I have received the OK from Mr Bill Teel, of TRW, to quote his memo:
"Thank you for your input and concern. TRW does not advocate the use of fast roping from the hoist hook for two reasons. The first is that fast roping can produce large shock loads depending on how quickly a person stops. The hoist was designed to minimize these loads by providing "soft" acceleration and deceleration rates when starting and stopping. This reduces transient loads from being imparted to the hoist cable and structure.
When using the fast rope method, the hook is horned and the load is reacted by a short (less than 18 inch) section of cable between the hook and the drum. If a transient load occurs, the short cable length is stiffer and tends to lengthen the cable when reacting the instantaneous load. This action tends to "unwrap" the cable helix. This is much more damaging to a cable than if the same shock load were applied when 10 or 20 feet of cable were extended. Consequently, the cable is more susceptible to damage and unwrapping with a short cable length, just as you have observed.
Worst yet, shock loads tend to break the inner wires first, even though the outer wires may still visually appear acceptable. This is due to the relative motion that occurs between the inner strands/wires. As time and use progresses, the cable will begin to permanently stretch or neck down in a localised area. By the time this occurs, the cable has lost much of its strength. This is why TRW strongly advocates inspecting the cable frequently including measuring the cable with a micrometer.
The second reason TRW does not advocate fast roping from the hoist hook is that this produces a possible single point of failure. When the fast rope is attached to the hook, the hook is only reacted by the steel hoist cable. If the cable were previously damaged (again note that the damage could be undetected, internal damage), then a large shock load could cause the hoist cable to fail and there would be nothing to react the person attached to the fast rope.
For these reasons TRW does not recommend fast roping from the hoist hook. TRW recommends fast roping from a structural feature only."
Please feel free to distribute this memo as widely as possible.....as it pertains to all company's hoists with a 3/16 diameter cable.
Sincerely yours,
WIlliam Teel
Senior Design Engineer"