Fall from cliff on Dyneema

The objective

Here we will analyze accident happened in South Africa during shooting HBO series by Ridley Scott. Tracers stunt vest got damaged and we will look into reasons and consequences and try to understand what was the problem.

video: FF1 fall on Dyneema rope, no shock absorbing devices
Source: Video from Scotty Kirkbride
gallery: Inspection after fall
On photo: Tracers stunt harness (model from year of 2019) after cliff fall from the video. We see that webbing is fine but fabric is torn. Is it good or bad? Follow the analysis below.
calc: Calc for the cliff fall
Related:Impact force in a fall

Let’s calculate the approximate load on falling from the cliff:

t (impact time) on Dyneema: 0.0175 sec
(no shock absorbing devices were used in the system)
g (gravity) is constant: 9.8 m/sec
m (mass): let’s say 75 kg
h (height of the fall): 1.75 m

F = $\frac{m*\sqrt{2g*h}}{t}$
t (Impact time on Dyneema) =
g (Gravity is constant) =
h (Height of the fall) =
m (Mass) =
Result:

Result is in Newtons.

Approximate load in the cliff fall accident

We have calculated the load of 25kN here but this is for ideal fall conditions.

Let’s deduct shock absorbed by:

  • broken stitching/fabric: ~6kN
  • little pendulum, body and muscles stretching, nylon in harness: ~9kN

We can roughly estimate based on our experience that webbing in the attachment point was loaded with the force around 10kN. Stitching and fabric as well as the body acted as absorber.

This type of load is called shock load. Here we two and half times exceeded stunt vest WLL of 4kN.

Cliff accident analysis

To understand the reasons of the accident we need to fully understand the task. But if you need a fall like this you should get proper rope for dynamic load or use shock absorbing devices or brake bar. To understand how big is the difference in loads between dynamic ropes (sport climbing nylon ropes) and ultra static ropes (Dyneema, Technora) please follow this link: Impact time

Top side point is definitely bad attachment to absorb the shock with the body. Helmet is adding some load to the neck as well. Luckily the stunt performer is a good athlete and didn’t get any serious injuries.

If fabric or stitching would not torn then webbing would take another ~6kN of load. It could lead to webbing breakage.

The fact that stitching and fabric are weaker than webbing was very important on first years of our manufacturing. In this case fabric and stitching worked as absorber. Don’t put your life in danger with use if improper rigging techniques. Understanding of the physics and anatomy are the best friends.

Take care and stay safe.

Solution example

Do you want to see how you can get only 3.5 kN on nice looking fall? Check this video with unwrapping stunt made with 2.5 meters Nylon sling where we get NO shock load in the system: Unwrapping stunt impact.

Sources 

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