I came across an interesting looking device on the Petzl website. As per the description,

the mobile fall arresters follow the user automatically as he moves, whether on an inclined or vertical surface. In case of a shock load or sudden acceleration, they lock onto the rope and stop the fall

It seems to me this sort of device might be potentially useful to arrest a fall into a crevasse, or even while climbing on fixed rope (smoother than using a jumar or prussik which needs to be manually pulled up continually). Has anyone heard of this being done in practice? If not, does it seem viable to be tried out for such applications?

  • 1
    You noticed that was in the Professional section of the website right?
    – endolith
    Jan 19, 2020 at 22:46
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    Yes of course. And the demo video shows that too. Hence the wording of the question "it seems to me this sort of device might be potentially useful......does it seem viable to be tried out for such applications"
    – ahron
    Jan 20, 2020 at 1:45

2 Answers 2


Professional use

These devices are intended for professional rope work. When doing rope access work, there are typically two ropes involved. One is the working line which is loaded with the worker's weight. The second rope is a backup line which is only loaded in case the working line fails. An arrester device is running along on the backup rope and moving up and down the backup line with you. If you fall because your main rope fails, it will lock and catch you. As this may easily result in a factor 2 fall on a static rope, a shock absorber is used with such devices. I have used such a setup when doing route-setting and it is really working fine.


However, in mountaineering, the situation is a bit different. For weight reasons, there is typically only a single rope fixed[2]. While mountaineering, one is always moving. When following a fixed rope, there are 3 possibilities:

  1. Going up: A simple ascender is sufficient, if the fixed rope section is short enough a prusik knot will do just fine as well.
  2. Traversing: No need for an arrester at all. A locking carabiner clipped into the fixed rope is enough to prevent a bad fall.
  3. Going down: Rappel the fixed rope. As there is no backup rope, an arrester would not be helpful.

For mountaineering, I cannot really see a use for this.

Rock climbing The only potential use I can see is solo top-rope climbing, for example practicing the crux of your project. In this case, one is typically self belayed by an ascender. If there is the possibility that the rope is running over edges [3] and might break, a backup line can make sense. The fall-arrester would allow to climb up and down without any hassle and still provide more security. However, the safety standards are a bit different in climbing, mountaineering, and professional rope work. Therefore I have never seen anybody actually use a backup line in this case. Doing a rebelay [4] is typically sufficient for this case as well.

[2] Meaning a single rope that one can trust. Old ropes from last year's expedition do neither count as a primary line nor as a backup.

[3] In case of sharp edges, a rope protector should be used

[4] This means fixing the rope to an intermediate piece of gear or bolt after the potential sharp edge with minimal slack. The weight is now on this piece and the rope will no longer rub along the edge. This is nicely explained in Dave McLeods video on self belay


This kind of device is rarely used in sports as it is only useful when used with fixed ropes. You mentioned crevasses: The rope is not fixed here and you are tied in, so it is hard to image how such a fall arrestor would help.

However, one kind of climbing frequently uses vertical fixed (steel) lines: Via Ferratas! In typical Via Ferrata kits, shock absorbers are used since falls will generate big shock loads. Skylotec makes a kit that basically has a fall arrestor on one arm of a standard Via Ferrata kit. This device seems inspired by the professional devices you mentioned.

(Note: I have never used this device, so I can't comment on how well it handles etc.)

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