mikev wrote: ↑
Tue Aug 13, 2019 2:23 pm
Where the dynamic amplification comes in, is enhanced vibration and resonance because there is now a constant airflow over the rope and the flag towed behind etc. This is not part of the design criteria for rock climbing.
My point is not exclusive to the rope, also inclusive of carabiniers and/or other rock climbing equipment and methods whereby aluminium carabs may be hooked up to steel hooks/clamps, resonance and vibration from the airflow over both rope and flag will be directly impacting on that hookup, dissimilar metals will have an enhanced metal fatigue effect, hence causing failure.
What a tragic accident.
Without any specific information, one will have to go back and consider what are the most likely causes of high angle and helicopter rescue accidents. The details may differ but the overarching category of the most common weak link is the same as in aviation: human factors. And that should be the prime suspect here too. This could range from choosing the wrong equipment, rigging/connecting it incorrectly, neglecting inspections, complacency etc.
The wind loading of the flag would be very different from normal what would be the case for a short-haul in a rescue context (which is effectively an external human cargo slinging operation) but maybe the banner towing guys can help with some indications of the kind of forces/issues that can be expected.
To me there is nothing in the video esp. the moments leading up to the accident that gives any hints as to a problem developing. There also does not appear to be any significant jerking/bouncing or cyclical loading of the rope (or cable). To me, it appears that the complete assembly (flag + humans + rope (or cable) etc.) suddenly and without much warning separates from the helicopter. What remains attached to the helicopter (or not) will probably be key to understanding what happened and where to go and look for the cause.
Speculation: If one assumes (a big assumption considering the kind of problems in that country) that proper equipment has been used (i.e. along the lines of accepted practices), it would be very strange to see the outright failure of the non-helicopter complement of the equipment being used here as the cause of the accident (ie. assuming appropriate equipment for the job was used and the equipment itself failed despite it being used and rigged correctly). My thinking: short hauls are a relatively well-understood technique under many combinations of longer and shorter ropes, different weights, durations and distances and most of the equipment limitations are, relative to helicopter rescue operations, mitigated to an extent that permits it being used in the context of live human loads. However, this is but for one very important and overriding characteristic of helicopter rescue techniques: there are many single points of failure and very little redundancy in the overall system to safeguard against a failure of any one of the components. Any weak link in the chain can cause the catastrophic failure of the complete system/operation.
The human factors that could be at play in this type of exercise are many-fold: you typically have more organisations involved, like many displays, there are time and budget pressures, there more people involved (and importantly typically from different organisations), there is a higher reliance on good communication, and the list goes on. Compared to a hoist style helicopter rescue, short-haul has a much higher risk profile despite it on the surface appearing to be a very similar style operation. Whereas terrestrial rescue has developed many safeguards against a failure, helicopter rescue has little to mitigate the failure of one of its critical components: humans that make independent decisions and judgment calls.