In a recent thread "Heli down in Cumbria" there was a discussion on the merits of using certain strategies when faced with worsening weather and the onset of a degraded visual environment. This paper deals with just that area.
To better understand the paper (for those not willing to plough through it), it is best to show illustrative text rather than Recommendations; firstly some text from the Foreword:While this work was being completed, a new international initiative, the International Helicopter Safety Team (IHST), was launched in 2006 with the goal of reducing helicopter accidents by 80% within 10 years. It is clear from the accident statistics that, if this goal is to be achieved, a large part of the safety improvement will need to be realised in small and medium helicopter operations. Early on in the IHST initiative, helicopter flight in degraded visual conditions emerged as a significant cause of accidents in the USA, Canada and a number of states within Europe. In the UK, it was shown to be the largest single cause of helicopter fatal accidents in a review published by the CAA (HELI-GASIL, December 2002). Small helicopters are particularly vulnerable to the hazards associated with helicopter flight in degraded visual conditions as they are not generally equipped with automatic stabilisation equipment. The UK is participating in IHST via the European Helicopter Safety Team (EHEST), which forms part of the European Strategic Safety Initiative (ESSI).
The work reported in this paper will be promoted within the IHST and EHEST which, it is hoped, will result in initiatives on an international basis to reduce and/or mitigate encounters with degraded visual conditions. In the UK, as an initial step, the CAA has already amended the Air Navigation Order (ANO) to introduce a minimum visibility of 1500 m for VFR flight, and a requirement for VFR flight to be conducted "with the surface in sight". The latter requirement is interpreted to mean "…with the flight crew being able to see sufficient surface features or surface illumination to enable the flight crew to maintain the aircraft in a desired attitude without reference to any flight instrument". An Aeronautical Information Circular (AIC) is being produced to underpin the change to the ANO, and to assist pilots towards a better understanding of the problems that can be associated with attempting flight by visual reference in unsuitable conditions.
This project has firmly established a direct link between flight safety, visual cueing conditions and helicopter handling characteristics. At the heart of the high accident rate is the inherent instability of many small and some medium helicopters which can rapidly lead to excessive pilot workload when attempting to fly in degraded visual conditions. An obvious step to reduce the accident rate would be to improve the handling qualities of these aircraft, but it is recognised that such a step would likely be impractical for many existing aircraft. Other means must therefore be found, and a number of recommendations are made in Section 9 of this report. One possible form of mitigation, not mentioned in the report, is the provision of a 'headup' attitude reference; devices such as the Malcolm Horizon have previously shown some promise and might be practical to retro-fit to existing aircraft.
One immediate gain in this paper is in the parsing of the accidents (all of which could, under a looser categorisation, be regard as CFIT) into three logical sets:Scenario 1 – cases where the primary causal factor was obstacle/terrain strikes in low level flight;
Scenario 2 – cases involving controlled flight into terrain (CFIT);
Scenario 3 – cases involving spatial disorientation and loss of control.
This ensures that the recommendations are not confined to a single solution (such as EGPWS/SVS/EVS) and are targeted at breaking the links in the causal chain in a number of places.
Further the language that is used to describe the problem scenarios is easy to understand:
It is of note that all 7 of the cases identified for more detailed examination took place under VMC and, in many, it seems that the pilot was justified in initiating the flight. Problems occurred because of the background visual conditions encountered en-route, or a sudden and unexpected deterioration in the visibility conditions. Pilots either found themselves suddenly immersed in cloud or fog where VMC flight was no longer possible (i.e. IIMC), or in a DVE condition that rendered flight using only external visual references unsafe.
For the IIMC situation, three types of pilot reaction can be identified from the data that could potentially result in a serious accident. Generally, pilots responded to the situation by endeavouring to backtrack, climb above or descend below the visual obstruction. In the first circumstance, unless they have adequate instrument training, are current and refer to instruments to effect a recovery, they can become disorientated very quickly and lose control of the situation. In the second circumstance, it is possible that the pilot will manage to maintain control of the aircraft but, because of poor situational awareness, fly the aircraft into an unsafe condition. In this case, divided attention between flight instruments and the outside visual scene can lead to a situation where the pilot fails to notice a gradual and insidious loss in height and consequently fly dangerously close to the ground. Alternatively, there may be a similar, unintended loss of speed and consequent loss of aircraft attitude stability, ultimately leading to loss of control. Division of attention is particularly relevant to low level flight where the pilot is more reliant on external cues for maintaining a safe flight path, or for maintaining position and attitude stabilisation in the hover. In the third circumstance, the pilot may attempt to establish on instruments but, because of the time needed to effect the transfer of attention, the aircraft may again be placed in a dangerous position. This is also relevant to low level flight in close proximity to the ground and obstacles.
In the DVE situation the pilot may be unaware of a loss of visual references, or be drawn into concentrating too much on external references at the expense of flight instruments. Generally speaking, the level of DVE will result from factors such as low levels of light, the presence of atmospheric haze or sun glare, lack of surface texture or features such as buildings, roads and rivers, or poorly delineated sloping or rising ground contours. These factors may combine to have a critical impact on the pilot’s situational awareness through giving misleading or inadequate information regarding the aircraft’s flight condition. For example, they may result in a poorly defined or missing visual horizon which will affect the pilot’s ability to judge and stabilise aircraft attitude, particularly in the hover and at low speeds, or reduce the pilot’s ability to detect changes in height, rate of change of height, or speed and position. Pilots have no way of knowing that they should not be relying solely on external cues, and if the cues are sufficiently degraded they may ultimately lose control of the aircraft, or fly into the ground or sea.
It is worth struggling through the 200 pages; but even if consideration is constrained to the 'Phase 1' part of the paper, and the Summaries and Conclusions, it provides an excellent background to the understanding of accidents which are prevalent in the smaller end of the operational spectrum and which have a high number of fatalities.
Jim