Enhanced cognitive performance and visual-motor skills are key attributes of a good pilot; night flying with the use of night vision devices (NVD) tests these abilities to a greater degree, through reduced field of view, altered depth perception and a monochromatic image of the external world.

Societal and professional disruptions to the circadian rhythm, and the body’s natural demand for sleep, compound the difficulties of night flying. What follows is an anecdotal view that there is an inevitable interaction between medical science and the aviation industry, and that an educational and financial investment into NVD, now more than ever, is paramount for safe and effective armed and emergency service helicopter operations. 

Unique pilot skills

Enhanced natural cognitive performance and visual-motor ability are both assessed prior to commencement of flying training. The delta between this enhanced ability, and the required ability to operate, is assessed qualitatively throughout training known colloquially as ‘spare mental capacity’. It is crucial to the effective and safe operation of aircraft to achieve a measurable outcome or dynamic task.

Since the birth of the aviation industry, research and development has consistently focused on automation, which has drastically improved flight safety and the capability of aircraft – full-axis autopilot systems and full authority digital engine control (FADEC) have eased the handling characteristics of aircraft, while instrument landing systems (ILS) have enabled aircraft to land in extremes of poor visibility. The uncredited common effect of the aforementioned systems is increasing spare mental capacity of the operating crew; the same can be achieved by procuring and researching NVD.

Engage hover

The principles of hovering a helicopter, from the perspective of our grey matter, is the processing of information acquired visually of the outside world and the aircraft flying instruments while simultaneously employing visual-motor skills.

A perception of the helicopter’s position and relative velocity in space is built by selecting and scanning references, both in the fore and background for all three-dimensional axis. For example, to hold a stable hover in height, a pilot could select the top of a lamppost in the foreground and its relative position against a building in the background. If the relative position is constant, the helicopter is stationary in height, though this must be cross-referenced against either flight instruments or best judgement by rearcrew to accurately determine a safe hover height. If the position of the helicopter is not stable, a rate of change in the relative position of the references will be apparent and a control input is required. 

A stable hover requires delicate and timely control inputs to compensate for changes in relative wind velocity, centre of gravity (medical team movement within the cabin or loading casualties onto a winch) or simply the inherent instability of a helicopter. This requires a sufficient scan rate (timely movement of the eyes) combined with accurate perception. Every helicopter pilot can relate to an occasion when their scan rate had room for improvement, especially when first learning to hover!

Despite the advancements of automation, the principle of scanning references and processing information acquired visually is still prudent to all phases of flight where use of the autopilot is either unavailable, unsuitable or simply because it’s commonly considered good airmanship to not delegate authority to the autopilot entirely. Such examples are approaches to and from the hover in unlit confined areas or mountainous regions – often places air medical or coastguard helicopters are required to land. 

Why is flying on NVD more difficult?

NVD offer the ability for aircrew to operate at reduced height and terrain separation minima comparable to those for daylight flying, ultimately allowing helicopters to make approaches to land in mountainous and austere locations at night. Whilst there are several types of equipment developed to enable this capability, this article will focus on the helmet-mounted binocular night vision goggles (NVGs) commonly employed by UK armed forces and emergency service helicopters.

Considering the human mechanics of flying a helicopter as described above, NVGs impose limitations on both the visual scan rate and visual perception; image clarity, reduced field of view (FOV) and illusionary effects. The binocular design of common NVGs reduces the FOV to approximately 40 degrees. To select and scan references requires movement of the head, rather than movement of the focal point of the eyes. This reduces the timely scan rate and / or the opportunity to visually perceive the outside world. This issue is further compounded with the weight and mounted position of NVGs, which restricts movement of the head and places strain upon the muscular system, primarily the neck. With practice, a suitable scan rate can be achieved for safe and effective flying, but at the expense of spare mental capacity.

Secondly, timely visual perception is more difficult while wearing NVGs due to reduced image clarity and illusionary effects. Common, widely available NVGs provide aircrew with a monochromatic image of the world that relies on contrast vision, rather than colour vision, for visual perception. Thus, the constructed image is dependent on the reflectivity of a surface, rather than colour; this can manifest in various hazards when low flying or making an approach, such as hidden ridges or unseen obstacles.

Additionally, NVGs construct an image that is a finite distance away from the retina, as opposed to an image constructed by the retina itself, thus, objects appear further away. Known as Emmert’s Law; an image of an object that is perceived as further away is interpreted as being larger than an identical image that is perceived closer. The opposite is also true, which manifests as an overestimation of distance for NVG users. Notably when operating in confined areas where close obstacles exist, an overestimation in distance could cause an infringement of safe separation or minimum separation clearance (MSC).

An optimal design for NVG could be one that allows movement of the eyes independent of head movement, and an increased FOV without a degradation of image clarity or increase in illusionary effects. Manufacturers already recognize the limitations described; panoramic NVGs increase the field-of-view and white phosphor NVGs increase visual acuity and contrast. Both signify a decisive step in the right direction, but these lines of research and development require continued user demand and investment. Perhaps then, a night vision system comparable to natural daylight vision would be available, which would undoubtedly optimise night flying capability 
and safety.

Night flying culture

Cultural limitations are also worthy of mention. Well-established methods of flying training teach day flying first. This apparent logical approach results in inequality of experience; few pilots will achieve night flying hours equal to those flown by day, resulting in relatively little experience in an inherently difficult skill, which is required just as often. For air assets required to operate 24 hours a day, half of all flying hours could be attributed to night flying, though reality may favor either day or night flying. Helicopter operators and aviation authoritative bodies often stipulate a minimum amount of night flying hours to ensure aircrew remain proficient. Seldom does it specify it should be conducted in a handling or non-handling role, or that it should equate to 50 per cent of the total required flying currency.

Irregular shift patterns and flying at night disrupts the body’s natural body clock, the circadian rhythm. Even with an adapted sleep pattern, external cues of time, meal sittings and darkness can continue to confuse the circadian rhythm. Professional and societal constraints, such as childcare, human resources and operations staff availability, meetings or a call with the bank manager, place pressure on aircrew to adhere to a typical working day; not only does this prevent the circadian rhythm from adjusting, but aircrew also experience a greater internal sleep pressure when night flying. Whilst aircrew are usually restricted to a Crew Duty Period (CDP), similar in principle to drivers’ hours for logistical companies, anecdotally, these pressures contribute to an increased state of tiredness when night flying; arguably a skill that requires elevated alertness.

Clearly, crews held on a readiness state responding to emergencies have an erratic and unavoidable flying schedule; providing a regulatory framework, CDP protects aircrew from fatigue, ensuring crews respond only to calls within a period of wakefulness, though this is an ideal. Often in the case of a night shift, a CDP will be skewed to the right of total time awake. Training and currency flying provides greater flexibility to the CDP, allowing crews to arrange their CDP at the convenience of other pressures. Whilst night flying for training and currency can take place as soon as it’s dark, and the risk and perceived pressure of the task is much less than operational flights, flying often becomes the final activity in the CDP window, or rather, a CDP will be arranged so this is the case.

Typically a CDP concludes with flying activity, at a time where the sleep pressure is greatest and the wake drive at its lowest, having made way for the morning school run and engagement with other colleagues working day shifts only. The facilitation of an aircrew working routine that discourages night flying to take place in a state of increased tiredness, and professional development that encourages a greater emphasis on NVG flying experience, will aid competent and safe operations.

Night flying requires increased safety and skills

NVGs have undoubtedly increased the capability and safe operation of military and emergency service helicopters overcoming fallibilities of the human sensory system. However, night flying remains an inherently difficult skill; alleviating some of these difficulties requires education, investment and a comprehensive dialogue between aircrew, the NVD industry and medical professionals.

The measured effectiveness of helicopters is the ability to provide a deliverable capability in a safe and timely manner. Until such time as holistic automation is viable and safe, addressing the issues of human sensory and visual-motor limitations is necessary to advance the effectiveness of helicopters, and the safety of the crews, who are operating at night.