Activities The Great Outdoors ATC Device for Belaying and Rappelling in Climbing Share PINTEREST Email Print Kitti Boonnitrod/Getty Images The Great Outdoors Climbing Gear Basics Highest Mountains Skiing Snowboarding Surfing Paddling Fishing Sailing Scuba Diving & Snorkeling By Stewart Green Stewart Green Stewart M. Green is a lifelong climber from Colorado who has written more than 20 books about hiking and rock climbing. Learn about our Editorial Process Updated on 03/07/19 An ATC or Air Traffic Controller is a type of belay and rappel device manufactured by Black Diamond Equipment. It is a tubular device, which gives it more area and steeper angles for creating friction and stopping power than a Sticht plate belay device. Tube devices are superior to plates for rappelling since they allow precise control of your descent speed. They are made out of aluminum. Development The ATC is the most commonly used type of belay device, and so the model name of ATC has become synonymous with tubular belay devices, much like Kleenex standing for facial tissues. The original Black Diamond Air Traffic Controller device debuted in 1993, designed by Chuck Brainerd, The improvement over plate devices is that plates could move down and lock against the carabiner when the rope was pulled hard. By using a tube instead of a plate, the slots were a half-inch above the carabiner and now the rope could be fed under tension. This made belay mechanics much easier. The sharper edge also generates more friction, giving the belayer better control. The ATC-XP is a variable friction device that lets you control how much friction and resistance you want for different situations when you’re belaying or rappelling and gives greater control when you’re using ropes of different diameters. It has deep cleated V-grooves on the high-friction side for belaying heavier climbers and using thinner diameter ropes in ice and snow conditions. You can use the smooth low-friction side to belay lighter climbers or make quicker rappels. However, it doesn't have an auto-locking feature as some other brands and models do. Further development includes the ATC-Guide, which has an auto-blocking feature, adding the ability to belay a follower directly off the anchor. This feature requires attention to the manufacturer's instructions to do correctly. Use To use an ATC belay device, a loop of rope is threaded through one of the slots. Then a carabiner is passed through the loop of rope and the keeper loop of the ATC. The carabiner is then attached to the belay loop of the belayer's or rappeller's climbing harness. When rappelling, one end of the rope is attached to the anchor, while the other is held in the rappeller's brake hand. While the ATC has two slots, if you only are using one rope, you use only one slot. There are two slots for cases where you are using two ropes. You do not thread the same rope through both slots. The biggest mistake that can be made is for the climber not to get the carabiner threaded through the loop of rope. This can happen due to inattention, clothing getting in the way, bad weather conditions, etc. Without the carabiner in place, the rope will simply come out of the ATC rather than providing any friction to halt a fall or slow a descent. If you are using two carabiners or two ropes, as is done in different situations, it requires more attention to make sure both rope loops go through the carabiner or carabiners.