Backing Up An Abseil

Talk to a dozen climbers and you'll likely get a dozen different answer's on how "best" to backup an abseil. Numerous methods abound. Each has advantages and disadvantages. I'll put a few far from original ideas forward here. This article represents merely my own opinion. It's hardly authoritative or complete and could well be inaccurate. I suggest you use your own judgement, consult an appropriate book and seek the advice of qualified instructors. Please read the full disclaimer.

Reasons To Backup An Abseil
There are a plethora of reasons to backup an abseil (and several not to do so). I'll just pluck a few out of the air to set the scene, so to speak. 

• You might foresee a tricky descent, possibly involving untangling the ropes. 
• You might be a beginner and can not trust your technique.
• It might be cold, windy, raining, icy or for whatever reason you don't trust your break hand. 
• You might need to be able to stop and let go to clean the pitch on abseil. 
• You might want to stop to take photos, check out the route next door, scope for babes ;-), etc. 
• An unexpected event may occur causing you to release the brake line, such as: being struck by falling rock, slipping on loose or wet rock, getting bitten by a wasp, accidentally knocking your brake hand against the rock, bumping your funny bone, loosing a contact lens, getting a cramped muscle, etc.
• You might be on a remote descent far from help.
• You might be carrying a heavy pack.
• The rappel is very long, and the device might heat up sufficiently for it to fail.
• You're in the process of bringing down an injured climber.
• You might choose to purely as a safety precaution.
• The list goes on....

Reasons Not To Backup An Abseil
In other words you're 100% reliant on your brake hand. This is the method employed by many climbers, given a certain type of descent, and has the advantage of speed and simplicity. A climber might not use a backup for numerous reasons, such as:

• The descent is straightforward and/or short and the climber is experienced enough to judge the risks.
• Speed is critical to safety. Eg, an alpine descent off a storm threatened peak. Again the participants are experienced enough to judge the risks. 
• You judge the potential difficulties that can be involved in continuing a descent once a backup friction knot has been weighted tight to be greater than any safety advantage gained. (More on this later).
• You judge that reliance on a rap backup might cause a relaxation of vigilance that is more detrimental.

Method's Of Backing Up An Abseil
The method you use will be dependant on the conditions of each descent. Let's assume you've decided to backup the abseil. What are the options? 

Abseil Backups Involving Another Participant
In this instance you want some from of backup and you have another participant ready to help. Here I can think of these methods:

Fireman's Belay
Very simple and fast given that someone is already on the ground (or bottom of the pitch), and available to hold the rope. The descender heads off as normal, except that their buddy down below holds the slack end of the rope (pictured right) and is ready to pull down, and thus lock off the descender's device (dependant on device used), at the first sign of trouble. I find this method very handy when sending beginners down or if one of the team feels a little spooked by the abseil. It's also just a great, painless and simple safety precaution. We use this often.

Belay From Above
Pretty obvious. Anchored climber at the top belays the descender on a second rope, as if they were lowering them, but giving the abseiler just enough slack so that they can control the rate of their own descent on their own rope. I find this is sometimes necessary when sending beginners down.

Abseil Backups By The Descender Alone
Here you're on your own, or have decided to backup the descent without involving your climbing partner. There are dozens of ways of doing this, but I'll mention only the ones I've used:

A Knot In The Ends Of The Rope
Not a backup as such, but at least it might stop you abseiling off the ends of the rope(s) if you're bailing off a multi-pitch in the dark. It's happened. Even well experienced climbers have died this way. Again, opinions vary as to when one should not include this simple safety measure. Many say it should always be done. Others argue that the knot can lead to stuck ropes (knot catches behind a flake in the wind for example), which can be of danger, especially if speed is critical to safety. I personally, almost always tie a knot in the ends of the rope(s), unless it's obvious that the ends reach the ground. When abseiling, most climbers like to tie a knot in each end (eg, a stopper knot), other's prefer a single knot joining the ends, but in my experience this can lead to twisting. Just be sure to remove the knots before pulling the ropes down. If you find the ends are uneven while descending, it's possible to lock off the short tail and let the longer line slide up, however beware of what this might do to your anchor above; you could easily burn through a sling if you were careless enough to be rapping off one.

Heaps Of Friction
You load the rope with enough friction to slow your descent to a crawl even if you totally let go of the brake end. This can be very handy when sending beginners down, or if, for example, you're carrying a heavy pack. If you're not lugging around a rappel rack and set of brake bars (pictured right), there are other ways to add friction. With a figure eight you can feed the rope through a second time. With an ATC/Plate you can add a second carabiner. Or you could add a second descending device attached to the line above the normal one and tied off to your harness with a sling - sounds dumb but it works a treat. You could do something similar to the carabiner brake rappel, to add additional friction. You could be a HardMan TM, and wrap the rope around your leg - ouch. I won't go into detail on all the options, but I'm sure you get the idea. You could still fall to your death of course. It depends on how much additional friction you'd added, the weight, device(s) used, terrain, etc.

Mechanical Device
Technology provides the backup. For example, descending with a Petzl GriGri (pictured right) which will lock automatically when the handle is released. It's not a real smooth ride though. I only descend on my GriGri when the situation really lends itself to it's use, otherwise I use the a normal ATC style belay/abseil device. There are other devices on the market which achieve a similar result in mechanically backing up an abseil. The Petzl Stop Descender, and Petzl Shunt (both pictured right) spring to mind. There are others out there too. Check out the Petzl web site for details on the models mentioned.

Friction Knot Above The Descending Device
This is perhaps the "traditional" method of backing up an abseil. Here we simply add a friction knot of your choice, such as a prusik, above the descending device and clipped to the harness. Your guide hand goes above the knot, and pushes it down as you descend. You have to keep the knot pushed down or it will lock. Make sure the knot is not placed so high it's out of reach. In the event that your brake hand is released the knot locks off your descent - at least in theory. Many people dislike this method because the knot can be difficult to loosen once it's been weighted. Put a beginner in this situation and they might get themselves into more trouble than it's worth. If the prusik is locked off, and you can't simply put your feet on the rock and lift enough weight off to loosen the knot, you can employ other techniques, such as wrapping the rope around your foot a few times and standing up.

Also studies have shown that in a panic situation the instinct is to grip the rope, not release it. If your guide hand is over the knot, as opposed to above it, gripping the knot may cause it to fail (depending on the friction knot you chose), thus resulting in the climber "riding the knot" all the way to the ground. Further-more, depending on the friction knot you've chosen, it may also fail if the ropes are icy, or if you've somehow built up enough slack to take a small fall, or if the knot was never seated correctly, etc, etc - you can see why many people dislike this method. It's far from fool proof. There is no guarantee that it will save you even with the above issues aside. Never-the-less this is the method I personally use if I deem a friction knot backup to be required. If I'm cleaning gear on abseil I'll also get a fireman's belay. To quote John Long in "How To Rock Climb": "If you don't know how to rappel, get a belay. If you are doubtful that you can make a certain rappel, don't make that rappel. Only if you are doubtful and must rappel, and no belay is possible, should you consider the prusik backup as an option".

Friction Knot Below The Descending Device
As an alternative to the aforementioned method you can place a friction knot (probably a Klemheist or autoblock), below the descending device on the slack, brake end of the rope and attached to a leg loop of your harness. Your brake hand goes just above the knot sliding it down as you descend. In the event that you release your brake hand, the knot slides up, cinches tight, and does the job of your brake hand for you.

This is a more recent means which is growing in popularity and appears in several books as the "recommended" way. It has advantages over the "above the device" method. The knot only has to have enough holding power to lock off the brake end of the rope, the descending device takes the brunt of the load. Also, once the knot has locked off the climber can simply regain control of the brake end of the rope and loosen the knot once more - the hassle with a jammed friction knot is removed. Also in this method the brake hand is doing everything, leaving the guide hand free to deal with obstacles if need be, where-as the "above the device" method requires a function from both hands. 

There are, however, disadvantages with this method. The friction knot must never be allowed to enter the descending device. If it does, it could cause both the device and the knot to fail. To avoid this potential hazard the friction knot must be formed with a loop of cord sufficiently small enough, or (and this appears to be the more common means), the descending device needs to be extended away from the harness via a sling. Another disadvantage is that should you abseil off the end of the rope this method won't save you, where-as the "above the device" method might (if you've very lucky). Tie a knot in the end of the rap ropes to avoid this problem. Also beware of rapping over a roof or undercut section. With the device extended away from your harness it might be possible to pin it too the rock with your weight. 

Alternatively both ends of the autoblock (friction knot), can be clipped to the carabiner attached to the leg loop, rather than having one end girth hitched to the leg loop as shown above. The former method is perhaps the more common.

Further Reading:
Prusik Rappel Safety - A discussion paper on using a prusik knot to backup an abseil from the rock climbing archive site.
Should I backup my rappel - Comments from climbers reproduced in Dawn's FAQ regarding backing up an abseil with a prusik (or similar) knot, above or below the belay device.
Rap Backup - From Karl Lews web site.
Autoblock - Backing up an abseil. From rockclimbing.com web site.
Zion Rap Accident - Knot in ends of rope failed?

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