vision and environmental factors

A.  vision

Good vision is of primary importance in flying, in judgment of distance, depth perception, reading of maps and instruments and should, therefore, be scrupulously protected.

Pilots; are exposed to higher light levels than is the average person. Very high light levels prevail at altitude because the atmosphere is less; dense. In addition, light is reflected back at the pilot by cloud tops. This light contains more of the damaging blue and ultra-violet wavelengths than are encountered on the surface of the earth. Prolonged exposure can cause damage to the eye and especially to the lens. Sunglasses should, therefore, be worn to, provide protection against these dangers and to prevent eyestrain.

Instrument panels should be dull grey or black, to harmonize with the black instruments, so that the eye does not have to adjust its lens opening constantly as the line of vision moves from the dark instruments to a light coloured panel.

When flying into, the sun, the eyes are so dazzled by the brightness that they cannot adjust quickly to the shaded instrument panel. This situation causes eyestrain and is fatiguing to the pilot. Sunglasses help to minimize the problem.

Atmospheric obscuring phenomena such as haze, smoke and fog have an effect on the distance the normal eye can see. The ability of the eye to maintain a distance focus is weakened. Distant objects are not outlined sharply against the horizon and after a short lapse of time the eye, having no distance point to fix on, has difficulty maintaining a focus at a distance of more than a mile or two, (a condition known as empty field myopia). As a result, scanning for other aircraft becomes difficult and requires special effort on the part of the pilot. With the pilot's focal range reduced, the span of time in which to perceive the danger and take evasive action is considerably shortened. Pilots must learn to recognize the limitations of the human eye under varying weather conditions and realize that the see and avoid maxim has limitations under some atmospheric conditions.

The Anatomical Blind Spot

The area where the optic nerve connects to the retina in the back of each eye is known as the optic disk. There is a total absence of cones and rods in this area, and, consequently, each eye is completely blind in this spot. Under normal binocular vision conditions this is not a problem, because an object cannot be in the blind spot of both eyes at the same time. On the other hand, where the field of vision of one eye is obstructed by an object (windshield post), a visual target (another aircraft) could fall in the blind spot of the other eye and remain undetected.

In order to find the blind spot of the right eye, it is necessary to close the left eye, focus the right eye on a single point, and see if anything vanishes from vision some 20 degrees right of this point. The following diagram has a set of characters on the left hand side, and black circle on the right. Keeping your head motionless, with the right eye about 3 or 4 times as far from the page as the length of the red line, look at each character in turn, until the black circle vanishes.

With increasing age, the blind spot enlarges. You may find that the black circle disappears when several of the characters are looked at. The size and shape of the blind spot can be found if a large enough grid of characters is used.

The same test can be done for the left eye. Close the right eye, and look at each character until the black circle disappears.

Note that when the black circle vanishes, you see only a white background where the circle was. What happens if the background colour is different? Say, yellow.

The blind spot appears as yellow. This is interesting, because it means that, although my eye can't detect anything in the blind spot, something knows that it is surrounded by yellow, and has guessed that what is in the blind spot is probably yellow. Smart!

How smart? If a thick horizontal line is drawn through the blind spot, what happens then?

The answer, it seems, is that if the line passes right through the blind spot, whatever is making shrewd guesses about colours is also able to work out that a line going in one side and coming out the other probably continues through the middle. The black circle disappears, but the line remains.

So what happens when a pen or pencil is pushed into the blind spot? It seems that as the tip enters the blind spot, the pencil appears truncated, as if it were vanishing into something (which, after all, it is). But when the tip emerges at the other side, the visual processing system fills in the missing part between. The following animation mimics pushing a pencil into the blind spot.

The first conclusion drawn from this little experiment is that, although each eye has a blind spot, some sort of intelligence is used to give this area not only a likely colour, but also to fill in lines that pass through the blind spot - rather than just have a fuzzy grey area. The net result is that, with one eye closed, it isn't immediately obvious where the blind spot is, because it has been given a suitable colour, and even pattern, based on what is adjacent to it.

The second conclusion drawn is that what we see is not just what has appeared on the retina, but is an image that has been reprocessed, tidied up. And if the human visual cortex is able to tidy up the blind spot, then it may well be that the same is being done for the entire visual field - that what we get to 'see' is not what appears on the retina, like a photograph, but instead something which has a whole bunch of special effects added.

If so, then we can't trust our eyes. We're being given doctored information, massaged figures. The world that we see is not something out there, but a world that we invent. The world I see is my idea.

The Night Blind Spot

The "Night Blind Spot" appears under conditions of low ambient illumination due to the absence of rods in the fovea, and involves an area 5 to 10 degrees wide in the centre of the visual field. Therefore, if an object is viewed directly at night, it may go undetected or it may fade away after initial detection due to the night blind spot.

The Fovea

The fovea is the small depression located in the exact centre of the macula that contains a high concentration of cones but no rods, and this is where our vision is most sharp. While the normal field of vision for each eye is about 135 degrees vertically and about 160 degrees horizontally, only the fovea has the ability to perceive and send clear, sharply focused visual images to the brain. This foveal field of vision represents a small conical area of only about 1 degree. To fully appreciate how small a one-degree field is, and to demonstrate foveal field, take a quarter from your pocket and tape it to a flat piece of glass, such as a window. Now back off 4 1/2 feet from the mounted quarter and close one eye. The area of your field of view covered by the quarter is a one-degree field, similar to your foveal vision.

Now we know that you can see a lot more than just that one-degree cone. But, do you know how little detail you see outside of that foveal cone? For example, outside of a ten-degree cone, concentric to the foveal one-degree cone, you see only about one-tenth of what you can see within the foveal field. In terms of an oncoming aircraft, if you are capable of seeing an aircraft within your foveal field at 5,000 feet away, with peripheral vision you would detect it at 500 feet. Another example: using foveal vision we can clearly identify an aircraft flying at a distance of 7 miles; however, using peripheral vision (outside the foveal field) we would require a closer distance of .7 of a mile to recognize the same aircraft. That is why when you were learning to fly, your instructor always told you to "put your head on a swivel," to keep your eyes scanning the wide expanse of space in front of your aircraft.

Depth Perception.  Clues for accurate depth perception are often absent in the air. Clouds are of varying size and there is no way to estimate their distance. Landings on glassy water or on wet runways are a problem as is the condition known as white out that occurs in blowing snow and other winter situations.

Night Vision.  At night, the pilot's vision is greatly impaired. The cones that are concentrated in the centre of the lens need a lot of light to function properly. As a result, there is a blind spot in the center of the eye at night. This blind spot is sufficiently large to block out the view of another airplane some distance away if the pilot is looking directly at it.

At night, it is necessary to develop the technique of using peripheral vision. One sees at night by means of the rods that are concentrated on the edges of the lens and are responsible to peripheral vision. It takes the rods about 30 minutes to adjust full to darkness. Even a small amount of white light will destroy the dark adaptation.

Pilots should wear sunglasses during the day and avoid looking at bright lights when they propose to undertake a night flight. Wearing red goggles for 30 minutes prior to a night flight helps their eyes adapt to darkness.

Night vision is also sensitive to hypoxia. Supplementary oxygen should be used above 5000 feet to avoid depriving the eye of oxygen.

Dirt and reflection on the windshield cause confusion at night A very clean windshield is important.

Colour Perception And Visual Acuity

Two aspects of the human vision that you will need to have are colour perception and visual acuity.  Included below are two quick tests for both colour and acuity:

colour perception:

Shown above is a sample of the type of colour images that you will be asked to identify by your medial examiner.  In each of the above circles is a number. If you can identify the numbers of each of the circles, then chances are you have no colour vision deficiencies.  Myself, I cannot see the 0 that is in the centre circle.  I failed to identify the colour differences associated with those of the centre circle and therefore failed that portion of my medical exam.  The restrictions to a pilot's license that apply for such a vision deficiency are "no night flight" and "not valid for colour control signal".  If you have a similar problem and still have the restriction, click here to learn about the process to obtain a S.O.D.A. ( Statement Of Demonstrated Ability ).




Federal Aviation Regulations, according to the third-class qualifications, sec. 67.303 says: Eye standards for a third-class airman medical certificate are: (c) Ability to perceive those colors necessary for the safe performance of airman duties.

** Note: This actually means the ability to distinguish between red, green, and white lights.

visual acuity

Shown here is a chart that you can use to give you an close estimate of your visual acuity.  To use this vision chart, follow these rules:

1.) Measure the length of the blue line on the chart in CENTIMETRES as it appears on your monitor.

2.) From your monitor, measure a distance backwards in FEET the number of centimetres the line is long (i.e. if the line is 9cm in length stand 9 feet back from the monitor) 

3.) Read the smallest line on the chart with each eye separately.  If you use corrective lenses, wear them for this test.

  • Very bottom line = 20/10 vision
  • Second line up from bottom = 20/20 vision
  • Third line up from bottom = 20/30 vision
  • Fourth line up from bottom = 20/40 vision
  • Fifth line up from bottom = 20/50 vision
  • The "T" and "B" represent 20/100 vision
  • The "E" at the top represents 20/200 vision

Federal Aviation Regulations, according to the third-class qualifications, sec. 67.303 says: Eye standards for a third-class airman medical certificate are:

(a) Distant visual acuity of 20/40 or better in each eye separately, with or without corrective lenses.

(b) Near vision of 20/40 or better in each eye separately, with or without corrective lenses.

** Note: if corrective lenses are required to obtain the minimal 20/40 vision, then the person is eligible only on the condition that the corrective lenses MUST be worn while exercising the privileges of an airman certificate.

Night Lighting of Instruments. 
Lighting of instruments is a problem in that the instruments must be well enough lit to be readable without the light destroying the pilot's dark adaptation.

Ultraviolet flood fighting of fluorescent instrument marking is probably the least satisfactory. The instruments are marked with fluorescent paint that shows up under fluorescent lighting as a bluish green colour. The disadvantages are that the instruments can't be kept in focus, dark adaptation may be lost, eyes are irritated, vision becomes foggy.

Red lights. Lighting of instruments by indirect individual red lights; is unsatisfactory because uniform light distribution over al, parts of the instrument cannot be achieved. There is no illumination of knobs and switches. Red flood lighting of the whole instrument panel is more satisfactory. However, the ability to distinguish colours one from another is lost. Coloration of maps is indecipherable and information printed in red becomes unreadable.

White lights. Low density white, light is considered the best cockpit lighting system. The instruments can be clearly read and colours recognized. Because the low density white light can be regulated, dark adaptation is not destroyed although it is somewhat impaired.

Thunderstorms.  It is not advisable to fly an airplane through or near thunderstorms. The blinding flashes destroy night adaptation. Turn the cockpit lights full bright if you are in the vicinity of lightning activity in order to prevent lightning blindness.

Anti Collision Lights.  When flying in the clouds, strobe lights and rotating beacons should be turned off as the reflection off the cloud of the blinking light is irritating to the eye. 

B.  noise, vibration and temperature


Noise is both inconvenient and annoying. It produces headaches, visual and auditory fatigue, airsickness and general discomfort with an accompanying loss of efficiency. Even at levels which are not uncomfortable, noise has a fatiguing effect, especially when the pilot is exposed for a long period as on a lengthy cross country flight. To arrive at destination suffering from noise induced fatigue and have to, make a landing under minimum conditions is clearly an undesirable situation.

Noise levels in the range of 130 decibels or above are very dangerous and should not be experienced without ear protection . (The unit of sound intensity or loudness is called the decibel or db.) Yet, little has been done to reduce and control noise in aircraft cockpits. Tests; have measured the sound level in modern aircraft at 90 to 100 decibels. Noise levels in jets can approach 140 decibels.

With noise levels of this magnitude, hearing damage is a distinct problem unless some sort of hearing protection is used. Many pilots report temporary loss of hearing sensitivity after flights. Still others have reported an inability to understand radio transmissions from the ground, especially during take-off and climb when the engine is operating at full power. In fact, there is documented evidence to show that continued exposure to high levels of aircraft noise will result over the years in loss of hearing ability.

The detrimental effect of noise is not a sudden thing but builds up progressively over years of exposure. Pilots of helicopters and aerial application aircraft are particularly susceptible because of the relatively high levels of noise experienced in these cockpits and the long duration of exposure. But even pilots, who put in only three or four hours a week in their airplanes, have been found to have slightly impaired hearing after several years.

Everyone experiences some hearing deterioration as the process of growing old. Add this to a level of deafness caused by exposure to noise and it becomes obvious that a pilot reaching middle age could have a serious hearing deficiency.

Protective devices against noise are therefore important, first of ail, in helping to reduce fatigue during individual flights and, secondly, in helping to minimize the possibility of hearing loss or deterioration in later years.

The best protection is a pair of properly fitting earplugs. They lower noise levels by as much as 20 to 30 decibels. The use of ear covering devices, such as earphones. can also help if they are tight fitting. If they fit poorly, they can be worse than nothing, in that they give the wearer a false sense of security. The use of earplugs as well as earphones is recommended.

The wearing of earplugs does not impair ability to hear.  In fact, speech intelligibility is improved because the earplugs filter out the very noises that interfere with voice transmissions.

The regular wearing of earplugs, especially by pilots but also by passengers, is therefore a good precautionary measure to ensure continued good hearing throughout a pilot's lifetime.


The power plant of the airplane is the principal source of vibration. At subsonic speeds, this vibration is responsible for fatigue and irritability and can even cause chest and abdominal pains, backache, headaches, eyestrain and muscular tension. If the vibration happens to occur in the frequency of about 40 cycles per second, the eyes will blur. It is even possible to become hypnotized as a result of rhythmic and monotonous vibrations.


At temperatures over 30° C, discomfort, irritability and loss of efficiency are pronounced. High temperatures also reduce the pilot's tolerance to mental and physical stresses, such as acceleration and hypoxia.

At cold temperatures, the immediate danger is frostbite. Continued exposure will result in reduced efficiency to the point where safe operation of the airplane is impossible.


The most serious result of extended exposure to extremely cold temperatures is a condition known as hypothermia. Hypothermia is a lowering of the temperature of the body's inner core. It occurs when the amount of heat produced by the body is less than the amount being lost to the body's surroundings. As it progresses, vital organs and bodily systems begin to lose their ability to function. It is a condition that can develop quickly and may be fatal.

In the early stages, the skin becomes pale and waxy, fatigue and signs of weakness begin. As the body temperature drops farther, uncontrollable intense shivering and clumsiness occur. Mental confusion and apathy, drowsiness, slurred speech, slow and shallow breathing are the next stage. Unconsciousness and death follow rapidly.

Hypothermia certainly can attack a pilot in the cockpit of his airplane if there is no cabin heating system and if he is not adequately dressed to protect against very cold ambient temperatures. Usually, however, hypothermia is considered to be a danger to the pilot who has been forced down and is exposed to the elements. Cold, wetness, wind and inadequate preparation are the conditions which cause it. Wet clothing, caused by weather, immersion in water or condensed perspiration, acts like a wick and extracts body heat at a rate many times faster than would be the case with dry clothing. Immersion in cold water greatly accelerates the progress into hypothermia.

The best protection against this condition is adequate clothing, shelter, emergency rations and, above ail, knowledge of the danger. Every wintertime flier should have a survival kit that includes a lightweight tent, plastic sheet, survival blanket, etc., that can be used to construct a shelter. Always wear (or take along as extras) proper clothing for the worst conditions you might encounter. Several layers of clothing are more effective than one bulky layer. Protect high heat loss areas, such as the head, neck, underarms, sides of the chest. Carry effective rain gear and put it on before you get wet. High energy foods that produce heat and energy should be included in the survival kit. Hot fluids help to keep body heat up. Guard against becoming tired and exhausted, A tired person, exposed to a cold, wet and windy environment, is a prime candidate for hypothermia. 

C.  sensory illusions

Under normal conditions, the eyes, inner ears and skeletal muscles provide the brain with information about the position of the body in relation to the ground. In flying, however, conditions are sometimes encountered which fool the senses.

The eyes are the prime orienting organs but are dependent on reference points in providing reliable spatial information. Objects seen from the air often look quite different than they do when seen from the ground. If the horizon is not visible, a pilot might choose some other line as reference, such as a sloping cloud bank. Fog and haze greatly affect judgment of distance. Lights on the ground at night are commonly confused for airplanes. Even stars can be confused with ground lights.

The tension of various muscles in the body assists in a small way in determining position. The body is accustomed to the pull of one g force acting in only one direction. In an airplane. if a second force is introduced as in acceleration, deceleration and turns, and if there is no outside visual reference, illusions may result. For example, in a bank, both centrifugal force directed outward and the normal downward pull of gravity combine to give an illusion of level flight. Acceleration gives an illusion of climbing and deceleration of diving.

The three semicircular canals of the inner ear are primarily associated with equilibrium. They are filled with fluid and operate on the principle of the inertia of fluids. Each canal has tiny hair like sensors that relate to the brain the motion of the fluid. Rotation of the body tends to move the fluid, causing the displacement of the sensors which then transmit to the brain the message of the direction of their displacement. However, if the turn is a prolonged and constant one the motion of the fluid catches up with the canal walls, the sensors are no longer bent and the brain receives the incorrect message that the turning has stopped. If the turn does then indeed stop, the movement of the fluid and the displacement of the sensors will indicate a turn in the opposite direction. Under instrument conditions or at night when visual references are at a minimum, incorrect information given by the inner ear can be dangerous.

The following factors contribute to visual illusions: optical characteristics of windshields; rain on the windshield; effects of fog, haze, dust, etc. on depth perception; the angle of the glide slope makes a runway appear nearer or farther as does a very wide or very narrow runway; variations in runway lighting systems: runway slope and terrain slope; an approach over water to the runway; the apparent motion of a fixed light at night (auto-kinetic phenomenon). The visual cues by which a pilot makes judgment, about the landing approach are largely removed if the approach is over water, over snow or other such featureless terrain or carried out at night. A particularly hazardous situation is created if circumstances prevent him from appreciating ground proximity before touchdown.

The following factors contribute to sensory illusions: change in acceleration or deceleration; cloud layers; low level flight over water, frequent transfer from instrument to visual flight conditions (choose either VFR or IFR and stick with the choice); unperceived changes in flight altitude.

There is just one way to beat false interpretation of motion. Put your faith in your instruments and not in your senses.

Refer to the altitude Instruments constantly when flying at night or In reduced visibility conditions.

Always trust the attitude instruments no matter what your senses tell you. 

D.  spatial disorientation

Spatial disorientation means loss of bearings or confusion concerning one's sense of position or movement in relation to the surface of the earth. Disorientation rarely occurs without reduced visual references in such situations as fog, cloud, snow, rain, darkness, etc.

A type of spatial disorientation is caused in some individuals by flickering shadows. When, for example, letting down for a landing into the setting sun with a single engine airplane, the idling propeller can induce reactions that range from nausea to confusion and, in rare cases, complete unconsciousness. Other causes of this sensation are helicopter rotor blade shadows, the flashing illumination caused by anti-collision lights when flying in clouds, and runway approach strobe lights when viewed through the propeller at night.

The term vertigo is sometimes used in relation to spatial disorientation. Vertigo is a sensation of rotation or spinning, an hallucination of movement of either the individual himself or of the external world.

Coriolis effect is probably the most dangerous type of disorientation. The three semicircular canals of the inner ear are interconnected. If movement is occasioned in two of them, a sympathetic but more violent movement is induced in the third. This is known as tumbling and causes extreme confusion, nausea, and even rolling of the eyeballs that prevents; the pilot from reading correctly the airplane instruments. This situation can occur if, when the airplane is in a turn, the pilot suddenly turns his head in another direction. The rule should always be to avoid head movements, especially quick ones, when flying under instrument conditions.

Otolith-False Climb Illusion.  The otolith is a small organ which forms part of the inner ear, and vestibular apparatus. Its’ function is to sense and signal to the other organs the position of the head relative to the vertical. This signal has a profound influence on the balance and orientation of the body.

The otolith, simply described, is an erect hair with a small weight or mass at its tip. The base of the hair is embedded in a sensory cell which conveys to the brain information about the angle of the hair.

When the head is tilted backward, the small mass bends the hair and the message relayed to the brain indicates a backward tilt. If the head is held vertical but is subjected to acceleration, the hair bends owing to the inertia of the mass at the tip of the hair. Both tilt and acceleration, therefore, produce the same response by the, otolith. If there are no visual cues to compliment the information from the otolith, the brain is unable to differentiate between till and acceleration. If tilt and acceleration are experienced simultaneously the interpretation is that of a much steeper tilt. This is known as the false climb illusion.

In such a situation, a pilot is tempted to lower the nose of the airplane. This increases the forward acceleration component and increases the illusion of climbing steeply. Owing to lag in the altimeter and vertical speed indicator, the loss of height may go unnoticed.

There are three situations in which the false climb illusion may occur: (1) take-off at night or in IFR conditions, (2) an overshoot in reduced visibility or in IFR conditions, and (3) a climb from VFR into IFR conditions. During the latter situation, the illusion can be com pounded by turbulence, in a turn, or by reliance on an artificial horizon that is not quite erect.

All pilots irrespective of experience or skill are susceptible to the illusion. Pilots must learn to anticipate the illusion and ignore it, to establish a positive climb attitude and to rely on the aircraft instruments for confirmation of attitude.