TLDR: Buy a Stainless Steel karabiner with a strain rate of at least 2.5kN
When was the last time that you changed your karabiner? And why did you change it? Is it because it’s been two years? is it because you dropped it and you’re not sure anymore about the integrity of the device? How long as it been since the last time you changed your karabiner? How about the one that attaches the rescue to the harness? When was the last time that you inspected that one for marks of wear? How old is that karabiner?
When things go almost wrong
Take a look at this guy flying with an open karabiner throughout his whole 1h50m flight. The pilot mentions in the comments the following:
It’s a kortel mylon, it wasn’t fixed properly (my fault), and probably opened at start. It was almost a 2 hours long flight (towed start): And I didn’t notice it’s open until landed! I was lucky.
This pilot is both very lucky to have survived this incident and very brave to post the video online because although the intention is to raise awareness within the free flight community, he also knows that there will be haters calling him stupid, that he should quit the sport, etc.
Thank you Andras for sharing this important lesson with all of us.
How often people change their karabiners?
This thread on the Hang Gliding Forums asked that question and even though the amount of participants was underwhelming (57 in total), a total of 44 people said that they never change their karabiners!
Passive Safety
Whatever your gear has to offer that requires zero input from your during a flight is what we call passive safety. The keel strap on your helmet, the risers on your paraglider, your wires on your hang glider, etc., are a few examples of Passive Safety features.
Your karabiner, which requires zero input from you once you’re airborn, is also considered a Passive Safety device or feature.
Some of these devices will have so little strain that you will almost never have to replace them. For example, the clip on the chin strap of your helmet is under so little strain that you probably have never thought of changing it. And that’s probably fine. But your karabiner, though…
The karabiner under load
I weight somewhere between 70-73Kg. My hook-in weight is somewhere near 85Kg counting clothes, helmet, harness, reserve, water, food, radio, vario, batteries and what not. That means that whatever karabiner I use has to hold at least that much. What I mean by this is that whatever karabiner I buy, when I hook into my glider and hang from it, it shouldn’t break.
The term used for knowing how much a karabiner can take before it breaks is called breaking strain (although in Physics the correct term is Ultimate tensile strength). and is measured in Newtons. A karabiner that breaks under a load of 85Kg (my hook-in weight) has a breaking strain of 0.85kN (kiloNewtons).
But taking off with such a small breaking strain rating is rather scary, taking into consideration that most of our flying happens in turbulent conditions in which the G loading can increase considerably, specially when you get slack lines! Suddenly you have now 2 or 3 extra Gs added to your system, meaning that your karabiner doesn’t think that my weight is 85Kg anymore but 3 times that, 255Kg! whoops.
Reading on the Wills Wing website, a lot of the hang gliders out there are tested up to 5G before they break, so ideally our karabiner is somewhere near those numbers because I think it’s safe to say that if the wing has broken I think the main karabiner is the least of our concerns.
In a hypothetical scenario where I bring the load to 5G the stress put onto my karabiner would be at around 425kN (my hook-in weight 85 times 5).
Here are a few breaking strains of the most common makers in the industry:
| Maker | Model | Material | Weight | Breaking Strain |
|---|---|---|---|---|
| AustriAlpin | Stratus 10 | Aluminium | 64g | 18kN |
| AustriAlpin | Stratus Inox 11 | Stainless Steel | 130g | 28kN |
| AustriAlpin | Powerfly 12 | Stainless Steel | 139g | 26kN |
| Finsterwalder-Charly | Snaplock 5 | Titanal | 76g | 30kN |
| Finsterwalder-Charly | Pin Lock 6 | Titanal | 80g | 25kN |
| Finsterwalder-Charly | Quickout 7 | Titanal | 200g | 40kN |
| Maillon-Rapide | Square Quick Link 8 | Stainless Steel | 64g | 20kN |
| Maillon-Rapide | Trapezoidal Quick Link 9 | Stainless Steel | 129g | 49kN |
| SupAir | Self-locking 2 | Aluminium | 68g | 18kN |
| SupAir | Self-locking 3 | Aluminium | 72g | 16kN |
| SupAir | Self-locking 4 | Stainless Steel | 130g | 24kN |
| Woody Valley | Skyway 1 | Aluminium | 48g | 19kN |
A quick word about the Maillon-Rapide karabiners: these are the most cost-effective ‘biners that I can see but with the caveat that setting them up requires more caution than with the rest of the self-locking mechanisms of other makers.
All of them offer a breaking strain rating well above my hook-in weight even at 5G. What I can see though is that there are three materials in place: aluminium, stainless steel and titanal, an alloy made of aluminium that promises better material performance.
Steel vs others
André van der Lingen, Mechanical Engineer, hang glider pilot and sail glider pilot says:
Steel is very predictable in fatigue. When the repetitive force does not exceed say 50% of the Ultimate Strength (breaking point) of the material, it doesn’t matter how much cycles it is subjected to, it will never break. This limit is called the fatigue limit and is not influenced by numbers of cycles. Aluminium behaves way different and has a so called s/n curve. Aluminum could hold up to 70% of it’s Ultima Strength at 10.000 cycles but might drop to only 30% at 1.000.000 cycles. There’s no limit and it totally depends on the number of cycles. You should know exactly how much you will load an aluminium carabiner when you want to use it for free flight.
There are two reads that I would really recommend you go through to understand better how different materials affect the performance of a carabiner.
AustriAlpin has a nice summary here.
Another important piece of information can be found on the white paper written by Dipl. Ing. Thomas Finsterwalder on Risk of material fatigue in air sports carabiners with conventional snap gate.
To summarize it, our inherent problem in free flying is that these karabiners are normally made for static loading (imagine a climber just hanging from it) while we are constantly adding dynamic loading to it because of the fly that we fly in. This has a very critical impact on the overall performance of the material and the breaking strain of the karabiner.
Asking a bunch of friends and reading on the hang gliding forums I found out that the material that performs better under dynamic loading is Stainless Steel. This is what I’m going to stick to
Tests
The reason why some karabiners are more expensive than others is because in order to arrive to the final product, designers have to run stress tests to find out the sweet spot between breaking strain, weight and costs. As a designer if you run tests on a batch of a 1000 karabiners, that’s a 1000 karabiners that you’re “throwing away”, plus the costs of renting the equipment to run the actual tests, plus the man hours invested on analyzing the data, fixing the issues in the design, making more karabiners, etc. The costs add up. These costs are obviously passed on to the customer and that’s why some karabiners are much more expensive than others.
So next time that you see an aluminium karabiner with a breaking strain of 20kN in Amazon selling for only US$10, think twice.
Saftey Warnings
Good karabiners will come with a serial number imprinted on the material. If customers from a specific batch of karabiners notice malfunctions or deformations on a certain karabiner, it’s their responsibility to report this to the factory. If several people report the same problem to the factory the company will issue a Safety Warning explaining the issue with the product. If the problem is only present in a specific batch, the announcement will say so and the people that bought a karabiner from that batch will be able to return the product and receive a good one. If the whole product has a design flaw that makes it ultimately too risky, the factory will recall the whole line and issue a refund to each customer that bought such a karabiner.
Parting thoughts
Initially I just wanted to write down a comparison list of alu vs steel karabiners. The more I pulled from the thread the more information I found, so I had to keep the scope as short as I could without skipping vital information.
If you think that there is important information that is missing from this article do not hesitate to drop me a line via social media and I’ll be happy to amend the article.
Special thanks to my friends Fran Ruiz and Andre van der Lingen for their input.