Tuesday, March 29, 2011

Abseiling with prussik warning

This came to me via the British Mountain Guides newsletter and makes interesting reading:

Summary - Using a prussik loop back up on your leg loop can open the buckles which are 'cinched' tight (by pulling), now increasingly found on modern harnesses, with dangerous results.
With this in mind it makes sense to follow Petzls own guidelines on attaching the prussik to your central belay loop and extend the belay plate away from the belay loop using a cowstail in the normal way. Here is a diagram from Petzl that uses a shunt or prussik knot attached to the belay loop: 
From a British Mountain Guide: 

" Last summer on a Technical Alpinism course I taught clients how to protect an abseil with a French prusik clipped into the harness leg loop, a standard technique used almost universally. While using this later abseiling from a gendarme, one client allowed the prussik to bite    whereupon the harness leg loop opened and he was left dangling from his waist belt with the prussik jammed against his belay plate. Luckily he was nearly at the end of the abseil and other clients helped him to unweight and free himself.
Wanting to see how this had happened I tried it using his harness and the leg loop undid every time. We then tried other harnesses in the team (which were not identical but similar construction) and again this happened.
All were new harnesses which had leg loops with buckles which 'cinched' tight (by pulling) rather than being back-threaded in the old style (as the leg loop buckles on the harness I was wearing). What happens is that the karabiner of the prussik lodges at the buckle, which then opens because it is being pulled at an angle which 'cinching' buckles won't take, i.e. a force from behind the buckle. This is much easier to demonstrate than to explain!
So many modern harnesses now use 'cinching' buckles on the leg loops, or are in the “Bod” style, that we may have to revert to the method of protecting an abseil with a prussik by clipping the prussik into the belay loop of the harness (usually with a quickdraw or cow’s tail to extend either the prussik or the belay plate), I have always thought this was a safer method anyway, but a bit less convenient and involving more gear.
What worries me is the number of people out there who have been taught to abseil with a French prussik clipped into leg loops with 'cinching' buckles - a lot of people I suspect."



The technique described has always worked well in the past as harness leg loops have been the double back type. In light of this experience it makes sense to review what we are doing! 



2 comments:

Anonymous said...

If you use a harness such as a Wild Country syncro, which has a loop on the leg loop, which can be used to attach a prussik, then I think you'd get round this problem. I don't know if that's the meaning of having that loop there, but I don't think it pulls on the buckle. Lots of harnesses seem to have loops there which are often in front of the buckle and I don't think they'd cause this problem.

Unknown said...

Thats correct on more recent harnesses but the first wave of cinchlock buckle harnesses don't have them. Its interesting that the manufacturers don't describe that loops use although I agree that the triple stitching will mean it is strong enough for the job.