The dangers of simul-rappelling

Question

To simul-rappel, the technique goes as follows:

1. Prepare to rappel as usual, passing the rope through bomber bolts;
2. Tie knots on both ends of the rope;
3. Attach your personal anchoring system (PAS) to your partner's PAS or belay loop (or remain tied to him in any other way);
4. Set the gear to rappel from a single strand of the rope simultaneously with your partner, who's going to rappel from the other one;
5. Weigh the rope and be absolutely sure neither you nor your partner has its weight placed on the PAS, but on the rope (standard procedure: never untie from something that's hard to untie from). In other words: make sure you're both tied to the rope and balanced only by your weights;
6. Untie both PASes and rappel down singing a beautiful song, preferably in a sonorous language like Italian, Portuguese of Russian (German and Polish songs may cause accidents);

This method is faster than usual rappelling. Many people use it frequently, and even more people have used it at least once. I like using it a lot, and one of these days a friend came to me saying that another climber got freaked out after hearing he used simul-rappelling in a daily basis. According to this guy, simul-rappelling is more dangerous than rappeling.

This got me thinking: imagine the belay is bomber and your rope is trustworthy (if those are not matched, then rappelling won't be safe regardless of the method). Then why would simul-rappelling be more dangerous than rappelling? I can only think of one situation where this is true: the weight difference between the climbers is considerable plus they're not attach to each other (as when rappelling from opposed sides on a spire). Overall, this is easily solved by remaining attached to your partner - actually, sometimes going down is only possible if simul-rappeling (like when one of the climbers is so light that the rope won't run through the brake - this happens sometimes to children).

Does someone have a clear, precise reason for simul-rappelling to be more dangerous than rappelling? Since most climbing accidents happen during the rappel and many of them are due to the fact that one of the climbers didn't actually pass his/her rope through the breaking device, I'd say simul-rappelling is actually safer than standard rappelling, since both climbers need to be balanced on the rope (using their braking devices) before lowering themselves.

Answer 1

I think it isn't fair to simu-abseiling to pull out horrific accident reports and use them to say it isn't safe, because there are just as horrific accidents with "traditional" abseiling. The only fair comparison would be if there were numbers of rappels done and numbers of accidents on both styles - I am pretty sure this kind of data doesn't exist.

Using the exact procedure and situation described in the question I agree, simu-abseiling is just as safe. However it is in my opinion not meaningful in this case to restrict the scenario to a perfect execution. There are situations where different techniques differ in safety given a defined scenario and not allowing mistakes - this is not one. I don't have numbers ready, but I strongly believe that mistakes are the prime reasons for accidents while abseiling. So what you can meaningfully compare is the complexity of a system and thus chances of making a mistake and the consequences of a mistake.

The following is based on armchair thinking: I have done plenty "normal" abseils, but only one simu-abseil (in a controlled environment for fun and the experience). It concerns "risk" which is an extremely complicated issue in itself and the tolerance for risk is a personal choice.

In my opinion the first part about higher complexity and thus options to make a mistake is not that bad for simu-abseiling. Then again for me it is unthinkable to not use a backup friction knot on long/multi-pitch abseils (ever had to untie a tangled rope during abseiling?): A backup knot completely removes the risk of unloading the rope while the other end is still loaded (you can't remove a friction knot when loaded).

What weighs more in my opinion is the consequences of a mistake. History proves that everyone can make mistakes, so ignoring this aspect would be naive. If you do a mistake (e.g. the classic: going over the rope's end due to missing knot) two people are dead instead of one.

So if you consider chance of a mistake to be equal between both options and define risk as chance times consequences you have twice the risk for simu-abseiling. This is likely also the reason why many people consider simu-abseiling instinctively as much less safe. However this is also where human risk perception usually breaks down: Low probability, high consequence. To be fair, not only human perception, this also makes it extremely hard to quantify it. Even if there was data on it, you would need a huge amount of cases to get any significant results.

And one aspect that is not safety related: If you do remain attached to each other while abseiling this is a pain. I feel like it is already annoying when just abseiling straight down. However very often I need to search for the next belay or do pendulum to reach it. Doing this in perfect synchronization appears to be an art-form very hard to master.

Edit:
I probably didn't really answer the question directly, but that also intended. Many of the above is based on arguments, a final answer is a judgment call. And everyone has to do it for themselves. In my opinion there is no reason to demonise simu-abseiling. I don't do it because I believe in keeping to one system that everyone knows. I would be feigning if saying I did it for safety, as I often leave out knots in the end (start the stoning) when going down first myself.

Answer 2

A very important point here is rappelling/abseiling is the most dangerous part of a climb. You're very exposed, typically relying on your rope alone and whatever you're anchored to and nothing else, your backups are minimal. An accident at this point is more likely to be serious or even fatal at this point then any other point on your climb. So most climbers do whatever they can to mitigate these risks. If you want to not do this to save some time (the only upside to rappelling at the same time is that it's faster), then that's up to you.

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Rappelling at the same time is more dangerous than standard rappelling, this is highlighted in this accident report http://www.rockandice.com/climbing-accidents/climber-killed-in-simul-rappelling-accident-on-the-goat-wall.

Here the people rapelling ended up in this situation:

                  /|\
                  x| |
                   | |
                   | |
                   | y

So y had reached the base of the cliff. He naturally unweighted the rappel. This caused a sudden shift in the rope though his device that he wasn't expecting. Unfortunately the rope came out of his device entirely and this caused the fatality of x (a fall of around 300 feet+).

There were likely mitigating factors here, y didn't have any backups (prusik) and they should have rappelled at the same distance keeping the rope even. But it does highlight an extra complexity that can be mitigated simply by rappelling individually.

Here's another accident where one person has again lost the rope. http://www.rockandice.com/climbing-accidents/simul-rappel-goes-tragically-wrong-reed-s-pinnacle-yosemite this article also contains some prevention information:

This accident would have been prevented if the climbers had tied knots in the ends of the rope before rappelling. Note, however, that knotted ends will not always prevent rappelling accidents. For example, on rappels where the rope ends reach the ground or a ledge, losing control could still result in a ground/ledge fall. A safer method involves both knotting rope ends and using a hands-free backup such as a prusik hitched on the rope below your rappel device, as shown in the photo here.

Simul-rappelling is rarely necessary—perhaps the only time it is actually required is to descend a spire where there is no anchor, and climbers must rappel different sides to get down. Simul-rappelling also presents extra risks. It increases the load on the anchor—a consideration in scenarios where you’re rapping on old fixed pitons, bolts or gear that could fail under high loads, such as those generated when two climbers jounce down uneven terrain simultaneously. Also, if the two climbers are using different devices or one of them is significantly heavier than the other, the rope might be differentially loaded, allowing more slack to slip through the anchor on one side and causing the rope ends to become uneven. The real danger in simul-rappelling, though, as evidenced by this fatal accident, is that if one rappeller unweights his side of the rope, either by reaching the ground or a ledge ahead of his partner or by losing control of his brake hand, the rappeller on the other side of the rope will free fall.

Avoid any scenarios where you unnecessarily place your life in the hands of another. In this case, David could simply have tied off the rope and rapped on a single line using his Grigri, then John could have rigged for a double-rope rappel. Or, knowing that the 80-meter rope was long enough, John could have, after leading, simply lowered from the anchor (quicklinks and rings) and belayed David up using a slingshot toprope.

According to Accidents in North American Mountaineering in 2013, rappelling accounts for a high percentage of climbing fatalities every year. Hedge your bets by practicing safe rappelling: knot your ropes, use a hands-free backup and take responsibility for your own safety.

I remember reading another incident (can't find the article) where a mitigating factor was the use of a gri gri. Again the people rappeling we're uneven and this caused one of the gri gri's to slip. The unevenness accelerated the rope slip though the gri gri and again one person died.

I'd say simul-rappelling is actually safer than standard rappelling, since both climbers need to be balanced on the rope (using their braking devices) before lowering themselves.

This is exactly why it's not safer, both climbers need to be balanced on the rope, at all times, evenly, or else someone falls. If your on your own you have control of both ropes and it's even because it's just you controlling both at the same time.