middle of a t-loop.

Even though it states to be a unique appearance on this webbing (nothing comparable happened to the other 20km + we’ve sold), we feel the responsibility to share openly with the community the current status and all details of our ongoing and closed investigations and to inform our customers, who’s t-loop may be concerned (around 15 t-loops made in 2018).

Many factors have been explored and tested since July, but so far, no single factor has given us enough comparative results to be declared with certainty as the one, exact origin of the failure. We can only state and explain what appears to seem the “ most likely” hypothesis in our report below.

Find all facts and details about the mainline failure: the webbing data, the process of rupture, the tests already conducted, the conclusions already established by Florent B. and all ongoing analysis in collaboration with the ISA and webbing manufacturers in this report.

As we have also noticed during testing, that specific factors concerning the care taking of webbings may have potentially played a role in the reduction of its MBS, we find this a good moment to use this incident also as a general reminder again. A reminder to all webbing owners, of the importance of usage tracking and good treatment of your slacklines, especially when used for highlining. Find therefore, on the bottom of our report, the recommendations of our development engineer Florent Berthet, on how to extend the life of your webbing to a maximum.










  • Total length of setup:  265m Setup Main line: 150m joker / 120m maverick (junction via sewns loops and a quick link )
  • Backup line : left overs of mav 80m / white Magic 200m (junction on sewn loop / quick link)
  • Slackliner weight: 95 kg
  • Fraction: no fractio on the mainline (there was one T loop on 150m main line joker, but not connected to backup)
  • Tension: Two people tensioned with jag-type haul (both people had a basic ascender to pull, microtrax on the strap side (5:1))
  • Duration of the rigged set up:  1 day
  • Frequentation: 2 runs before the backup fall run (3 runs in total).
  • First run: walked until the junction mav/joker (120m) including lots of leashfalls
  • Second run: For- and back of total length with some leashfalls.
  • Third run: Some leashfalls and mainline failure during the movement of getting up on the line, just next to the junction of mav/joker
  • Place of breakage: 1st failing reaction between the T-Loop stitches at around 75m from the slacker.
  • Additional Facts: T-Loop had never been served since the day of purchase. Had been kept taped since the beginning. Joker webbing seems in a good state without any visible marks. Sewing made by a non automatic machine (technology dismissed by Slack Inov' since 2019).

FLORENT BERTHET - Developer and founder of Slack Inov’:

“The webbing broke first exactly in the middle of the T-loop, at the level of one central bartack (first breaking - below img.: 1ere rupture). This therefore generated a force which tore the 7 central bartacks of the T-loop apart by opening it entirely (second breaking - img.: 2ème rupture), then lastly the strap of the T-loop broke at the level of the last bartack of a series of 9 (third breaking - img.: 3ème rupture).”


  • A sewing defect may have weakened the webbing.
    According to what the slackliners stated, the line may have broken at forces around 500kg max (this stays a rather unconfirmed number, as there was no absolute measurement device in place).

  • Weakening of the fiber.

    The T-loop that had always stayed taped may have stored moisture and developed mold under the tape: nothing visible to the eye though.

  • External parameters may have considerably increased the estimated load.
    Gusts of wind, strong resonance…? This option seems quite improbable, since the witnesses did not notice anything in particular.

  • Possible cyclic loading + a sewing defect as a combination.
    This may have influenced a phenomenon of fatigue in the webbing fibers: ISA cyclic loading on t-loops 1st results confirm no specific influencing of general breaking strength of a webbing.


(Webbing data above 1. )

In order to better understand and find answers to this very unusual rupture of the webbing, we conducted several, different tests and measurements along the remaining webbing, at very different spots:

  1. When we measured the remaining resistance, MBS values of this strand dropped in certain areas to around 17.2 kN as its lowest.
    Those occurred towards a sewn loop, the rupture happened on the line itself, not within seams.

  2. In other areas, as in t-loop free portions, we received results of around 23 kN.

  3. Resistance measured in the remaining portion of the T-loop: 20.1 kN

-> According to our own long term tests on webbings, we can state this webbing measures usually after 6 months of constant UV exposure around 21.4 kN.


(Purchased at the same time by the same customer)

In order to have values of comparison, we tested those resistances also on an identical webbing strand, bought at the same time, with t-loops made at the same time, using the same (today outdated), non automated sewing technique.

  1. When we measured the remaining resistance, this strand presented at random spots (not in a t-loop area) along the total length MBS values of around: 20 - 22kN

  2. When pulling the t-loop of this webbing, we measured a resistance of one of the t-loops at: 18.98KN, where the rupture occurred outside of the t-loop sewing (not like above, on the webbing in question, in the middle of the t-loop sewings).

c.  When pulling on the “main” (blue) webbing and the loop of the t-loop, the rupture appeared as stitches simultaneously opened on one side.

     We measured the breakage at 16.18kN



As of this moment within our testing, we are still not 100% convinced of the exact factor for this rare event of a line failure. As none of our comparative measurements have achieved absolute proofing results, we can not state, nor declare THE ONE reason for the breakage.

Including all details we had access to and considering all conducted tests, Florent considers that most likely, one specific factor concerning the sewing had weakened both layers of webbing in that area and therefore reduced its MBS within that t-loop massively. He considers that most likely a certain defect during sewing on one of its central "bar-tacks” has led to the failure later on (slightly broken or damaged needle tip?), however he has not found 100% proof to state the exact type of sewing defect with absolute certainty.

We also were not capable, even though in close collaboration with the slack association, to determine the exact data concerning further, possible influences that may have impacted the webbing’s state:

  • Time exposed to UV
  • Time the webbing was actually rigged (days under tension)
  • If tensioned rather in small or on big set-ups (low or high tensions)
  • Heavy or low type of sessioning (10-15 sessions p/day or rather 3-7)
  • Not enough details about the way it was stored, nor where (in a box, in a car, on a balcony, in the dark..)
  • Has it always been stored totally dry, not humid?
  • Did animals like dogs have access to it (urine smell) etc etc..

..a collection of unknown factors that we were unable to analyze and not capable to take in consideration their influence on the weakening of the webbing.


*Since early 2019 the pattern and technique of this t-loop in question has been dismissed, even though it had been properly prototyped and validated in breaking tests, before offered for sale back in 2018. Since then, all new SLACKINOV’ webbing sewings are being performed by automatic and programmed machines that have been thoroughly tested.


As our records recall, around 15 customers have ordered t-loops in 2018, please feel to send us your webbing or images of it ([email protected]), so we can conduct further sewing inspection of the t-loop and potentially test it’s strength by pulling up to a non destructive validation of it’s MBS.


According to field’s expert and ISA President Thomas Buckingham, we are missing another possible hypothesis: electrostatic damage.

It could be possible that areas underneath some tapes present a potentially higher risk for damages from electrostatic discharges. Those areas can happen to be in the exact opposite condition (dryer or more humidity), than the rest of the webbing, also possibly happening in areas of a t-loop.

“..We have seen this now a couple of times. At first sight, your damage looks like it saw some higher temperatures and it even seems like it got partially burned a bit..We don’t understand the mechanisms involved, nor do we know of a testing-setup to simulate this.

Further points that support this hypothesis:

- The t-loop section was taped for a longtime, so not wet/less wet then surroundings

- We only have seen damage to longer lines, so this fits here as well.

.. in theory the line could have also been damaged partially another time and hung again without noticing the damage (since the t-loops were not checked under the t-loops, I assume)..”

Following his recommendation, we will continue further inspection of all circumstances that may help better understand and analyze this hypothesis of electrostatic damage underneath tapes/in t-loop areas.


Proper use and maintenance of your webbing

Personal recommendations by our developer and engineer Florent B.

  • Read the manual properly
  • USE a tracking system of usage times
  • No prolonged UV exposure if avoidable
  • Absolute avoid potential contact with dangerous solvents and other chemicals
  • Avoid friction on rock, ground, tree barks Rinse the webbing after use in salty or dirty water
  • Rinse if dirty or contains sand, earth or other abrasive particles within its fibers
  • If need to wash - do so in a cold machine and without adding a product
  • Best to store it completely dry in a box with cover or a closed bag (avoid longtime exposure to dust and light)
  • Never dry a line in direct sunlight, dry it in the shade, opened up
  • Inspect before each use and after
  • Don’t let webbing for too long taped together with the same old tapes (moisture & glue)

Thank you.
The Slack Inov' Team