Good vision is of primary importance in flying, in judgment of distance,
depth perception, reading of maps and instruments and should, therefore, be
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
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
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
The Anatomical Blind Spot
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
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,
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
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
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
"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 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
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.
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
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
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:
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
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
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
** Note: This actually
means the ability to distinguish between red, green, and white lights.
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
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 =
- 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
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
(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.
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
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
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
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
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
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.
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
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
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
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
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
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 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.