The aspects of the human eye with particular relevance to flying at night are how the eye works in low light and how long it takes to adapt to a low light situation.
The eye contains two types of light-sensitive receptors known as rods and cones. In the centre of the retina is the fovea which is made up entirely of three types of cone, moving out form the centre of the retina the cones are replaced progressively by rods until the periphery of the eye where there are only rods. Cones are colour-sensitive and used for direct vision in good light, this is called photopic vision. Rods are used in low light situations, such as night flying and have no sensitivity to colour, the vision provided by rods is called scotopic vision (Hawkins, 1987).
Below is a graph showing the spectral sensitivity of the 3 cones (colored graphs) and 1 rod pigment (black graph). Note that the rod pigment is more sensitive to shorter wavelengths than the red and green cones which dominate day vision.
(Image retrieved September 9, 2009 from University of Florida)
When the light intensity entering the eye changes, the eye adapts, adaption takes some time and the elapsed time increases with age. There are two adaption methods used, the first is an adjustment by the pupil to allow more or less light into the eye, this is a reasonably crude adjustment and has variation capacity of only 30 times, while the eye has a working range of over a million to one (Hawkins, 1987). The second process involves sensing passing from the cones to the rods. This is a progressive action and can be seen as the break in the curve below. This occurs as low light vision with the rods involves a pigment called visual purple (rhodopsin) (Hawkins, 1987, p.111). This pigment is bleached in bright light and takes time to adapt to lower light levels (Hawkins, 1987). During the adaption time the cones cease to function.
A secondary consideration of using rods for vision is that due to their position around the eye the centre of the eye becomes blind and it becomes important to use peripheral vision. This means not looking directly at visual targets, to allow the rods to perform the sensing task (Hawkins, 1987).
Note also in the diagram below that to become fully adapted takes about 40 minutes and younger subjects (the lower part of the curve) are able to sense (or see) in a lower light intensity than older subjects.
(Retrieved September 9, 2009 from University of Florida)
For dark adaption it was originally believed that red light was best and wartime crews prepared in a red lit ready room. The red light was also believed to be less distinctive in the cockpit when viewed from the outside (Hawkins, 1987). Following World War II, studies found low colour-temperature white was only marginally less effective and acceptable for civil operations (Hawkins, 1987). According to Hawkins (1987) a more recent trend has been a move to brighter white light as it causes less drowsiness and fatigue. Brighter cockpit lights also provides protection to the crews vision during lightening storms, where full dark adaption can disrupted in a fraction of a second, leaving the crew temporarily blinded.
For light, general aviation aircraft, the standard cockpit light is still a red flood light with some instrument pillar or back lights. The general aviation pilot should therefore pre-flight and prepare early, preferably during daylight before avoiding bright lights prior to flight.
A secondary consideration is the use of red colours on charts which disappear under red light conditions. To minimise any potential problems, charts should be studied under normal light and cross checked with a shielded torch in flight.
Another consideration for pilots, particularily those who fly at night, is colour blindness. Colour blindness is a condition where a person can not identify of differentiate different colours. This can be a problem during the day but is definately a problem at night. The correct identification of coloured lights is critical for situational awareness (different aircraft light colours indicate the orientation of other aircraft), navigation (identification of coloured hazard lights) and aerodrome operations (approach lights, taxiway lights etc). A pilot who is diagnosed as being colour blind may not be allowed to fly, may be required to stay out of controlled airspace, may be restricted to daylight flight only or may be required to undergo a flight test to demonstrate the ability to interperate and recognise coloured lights.
This illusion occurs when a pilot looks at a steady single light source. The light may appear to move or ‘dance about” and is more likely if there are no other lights in the vicinity (Ewing, 1993, p.44). The illusion is created by the brain attempting to make sense of the single point of light by moving it about. This illusion can disconcerting but once understood otherwise inconsequential.
Night central blind spot:
Caused by the fact that at night rods are used for vision and they are located around the periphery of the eye. If a pilot stares at a dim point of light, the light may disappear (Ewing, 1993). By maintaining a scan and looking to the sides of the target light source, peripheral vision will be maintained. Ewing (1993) also notes however that bright lights will still be picked up by the rods.
Relative Stationary Lights:
In the same way that another aircraft which remains stationary on the windscreen relative to the pilot indicates that the aircraft are on a collision course by day; this situation is paralleled at night by the relative position of other aircraft lights. If the lights do not move, then the aircraft are on a collision course. If you are sure it is another aircraft, or think that it might be, then change course or altitude before attempting to confirm or make contact. Note that some stars appear to strobe or change colour and can be mistaken for aircraft lights.
Empty Visual Field “Blindness”:
This situation occurs when the eye is not simulated and encouraged to re-focus. The eye will transition to a resting focus of 3-4 metres. To avoid this maintain a scan that periodically involves close objects (the instrument panel) and further points (the wing tip and further afield) (Ewing, 1993). This illusion can also occur during the day in cloud, hazy conditions and over vast, featureless areas such as snowfields, deserts and water.
Is reported to occur during flight in areas of low visual stimulus, often with a low workload, including at night (Ewing, 1993). Ewing (1993) states that solo pilots note a feeling of detachment and floating free of the aircraft, or that the aircraft feels like it is balancing on a knife edge, about to fall off. This feeling is short lived, and pilots who are more anxious than average may be more susceptible and it is relatively common in situations of low visual stimulus (Ewing, 1993).
Black Hole Effect:
A common situation that is especially prone to developing when on a long straight in approach over the sea or unlit terrain, Black Hole Effect is something all pilots should be aware of. The judging of one's height on the approach path relative to terrain is difficult and can be exacerbated when lighting or terrain exists beyond the runway. This creates the effect of being to high on the approach, causing the pilot to steepen the approach and potentially resulting in impact with terrain. If encountered, this effect must be ignored and pilots should make use of PAPI or VASI guidance lights to ensure the correct approach path is maintained.
As most general aviation pilots will conduct their night flying at the end of a “normal” day, individuals should be aware of their personal requirements for sleep and rest. Pilots should follow the IMSAFE checklist and ensure they are suitably rested and nourished prior to any flight. Special attention should be paid to situations where night flying is intended to follow an early start and/or full day of activity, whether the activity is physical or mental as both are known to induce fatigue (Anderson, 2005).
Anderson (2005) also notes that inadequate sleep and disturbed biological rhythms will cause fatigue. These factors are significant for pilots who work either on the ground or in the air at night. Commercial pilots should adjust their day and activities to allow sufficient rest and sleep to reduce the onset and effects of fatigue as well as maintaining an adequate diet and fluid uptake to maintain energy levels and reduce dehydration. This may mean deferring or planning activities so that sleep and rest are paramount.
Of interest to pilots who fly at night, particularly at altitude or in aircraft with a high cabin altitude such as is common in the older aircraft often used for night freight operations, is the observation by MacAllister, which states that;
"Subtle hypoxic effects may be noticeable at 5000’ at night. In the average individual, night vision will be blurred and narrowed. Also dark adaption will be affected. At 8000’ night vision is reduced by as much as 25% without supplemental oxygen. Few or no effects will be noticed during the day at these altitudes" (MacAllister, 1997, p.130).
This indicates that pilots flying at night at altitude must be more aware of the existence and effects of hypoxia, as well as ensuring that other factors that may make mild hypoxia more dangerous, such as sleep deprivation, fatigue, inadequate nourishment and dehydration are minimised and controlled as much as possible.
Without the warmth of the sun, it can get cold very quickly. Always ensure that there is adequate clothing on board to keep both the pilots and passengers warm. Getting cold can effect a person's decision making but also bring on dangerous attitudes like "Get Home-itis".
Anderson, H. (2005). Fatigue and sleep factors for shift workers (3rd ed.). Christchurch, NZ: H.Anderson
Ewing, R., L. (1993). Aviation medicine and other human factors for pilots. Auckland, NZ: David Ling Publishing.
Hawkins, F. H., Orlady, H. W. (ed.). (1987). Human factors in flight. (2nd ed.). Aldershot, UK: Ashgate.
MacAllister, B. (1997). Crew resource management. England, UK: Airlife.
Want to know more?
More about night flying can be found here Night Flying-Part 1-Rules and here Night Flying-Part 2-Airmanship
More about the eye can be found here Human Physiology
More about fatigue can be found here Pilot Fatigue
More about visual illusions can be found here In-flight Visual Illusions
More about colour blindness can be found here Colour Blindness
More about Hypoxia can be found here Hypoxia
More about Hypothermia can be found here Hypothermia
More about Health Risk can be found here Health Risks for Pilots