Avoid the prussik as a rappel backup knot

Some newbie climbers think adding a prussik knot on the rope above their rappel device will act as a backup - if they lose control of the brake hand on rappel, they assume the prussik will catch them. This sounds reasonable, but many accidents later, we have evidence to show this backup does not always work.

For the prussik to slide freely, the non-brake hand must be on it during the rappel to slide it along. To catch on the rope and stop the climber, the non-brake hand needs to be off the prussik. Problem: In the event of a loss of control, our instinct is to grab tighter on the rope, not let go of it. This grabbing keeps the prussik loose, prevents it from grabbing the rope, and may cause the climber to accelerate down the rope, possibly causing injury.

A better rappel backup method is to use an autoblock knot, which is tied below the brake hand rather than above it. If the brake hand comes off, the autoblock immediately grabs the rope and stops the climber. See some of the better climbing references for diagrams on how to tie this simple knot, or this web link.

When to use a rappel backup?  More conservative climbers might say always. For others, consider a backup when: beginners are rapping, when your hands are cold, if the rope is wet, if you are rapping on a single strand or a skinny rope, if you need to swing or pendulum to reach the next rap station, if you are rapping with a heavy pack, when you can see the rope is hung up and you need to free it, or if you are not sure exactly where the next rap station is.