Over the weekend I was climbing a multipitch trad route with bolted belays. There were four of us climbing in two teams. After 7 pitches it was getting late so we decided to back off, with the knowledge that there was an escape route at the top of pitch 5. The other pair went first and didn't find the walk off route, so they continued abseiling. They had two 50m ropes of different diameters. My climbing partner and I abbed 2 pitches then found the escape route and walked down. We got back to the base of the crag in the dark, with no sign of the other team. We grabbed some food and headtorches for them and walked back up to the top of pitch 5.
Turns out they'd ended up having a few issues with their ropes. Firstly, they had threaded the thinner, stretchier rope through the anchor. As they abseiled the thin rope stretched and the knot moved further down on the thick rope side, meaning that thin end of the rope ended up being considerably shorter. I think they had a bit of faff reaching the next anchor as a result. We met an instructor who helped them out and told them that when abseiling on two ropes of different diameter it is best to thread the thicker diameter rope through the anchor so that if there is any movement due to differences in rope stretch, the knot will come up hard against the anchor (potential warning here for cases where the knot can pass through the anchor). This was new information to me and is lesson 1.
Secondly, while abseiling they hadn't been keeping track of ends. When finishing an abseil the two strands will be nicely separated by your belay plate (provided you haven't been spinning around on the way down). The first person can keep the strands separate while taking ab gear off the rope and has some time to get rid of any twists all the way to the ends of the ropes. This can happen while the second person is abbing down- it doesn't take long and can save a lot of faff later on. As soon as the ropes are released, the twists are free to go back up the rope. This is the second issue my pals had.
With both of them at the lower anchor, they tried to pull the ropes and discovered they were stuck. It was dark at this point and they didn't have headtorches, so it was a little unclear whether this was because the thin rope got pinched in a groove near the top, the twists introduced to much friction or the knot got stuck (they were using two overhand knots). At any rate, they then found themselves having to climb back up the ropes.
For this, they free climbed one at a time with guide mode self belay. This was working nicely until they came up against the badly twisted ropes and the knot that had moved down. For the twisted ropes they untwisted them by spinning around, and with the knot they stood on a ledge, released the guide mode plate and asked the instructor who passed them to feed the rope through the anchor until the knot was back in the right place. Eventually everyone made it to the ledge and we all walked off from there.
In short, my friends faced a few problems as a result of stuff that is a little subtle and I haven't thought about before. Abseiling with different diameter ropes means that you need to consider rope stretch as well as knot choice carefully. Twisted ropes can be a pain when abseiling, but if you have to reascend the ropes they can make it much more awkward and it is worth untwisting them when the opportunity arises. Headtorches are awesome (less subtle).
I am curious to hear the thoughts of the hive mind. Have you encountered any issues with abseiling using different diameter ropes? Is there anything you would have done differently if you had been in my friends' shoes?