Perception is everything
Allowing history to repeat itself is something that, as an industry, we are quite good at doing – and the continuation of rotor-strike events is something that has been exercising the safety workings of the mind of Claude Vuichard. And he has a point.
According to the EASA annual safety review published in the summer for commercial air transport operations, the most common type of accident for the period 2020-2024 was collision with obstacles during take-off or landing – 2024 being a particularly bad year for this type of accident.
Unsurprisingly, given the very high percentage of off airport/heliport operations, the lion’s share of these events occurred in relation to HEMS operations.
According to Vuichard, these events that involve catching a rotor, be it main or tail, generally come down to pilot perception of where the rotors are in relation to obstacles.
Sometimes this is based on the illusion created by the blur of the rotor disk viewed ahead.
It is a human trait to assume that we are the centre of everything, I suppose, when the reality is that rotors stubbornly continue to rotate about their hub rather than the pilot, thus distorting the real clearance especially to the sides of the aircraft.
The illusion can be quite extreme, using a popular HEMS platform like the AW139 as an example.
To a pilot in the right-hand seat, the perception is that the blade arc is 4.7 metres ahead, which is accurate, leading to an assumption that it is the same all round, when the reality is that the arc on the right side is actually 6.8 metres from the pilot eyeline and 7.9 metres away – quite a difference.
While knowledge of the illusion provides some benefit, and arguably recognition of the phenomena may go a long way towards reducing risk, Vuichard contends that at airports and heliports clearer markings around structures like fuelling stations further mitigate the risk, while at other sites a fully trained marshaller showing you where the disk arc actually is or providing guidance will also go a long way towards risk-reduction.
Another frequent causal factor is the perceived threat of collision for one rotor, leading to a collision by the other.
Imagine a pilot lifting out of a confined landing zone being concerned with power or telephone lines they can see ahead, thereby allowing the aircraft to drift a bit further back “to be on the safe side” and thus the tail rotor contacts a tree limb behind the aircraft.
This of course may be a more problematic issue, since sometimes tight LZs cannot be avoided.
The solution here, when assistance may not be available from rear crew members in the HEMS scenario – who may be otherwise engaged in patient care – is to revert to old school techniques like placing markers in the form of sight transits to act as guides until safely clear of obstructions.
