Avy One – Chapter 1: Topic 5

Key concepts:

  1. Understand how to convert pounds to kilograms to kilonewtons
  2. Understand the difference between a rescue load and a personal load
  3. Understand how to determine a static system safety factor (SSSF)
  4. Understand how to calculate resultant forces on slopes


Most SAR and mountain rescue agencies in the US (and the rest of the world) use the metric system when analyzing and conducting safety checks on their rope rescue systems. The metric system also makes the math a little easier once you are used to the system. Below are the basic measurements you should be familiar with.

  • 1 kilogram (kg) = 2.2 pounds
  • 100 kg = 220 pounds
  • 1 climber/rescuer with pack = 100 kg weight (this is an approximation we use in rescue)

Determining Forces on your Rope Rescue System

1 rescuer (with pack) = 100 kg of weight = 

1kN of force when hanging on rope

When climbers and rescuers are suspended by a rope, we want to know the FORCE that the weight is putting on our system components like the anchors, rope, carabiners, etc. So, when examining our rope rescue system, we use kilonewtons (kN) when discussing how much force will be applied (by the weight of the load) to our hanging body (if we fall on the rope) or to the anchors (if there is a sudden shock-loading of the system).

2 climbers (with packs/gear) = 200 kg of weight 

= 2kN of force when hanging on rope

Another way of describing the force or weight that we will see on technical rope systems would be to say we will either have a PERSONAL LOAD (1-person load about 1kN or 225 pounds) or a RESCUE LOAD (2-person load about 2kN or 450 pounds). 

 This is extremely important information that we need as rope rescue technicians to determine our rope system safety factor and to try to limit the force on our system, so we don’t damage or fail  anchors or equipment.

Static System Safety Factors (SSSF)

The SSSF is the ratio between the equipment breaking strength and the maximum force that may be placed on the equipment. SSSF helps us determine a safe working load for our rope rescue system. If your rope had a breaking strength of 500 pounds and your load was 250 pounds, your SSSF would be 2:1. Most SAR and mountain rescue teams have adopted a 10:1 SSSF for vertical rope rescue operations, where equipment failure would cause a catastrophic  (fatal or near-fatal) event.

SSSF’s were developed based on engineering principles utilized when constructing structural load-bearing materials (like bridges). Engineers would use a simple formula stating that the maximum force a structure could be subjected to should never be over 1.5-2 times the strength of the material. Thus, if you take the worst-case scenario of a rescuer with a subject in a litter who stumbles and falls during an edge transition and generates a force of 10-12kn, the minimum breaking strength of our system should be about 20kN strong. You can see here how the SSSF of 10:1 was developed (system 20kN strong, load 2kN = 10:1 SSSF).

Resultant Forces

Resultant forces are the forces you will have on your system based on the mass of the load and the angle of the slope that you are lowering or raising on. Determining resultant forces will allow rope technicians to determine acceptable loads on the rope system based on the slope angle. In other words, how many people (including the patient) can we have hanging on the rope and still be within our SSSF of 10:1.


Resultant Force Table

Let’s conduct a sample resultant force calculation. You have an injured subject that needs to be raised up a 30-degree slope. You have a team of 4 rescuers with the injured subject. You are having a conversation with the litter captain about how many rescuers should be on the litter. The litter captain suggests having 2 rescuers on each side of the litter. You tell him to standby while you check your resultant forces calculation table. If you have 4 rescuers (400kg) plus a patient (100kg) that will be a total of 500kg mass on the system. At 30 degrees, the steepest part of the slope, that would have a resultant force of 2.45 kN on your system. Your team utilizes a strict 10:1 SSSF for all rope rescue applications. Would 4 rescuers be acceptable (check the table above)?

After checking your resultant forces table, you advised the litter captain that to maintain our SSSF of 10:1. We can only have 3 rescuers on the litter with the subject. So you recommend 1 rescuer on both sides of the litter and 1 rescuer at the tail of the litter. 3 rescuers (300kg) plus 1 subject (100kg) equals 400kg.  400kg on a 30-degree slope has a resultant of 1.96kN. This would meet your SSSF of 10:1 (19.6:1 SSSF for this application).

Drop Tests & Fall Factors for dynamic ropes

UIAA standards state that maximum impact forces on a dynamic rope may not exceed 12kN.

To calculate impact forces on a dynamic rope (fall factor), use the following equation:

Length of fall/length of rope fallen on = fall factor

A fall factor of 2 is the highest impact that can be generated on the anchors & climbers.

Note:  Falling on a 1 meter static cord (like a daisy chain) can generate 15kN of force & can hurt you!

Never climb above a static line where you could fall & shock load it!