You come to a rappel station and upon inspecting the webbing notice that there are no cuts or wear marks, and that the nylon feels soft - almost like new. But, the sun and elements have bleached the sling bone white. Is the webbing still strong enough for rappels? At what point should you replace webbing? —Richard Denker / via email
If you question the integrity of a sling, you can either apply the Law of Least Effort, do nothing and accept the consequences, or replace it. Your call, but the fact that you asked the question suggests that the webbing needs replacing. I will offer that only a fool or drunkard would rely on a single piece of gear in a mission-critical situation. Double up or die.
The fact that the sling is faded indicates little, as a sling in direct sunlight can bleach out in weeks, while a sling in the shade might be 10 years old and hardly faded. We do know that nylon is stabilized against UV radiation - sun light will bleach out the color, but nylon itself is affected at a slower rate, which depends on variables including UV intensity, which fluctuates around the world.
Kolin Powick, the quality assurance manager for Black Diamond has been testing slings he has scrounged off climbs. Powick reports that sun-bleached, dried slings that he estimated were three or more years old, and when new had a minimum breaking strength of 22 kN, failed in a range from 16 kN down to 5 kN. Sobering news considering it's possible to achieve a load in excess of 5 kN by shock-loading an anchor.
Sunlight, however, isn't your only concern. Pins and bolt hangers can cut slings. Always rig rappel stations so the sling is clipped or threaded through a smooth carabiner or lap link, and, as usual, rub your lucky charms before you kick into the void.