Knowing how to ascend a fixed rope is an important skill for many situations:
If you fall into a crevasse while traveling on a rope team and manage to remain uninjured, you can self-ascend out once a rope team partner has secured (AKA fixed) the rope to an anchor.
In a climbing scenario, if you find yourself following your partner's route but are unable to climb what they led, you can ascend after they've fixed the rope.
When ice climbing in a glacier moulin with a belay from above, if you've rappelled too far and now are faced with climbing over an intimidating overhanging bulge, you can ascend to above the overhang.
If you underestimated the length of the rope to the distance of a ledge below and are unable to reach the safe zone of the ledge during a rappel you can ascend until you find another safe zone while safely attached to the fixed rope.
Additionally, ascending is a huge part of big wall and aid climbing.
There are two main equipment options for ascending in a mountaineer setting:
A set of joined pieces of cordelette with a locking carabiner can be used, or you can use a mechanical rope clamp (like Petzl's Tibloc) which have a spring-loaded cam with teeth that allows the clamp to just move in one direction on the rope. In this blog post I will be covering how to use cordelette as a mode of ascending because it's more versatile. Mechanical ascenders are specific to rope ascension and are not useful in other situations, and they are capable of damaging the sheath of the rope is used incorrectly. The benefits of using cordelette for ascension is that each piece of equipment (the two pieces of cordelette with the locking carabiner) have other uses during glacier travel expedition settings, such as during a rope team travel as you belay your partners in and out of safe zones, during crevasse rescue situations with mechanical advantage, and as personal anchor cords, just to list a few. Since ascension is not a sought-after activity and is rather a skill set used to get you out of a sticky situation, it's best to conserve weight and bring the most versatile gear.
For the purposes of glacier travel, the AK Mountain Women plan to use two pieces of cordelette; one 3.5-4 feet in length for the waist and one 5-6 feet in length for the foot. Join the ends of each piece of cordelette with a double fisherman's knot to make two loops. Cordelette tied with a friction hitch, like the Prusik knot, to the fixed rope will hold better if the diameter of the cordelette is smaller than the fixed rope (around 6-7mm). If the standard 3 loops associated with the Prusik knot don't create enough friction to hold the rope, add a 4th or more loop to create more friction. Until the need for a fixed ascension scenario, the two pieces of cordelette can sit on the harness as designated gear, or in the case of using gear for multiple purposes, can be used on the team rope for belaying, and then taken off and replaced above the tie-in knot for ascension.
There are three knots that work for ascending a rope: the Prusik, the Klemheist and the Bachman knots (or friction hitches). I will explain ascending using the Prusik because it's the most versatile knot and can be used for belaying partners in and out of safe zones on a rope team as well as when building mechanical advantage. The Prusik knot is a friction hitch that adheres to the fixed rope when it's tightened under a load, and can be loosened to slide up and down (whereas the Klemheist and Bachman only move easily in one direction).
To perform an ascent secure yourself to the fixed rope and apply the shorter cordelette with a Prusik knot to the rope at chest level, making sure to offset the double fisherman's knot so it's not tied within the Prusik or where it will be holding weight (the knot is the weakest part of the cordelette). Attach yourself to this waist Prusik by locking it to your harness's belay loop with a locking carabiner. Next, apply the longer cordelette below the waist Prusik (again, make sure to offset the double fisherman's knot so it's not directly holding weight). This second Prusik is the foot Prusik and does not require a carabiner. The foot Prusik will hold just one foot, typically the dominant foot. If using crampons, tie an overhand knot at the base of the foot Prusik small enough so that the cordelette stays on the crampon car and does not get damaged on the spikes.
To move up the rope:
Weight your waist Prusik and make sure it's secure to hold your full weight while you adjsut the foot Prusik. You will be alternating your weight between the waist and foot Prusiks, so you want to make sure whichever Prusik is holding your weight is secure while you loosen and adjust the other.
Slide the foot Prusik up to your thigh height and weight it.
Start alternating between Prusiks by loosening the waist Prusik, straightening your leg while simultaneously helping to straighten upwards by keeping a hand on the rope above the waist Prusik. Haul the waist Prusik up to chest height and weight it, then repeat from the beginning until you've reached your destination.
Tie safety knots (overhand or figure eight knots on a bight) to the rope beneath your foot Prusik every 15-20 feet. If your Prusiks were to loosen and fail, the safety knots would catch the Prusiks.
Glacier travel is gear intensive, and often includes carrying a pack and haling a sled while traveling on skis. If you fall into a crevasse while on a rope team your chest harness would keep you upright. You can use your quick release ditch straps to take off your ski bindings easily while still keeping them attached to your boots, making using a foot Prusik for ascension possible. The Texas Prusik is an alternative foot Prusik that allows for the use of both feet while ascending, but requires a longer piece of cordelette that is less versatile.