A new approach to descending
Over the past 20 years, more than a dozen patents have been issued covering descender escape systems. Typically, instead of a rope, they use a steel cable that is attached to a reel. The reel is anchored to a window or other egress point and, as the person descends, some sort of a brake is used to control the reel and thereby govern the descent speed. Typical techniques include centrifugal brakes, air fans and even hydraulic pistons. In some cases, the cable is rewound for the next person, in others detachable reels are used to facilitate the movement of personnel.
Even this group of technologies is not without problems. First of all, most descender systems require a bulky apparatus that has to be permanently bolted at the egress site. Even if each user gets his own reel, he needs to wait in line to descend. Since people go down one at a time, the waiting time on higher floors may be thirty minutes or more. This may result in what experts call the clustering effect when people push each other and create confusion that interferes with safe escape procedures. Panic may result when somebody gets stuck on the way down and other people are temporarily prevented from using the apparatus.
Another problem is that once the person is descending, most systems do not permit him to slow down or stop until he has reached the ground. When one looks at the various sizes and shapes of buildings in a city like New York, it is immediately apparent that very few buildings are straight like the World Trade Center. Most buildings have setbacks, balconies and decorative protrusions. Getting stuck in any of these situations is the last thing that should happen to a scared person. This calls for a better approach.