If you ever have to rappel down a drop that is longer than your longest rope, you will have to tie two ropes together to reach the ground. Ordinarily this means that you would rappel down the top rope until you reach the knot, then you would have to pass the knot using extra gear while you disconnect from the upper rope and connect to the lower rope. This can be difficult and scary, especially for inexperienced rappellers.
A stacked lowering system allows all but the last person to combination rappel and be lowered without passing a knot mid-rappel. You can stack as many ropes as you need to using this method and not have to pass any knots.
To rig a stacked lowering system with 3 ropes, do the following:
Feed the top rope through the anchor with a short rappelling strand and the rest of the rope still in the rope bag.
Secure the top rope to the anchor with a rigging method that allows you to lower. This could be a Releasable Figure 8 Block or a Munter-Mule Contingency or a Figure 8 Contingency or ATC Contingency, etc.
Tie the middle rope to the end of the top rope's rappelling strand. Also tie a loop in the end of the top rope that you can attach a carabiner to. A combination figure 8 on a bight with a figure 8 bend works well to accomplish this.
Attach a carabiner to the loop.
Feed the non-tied end of the middle rope through the carabiner with a short rappelling strand.
Secure the middle rope to the carabiner with a rigging method that allows you to lower.
Tie the bottom rope to the rappelling end of the middle rope.
Toss the bottom rope to the ground.
Rappeller attaches their rappel device to the bottom rope just below the knot (bend).
Rappeller locks off their rappel device.
Person at the top lowers the rappeller by releasing and lowering the middle rope until the middle rope is fully extended and tight against the knot to the top rope.
Person at the top lowers the rappeler by releasing and lowering the top rope until the top rope is fully extended.
Rappeller unlocks their rappel device and rappels down the bottom rope to the ground.
This system works best on free hanging rappels and with the anchor attachment point in the air to avoid rope and rock damage.
Advantages
A stacked lowering system can reduce risk and reduce time by allowing all but the last person to rappel down long drops without having to pass a knot mid-air.
Disadvantages
A stacked lowering system has a lot of complex rigging that introduces complexity and potential for failure. A single rope is preferred where possible.
Lowering someone with knots in the rope causes the knot to rub on the rock and can damage the rope quickly and severely.
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