September 2001 - Tech Tip Supplement
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
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.
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
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
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