Mountain Guiding; Mountain Safety
An Experienced Professional

Guiding Newsletter

September 2001 - Tech Tip Supplement

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Rappelling Safety - The Autoblock

Important: This is not meant to replace hands-on instruction, nor to replace the need for hands-on practice. Some skills are necessary to implement this system and to trouble-shoot anything that goes wrong. As with anything in climbing, trying this is at your own risk! Disclaimer 1 - Keep in mind that this is a technique note, not instruction on application. Different configurations are potentially useful and it is reasonable to choose to apply variations or entirely different techniques in different situations. Disclaimer 2 - This is not meant to replace hands-on instruction, nor to replace the need for hands-on practice. Things can go wrong. A variety of skills are necessary to implement this system and to troubleshoot anything that goes wrong. As with anything in climbing, trying this is at your own risk! This note pertains to using a friction knot as a backup on a rappel, which is not the only means of providing additional security. Other methods include the "firemans belay" from below and a traditional belay from above.
		Prusik Above Device - Not Recommended !! The prusik loop goes to the harness and is on the rope above the rappel device. While rappelling the prusik is minded with one hand to keep it from locking. This has been commonly used in the past and may still be taught and/or found in common books. However, it has a couple problems. An inexperienced climber who allows the prusik to lock (grip) will find it extremely difficult to release. And the most common instinct when falling or losing control is to grab the rope, which will slide the prusik along with the out of control climber as well as burning their hands.
		Auto-Block Below Device - Better !! The method most commonly used and taught by professionals today is called the "autoblock" and consists of a friction knot (i.e. a "gripping" knot like the prusik") attached below the rappel device and connected to the leg loop of the harness. It may be necessary to extend the rappel device to prevent the friction knot from getting pulled into it.

Technique Tip(s)

You are out in the northern Canadian Rockies starting down a rappel in a summer snowstorm with gusty winds. You can't see where the rappel ends through the storm. There is loose rock around which could fall while you rappel, perhaps with some inadvertent help from another member of your team. Wouldn't it be nice to know that there is a backup to keep you from sliding down out of control if something happens to you? And to know that you can stop with your hands free before you reach the knotted ends of the rope? In this situation last summer, and in other similar situations, I use an autoblock below the rappel device.

First, let's remember that there are other solutions than a friction knot. All except for the last person can be belayed with one end of the rope while rappelling on the other strand. All except the first can be belayed from below with a "Firemans Belay". These methods all have advantages and disadvantages, and a comparison is beyond this note. Use what you decide is best given your training and experience and the situation at hand.

In the past many instructors, leaders and books used a prusik knot above the rappel device for a security backup. You will still see this in books and sometimes in use. But there are a few problems with that method. One is that if the prusik knot is allowed to grip it has the full weight of the climber on it and is therefore difficult to release. Since one common use for such a backup is for beginners this can be a major problem. (This topic was inspired a few weeks ago when I watched a beginner in a class at Smith Rock encounter this problem after her instructor set up this type of backup.) The second problem with this is that the instinct of most people, and especially of beginners, is to grab the rope if they begin to fall. If they grab the prusik on the rope they will just slide it down with them - to allow it to grip they need to let go of it.

A better method which is widely used today is to put a friction knot below the rappel device. This can be a prusik knot, although an alternate gripping knot called the "autoblock" is commonly used. The knot is commonly attached to the leg loop of the harness. As with the previous method the knot is held with the braking hand to slide it along while rappelling. Using this method the knot, if it grips, only needs to provide a braking force - much less that holding full body weight. Therefore it is easy to release.

There are two potential drawbacks to this method to be aware of. The first is that a knot below the rappel device has the potential to be pulled into the device. The best way to prevent this is to extend the rappel device from your harness to allow for more space between it and the knot. The second drawback is that it can be difficult to feed the rope through the rappel smoothly, especially on a long rappel with high friction to begin with.

This is the method I prefer to use when I feel the need for some extra security on a rappel. In situations such as that mentioned at the beginning. However, I don't always use it. If the rappel is a nice clean drop, the weather is good, and I can see the bottom clearly (with the knotted ends there) I may not use anything. What you choose to use in different situations will vary with your training and comfort level

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