An increasing number of pilots are discovering cracks in their karabiners as pictured above. Several karabiners have broken whilst launching. Are we dealing here with over-burdening, defective material, or metal fatigue due to high oscillation loading ?

Defective material could not be discovered in the case of the affected karabiners. Intact aluminium karabiners withstand a loading of between 5 – 7 KN even when the gate is open.
They become strongly disfigured due to this strain, but they should be able to withstand a loading of approximately 1,5 KN which arise under normal flight conditions. The karabiners broke however when launching, and without any special strain.
Overloading of an open or closed karabiner causes strong deformation as a result of which the anodised layer cracks open extensively around the breakage point. The anodised layer on the broken karabiners appears to be intact however.
Which leaves us with metal fatigue. On first inspection this would seem unlikely as the working load of the karabiner amounts to only 1/10 of its breakage strength at the most. Such marginal strain to the aluminium does not lead to metal fatigue even when affective for long periods, if this were not so then the whole of the aviation industry would have to be shut down.

We are dealing here with a special case caused by the play of the safety catch arising from construction / manufacturing tolerances. The tensions arising because of the gate play do not increase in a linear manner with additional loading. As the safety catch is not loaded in the area of the gate play, we are dealing here with the static system of a hook, in which case the tensions rise very steeply. Only when the safety catch engages by a loading of 60 to 200 kg (depending on the extent of the gate play and the position of the harness strap) are we dealing with a ring shaped carrying element with slowly rising tensions. In contrast to rock climbing karabiners, paragliding karabiners are permanently subject to oscillation loading. In contrary to most D-shaped rock climbing karabiners, paraglider karabiners have a broad harness support, as a result of which they can be subject to high bending tensions especially in the area of the gate play because of the long lever arm. If the karabiners are bent cold without subsequent heat treatment in the finishing process (this is usually the case with steel karabiners) then additional high tensions in the bends remain due to reset forces. Possibly the torsion forces due to twisted straps should also be added. These factors have obviously not been taken into account by the manufacturers when dimensioning the karabiner.

This fault would have been recognised at the DHV Harness tests if the karabiners had been tested in the same manner as they are burdened in flight : with an open gate. Only now has a material testing institute commissioned by the DHV proven the problems arising from gate play by carrying out oscillation loading tests. Tests have shown that even by oscillation loading as low as between 3 and 30 kg hydraulic fracturing can arise in a normal anodised karabiner as shown in the pictures. Due to the cross section weakening of a fractured karabiner, the safety catch engages during flight , the catch is seated solidly, the bending tensions are reduced and thus the karabiner does not break (hopefully) completely. When due to the fracturing the safety catch no longer engages properly at the next launch the karabiner then fails completely.

It becomes clear that the dimensioning of the karabiner is too marginal, when paraglider aluminium karabiners with an open gate begin to permanently misshapen by a load of between 200 and 300 kg. Paraglider steel karabiners also begin to permanently misshapen by a loading of 280 Kg. The karabiner manufacturers warn for this reason against flights with an open gate obviously unaware that the karabiner is always burdened as if the safety catch were open, due to the gate play. The dimensions of the karabiner would be sufficient if a „gate open“ burden of 5 KN could be withstood without permanent misshaping. The DHV was correct to demand this value in a karabiner test seal on10.04.2003.
How many load changes a karabiner can withstand before a breakage due to metal fatigue arises can only be forecast statistically. As far as critical stress exists, a breakage can occur in a wide range of load changes. With the karabiners presently in use, the risk is obviously low, but it does exist. For this reason, a limitation to the service life will not bring about the safety that pilots desire. In the case of the breakage to an anodised aluminium karabiner in the Czech republic, the karabiner was only in use for approximately two years.
In any case it is not only the karabiners from a particular manufacturer that approach a critical limit as regards breakage due to metal fatigue , rather all circulating paragliding karabiners with a gate play of more than 1mm - possibly also steel karabiners in as far as they were not subject to heat treatment after the bending process. Whether or not the karabiners presently in use should continue to be used will be shown by the oscillation loading tests which all paraglider karabiners must currently undergo.
None of the conventional karabiners currently (September 2003) in circulation fulfil the DHV paraglider karabiner norm recommended by the DHV!
Only with the Safe-in-Lock and the Quick Out karabiner from Finsterwalder is metal fatigue due to usage loading of no importance, because they have NO gate play and currently comply with the DHV stability and demands of security steps against unwanted opening from April 2003!