Passing A Knot On Abseil

It's dark, cold and raining. You're bailing off an epic multi-pitch route. Body fatigued and mind numb. Your buddy has already descended, leaving you alone. His voice trails off in the wind. You've joined two ropes to achieve one long, desperate rappel to the safety of the ground below. What the heck, you can always come back tomorrow and fetch the gear. As you fly down the steep descent, the free hanging rope lashing wildly in the grips of the storm, a nagging thought suddenly occurs: you've never passed a knot on abseil before.

This is not a topic to be taken lightly. It doesn't take much imagination to realise the consequences of a stuff up are very serious indeed. I've read of at least one instance in which a climber died in this manner. I'd not be surprised if there were several other incidents. I think you'll agree that high off the ground is not the best place to practice the technique for the first time. Never-the-less there are many methods to pass a knot, and if you think about it for a while you could probably come up with a reasonable solution.

Passing A Knot Using A Friction Knot Above The Device

Here's one method that was taught to me. I make no claim that it's accurate. (See full disclaimer). As with the other techniques described on this site, consult a good book (recommend Self Rescue - Good book by David J. Fasulo), or seek qualified, in person, instruction beforehand. This method assumes you already know how to abseil, tie friction knots, tie yourself off with a figure eight on a bite, etc.

Step 1: Rig a friction knot (eg, Prusik, or Autoblock), above the descending device. See: backing up an abseil, for details.
Step 2: Abseil on down to the point just before the knot joining the two ropes. The knot might be there for another reason, but for readability I'll henceforth refer to it as "the join".
Step 3: Weight the prusik, allowing you freedom to work with both hands and tie a backup figure eight on a bite, a reasonable distance below the join in the slack line. Clip this to your harness with a locking carabiner.
Step 4: Remove the descending device from it's position above the join and reattach it below the join, clipping to your harness as for abseil.
Step 5: At this point you can decide if you want a backup for the remaining portion of the abseil. If you're buddy is down below you might sing for a fireman's belay, or if not, perhaps rig a friction knot below the descending device. See: backing up an abseil, for details
Step 6: Take control of the brake hand as for abseil, then unweight and remove the prusik that you've been hanging off since step 3 above. If you're on a ledge or a slab this might be as simple as standing up. If the descent is free hanging you might need to loop the rope around a foot a few times and carefully stand up on it. However you achieve it, carefully transition from hanging off the prusik to being back on abseil below the join.
Step 7: Abseil on down to your backup figure eight and carefully untie it. This is where the backup from step 5 comes in handy. Continue down as per normal.