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Permalink Reply by Ben Waller on June 17, 2012 at 4:15pm

A good rescue, but there are much safer ways to rig the aerial ladder as an AHD (artificial high directional).

 

The belay should never be run through the AHD pulley system - a failure of the pulley or anchor sling will create a huge amount of slack in the belay, shock load it, and in this situation, probably drop the victim back into the water.

 

If the main line is rigged like this with only a direction change at the ladder tip, it actually doubles the down force on the aerial.  That can easily overload the aerial, even if the ladder doesn't get a shock load or a torsional load from the victim and/or the water pulling on the rope system.

 

A better rig is a 2:1 MA with the fixed end anchored to the ladder tip.  The dynamic end of the 2:1 should be run down to the load, then back to a direction-change pulley at the ladder tip, then redirect the rope down the aerial to another direction-change pulley at the turntable.  This cuts the dynamic line force by half, then redirects it in a way that does not create additional down force on the aerial; it simply puts the ladder box into compression, which it is designed to do.

 

Once again, a good rescue.  This kind of rescue can be done with a better safety margin,however.

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Permalink Reply by Rescue 2 Training on July 13, 2012 at 1:05pm

A little late, but...

A failure of BOTH pulleys and anchor slings would create a huge drop. If you can find me a case where this has happened, I'll buy you a beer.

Ladder failure...Maybe if you have an extreme torsional load

Ladders are designed as a cantilevered truss; with the bottom rail in compression and the top rail in tension. The "ladder box" is not designed to be in compression.

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Permalink Reply by Ben Waller on July 14, 2012 at 11:18pm

I'll take that in reverse order...

 

1) Aerial ladders taken as a whole are essentially a 3-sided box or 4-sided box, depending on whether they're a straight stick or a boom like a Sutphen tower. Regardless, they are indeed designed to be put into compression.    When the aerial is fully elevated, it's at an angle of around 75 degrees.  Even with no additional weight on the ladder, the weight of the fly sections puts the lower sections and the bed section into compression.

 

The rigging method I described takes advantage of this and removes the inverted 2;1 used in the rescue under discussion.  That lessens the chance that the rope system will exceed the ladder's tip rating, regardless of the ladder position.

 

2) The potential failures are not limited to hardware - a rope can fail, too.  Regardless of what fails, a single element failure in the main line or main line rigging is still going to create a drop, as the belay line will elongate when it is shock loaded by the main line/main line rigging failure.  Less rope in the belay means less stretch, and less chance that the load will impact the ground, or in this case, the river. 

 

Dropping the load into the river is an excellent way to torsionally load the aerial ladder, since it injects multi-dimension forces into the equation - gravity and the current.

This can also result in the rope system becoming the method that kills the patient if the load (patient and stokes basket) drops into the water. 

 

Speaking as someone who has done a very similar rescue, once again, I'm happy that this rescue was successful.  I stand by my point that there is a safer way to do it, though.

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