Figure 17-11. To avoid the bends, scuba divers must not fly for
specific time periods following dives.
Figure 17-10. Signs and symptoms of altitude decompression sickness.
DCS Type Bubble Location Signs and Symptoms (Clinical Manifestations)
BENDS
NEUROLOGIC
Manifestations
CHOKES
SKIN BENDS
Mostly large joints
of the body (elbows,
shoulders, hip, wrists,
knees, ankles)
Brain
Spinal cord
Peripheral nerves
Lungs
Skin
• Localized deep pain, ranging from mild (a “niggle”) to excruciating–sometimes a dull
ache, but rarely a sharp pain
• Active and passive motion of the joint aggravating the pain
• Pain occurring at altitude, during the descent, or many hours later
• Confusion or memory loss
• Headache
• Spots in visual field (scotoma), tunnel vision, double vision (diplopia), or blurry vision
• Unexplained extreme fatigue or behavior changes
• Seizures, dizziness, vertigo, nausea, vomiting, and unconsciousness
• Abnormal sensations, such as burning, stinging, and tingling, around the lower chest
and back
• Symptoms spreading from the feet up and possibly accompanied by ascending
weakness or paralysis
• Girdling abdominal or chest pain
• Urinary and rectal incontinence
• Abnormal sensations, such as numbness, burning, stinging and tingling (paresthesia)
• Muscle weakness or twitching
• Burning deep chest pain (under the sternum)
• Pain aggravated by breathing
• Shortness of breath (dyspnea)
• Dry constant cough
• Itching usually around the ears, face, neck, arms, and upper torso
• Sensation of tiny insects crawling over the skin
• Mottled or marbled skin usually around the shoulders, upper chest, and abdomen
accompanied by itching
• Swelling of the skin, accompanied by tiny scar-like skin depressions (pitting edema)
evolved gas can occur during exposure to low altitude and
create a serious inflight emergency.
The recommended waiting time before going to flight
altitudes of up to 8,000 feet is at least 12 hours after diving
that does not require controlled ascent (nondecompression
stop diving), and at least 24 hours after diving that does
require controlled ascent (decompression stop diving). The
waiting time before going to flight altitudes above 8,000
feet should be at least 24 hours after any scuba dive. These
recommended altitudes are actual flight altitudes above
mean sea level (MSL) and not pressurized cabin altitudes.
This takes into consideration the risk of decompression of
the aircraft during flight.
Vision in Flight
Of all the senses, vision is the most important for safe flight.
Most of the things perceived while flying are visual or heavily
supplemented by vision. As remarkable and vital as it is,
vision is subject to limitations, such as illusions and blind
spots. The more a pilot understands about the eyes and how
they function, the easier it is to use vision effectively and
compensate for potential problems.
The eye functions much like a camera. Its structure includes
an aperture, a lens, a mechanism for focusing, and a surface
for registering images. Light enters through the cornea at the
front of the eyeball, travels through the lens, and falls on the
retina. The retina contains light sensitive cells that convert
Figure 17-13. Types of vision.
Types of Vision
Types of vision used
Photopic
Mesopic
Scotopic
Light level
High
Medium/Low
Low
Technique of viewing
Central
Both
Scanning
Color perception
Good
Some
None
Receptors used
Cones
Cones/Rods
Rods
Acuity best
20/20
Varies
20/200
Blind spot
Day
Day/Night
Day/Night
Figure 17-12. The human eye.
The rods and
cones (film) of
the retina are
the receptors
which record
the image and
transmit it
through the
optic nerve to
the brain for
interpretation.
Rods and
cones
Fovea
(all cones)
Lens
Retina
Optic nerve
Optic disk
(blind spot)
Fovea centralis
Iris
The pupil (aperture) is the opening at
the center of the iris. The size of the
pupil is adjusted to control the amount
of light entering the eye.
PUPIL
Light passes through the cornea (the
transparent window on the front of the
eye) and then through the lens to
focus on the retina.
CORNEA
Rod
concentration
light energy into electrical impulses that travel through nerves
to the brain. The brain interprets the electrical signals to form
images. There are two kinds of light-sensitive cells in the
eyes: rods and cones. [Figure 17-12]
The cones are responsible for all color vision, from
appreciating a glorious sunset to discerning the subtle shades
in a fine painting. Cones are present throughout the retina, but
are concentrated toward the center of the field of vision at the
back of the retina. There is a small pit called the fovea where
almost all the light sensing cells are cones. This is the area
where most “looking” occurs (the center of the visual field
where detail, color sensitivity, and resolution are highest).
While the cones and their associated nerves are well suited
to detecting fine detail and color in high light levels, the
rods are better able to detect movement and provide vision
in dim light. The rods are unable to discern color but are
very sensitive at low-light levels. The trouble with rods is
that a large amount of light overwhelms them, and they take
longer to “reset” and adapt to the dark again. There are so
many cones in the fovea that are at the very center of the
visual field but virtually has no rods at all. So in low light,
the middle of the visual field is not very sensitive, but farther
from the fovea, the rods are more numerous and provide the
major portion of night vision.
Vision Types
There are three types of vision: photopic, mesopic, and
scotopic. Each type functions under different sensory stimuli
or ambient light conditions. [Figure 17-13]
Photopic Vision
Photopic vision provides the capability for seeing color and
resolving fine detail (20/20 or better), but it functions only
in good illumination. Photopic vision is experienced during
daylight or when a high level of artificial illumination exists.
Figure 17-15. The eye’s blind spot.
Figure 17-14. Central blind spot.
Pupil
Blind spot
Optic nerve
Retina
Center of vision
RightLEFT
The cones concentrated in the fovea centralis of the eye are
primarily responsible for vision in bright light. [Figure 17-12]
Because of the high light level, rhodopsin, which is a
biological pigment of the retina that is responsible for both
the formation of the photoreceptor cells and the first events
in the perception of light, is bleached out causing the rod
cells to become less effective.
Mesopic Vision
Mesopic vision is achieved by a combination of rods and
cones and is experienced at dawn, dusk, and during full
moonlight. Visual acuity steadily decreases as available light
decreases and color perception changes because the cones
become less effective. Mesopic viewing period is considered
the most dangerous period for viewing. As cone sensitivity
decreases, pilots should use off-center vision and proper
scanning techniques to detect objects during low-light levels.
Scotopic Vision
Scotopic vision is experienced under low-light levels and
the cones become ineffective, resulting in poor resolution of
detail. Visual acuity decreases to 20/200 or less and enables
a person to see only objects the size of or larger than the
big “E” on visual acuity testing charts from 20 feet away.
In other words, a person must stand at 20 feet to see what
can normally be seen at 200 feet under daylight conditions.
When using scotopic vision, color perception is lost and a
night blind spot in the central field of view appears at low
light levels when the cone-cell sensitivity is lost.
Central 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. [Figure 17-14]
As a result, it is referred to as the blind spot that everyone
has in each eye. Under normal binocular vision conditions
(both eyes are used together), 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 divider or another
aircraft), a visual target could fall in the blind spot of the
other eye and remain undetected.
Figure 17-15 provides a dramatic example of the eye’s
blind spot.
1. Hold this page at an arm’s length.
2. Completely cover your left eye (without closing or
pressing on it) using your hand or other flat object.
3. With your right eye, stare directly at the airplane on
the left side of the picture page. In your periphery, you
will notice the black X on the right side of the picture.
4. Slowly move the page closer to you while continuing
to stare at the airplane.
Figure 17-16. Night vision.
Figure 17-17. Night blind spot.
Cones active
Rods active
Night blind spot
Pilots must look 5°–10° off center of the
object in order for the object to be seen.
5. When the page is about 16–18 inches from you, the
black X should disappear completely because it has
been imaged onto the blind spot of your right eye.
(Resist the temptation to move your right eye while
the black X is gone or else it reappears. Keep staring
at the airplane.)
6. As you continue to look at the airplane, keep moving
the page closer to you a few more inches, and the black
X will come back into view.
7. There is an interval where you are able to move the
page a few inches backward and forward, and the black
X will be gone. This demonstrates to you the extent
of your blind spot.
8. You can try the same thing again, except this time with
your right eye covered stare at the black X with your
left eye. Move the page in closer and the airplane will
disappear.
Another way to check your blind spot is to do a similar test
outside at night when there is a full moon. Cover your left
eye, looking at the full moon with your right eye. Gradually
move your right eye to the left (and maybe slightly up or
down). Before long, all you will be able to see is the large
halo around the full moon; the entire moon itself will seem
to have disappeared.
Empty-Field Myopia
Empty-field myopia is a condition that usually occurs when
flying above the clouds or in a haze layer that provides
nothing specific to focus on outside the aircraft. This causes
the eyes to relax and seek a comfortable focal distance that
may range from 10 to 30 feet. For the pilot, this means
looking without seeing, which is dangerous. Searching out
and focusing on distant light sources, no matter how dim,
helps prevent the onset of empty-field myopia.
Night Vision
There are many good reasons to fly at night, but pilots must
keep in mind that the risks of night flying are different than
during the day and often times higher. [Figure 17-16] Pilots
who are cautious and educated on night-flying techniques
can mitigate those risks and become very comfortable and
proficient in the task.
Night Blind Spot
It is estimated that once fully adapted to darkness, the rods are
10,000 times more sensitive to light than the cones, making
them the primary receptors for night vision. Since the cones
are concentrated near the fovea, the rods are also responsible
for much of the peripheral vision. The concentration of cones
in the fovea can make a night blind spot in the center of the
field of vision. To see an object clearly at night, the pilot must
expose the rods to the image. This can be done by looking 5°
to 10° off center of the object to be seen. This can be tried in
a dim light in a darkened room. When looking directly at the
light, it dims or disappears altogether. When looking slightly
off center, it becomes clearer and brighter.
When looking directly at an object, the image is focused
mainly on the fovea, where detail is best seen. At night, the
ability to see an object in the center of the visual field is
reduced as the cones lose much of their sensitivity and the
rods become more sensitive. Looking off center can help
compensate for this night blind spot. Along with the loss of
Figure 17-18. Scanning techniques.
4
2
3
1
10°
sharpness (acuity) and color at night, depth perception and
judgment of size may be lost. [Figure 17-17]
Dark Adaptation
Dark adaptation is the adjustment of the human eye to a dark
environment. That adjustment takes longer depending on the
amount of light in the environment that a person has just left.
Moving from a bright room into a dark one takes longer than
moving from a dim room and going into a dark one.
While the cones adapt rapidly to changes in light intensities,
the rods take much longer. Walking from bright sunlight into
a dark movie theater is an example of this dark adaptation
period experience. The rods can take approximately 30
minutes to fully adapt to darkness. A bright light, however,
can completely destroy night adaptation, leaving night
vision severely compromised while the adaptation process
is repeated.
Scanning Techniques
Scanning techniques are very important in identifying objects
at night. To scan effectively, pilots must look from right to
left or left to right. They should begin scanning at the greatest
distance an object can be perceived (top) and move inward
toward the position of the aircraft (bottom). For each stop, an
area approximately 30° wide should be scanned. The duration
of each stop is based on the degree of detail that is required,
but no stop should last longer than 2 to 3 seconds. When
moving from one viewing point to the next, pilots should
overlap the previous field of view by 10°. [Figure 17-18]
Off-center viewing is another type of scan that pilots can use
during night flying. It is a technique that requires an object be
viewed by looking 10° above, below, or to either side of the
object. [Figure 17-19] In this manner, the peripheral vision
can maintain contact with an object.
With off-center vision, the images of an object viewed longer
than 2 to 3 seconds will disappear. This occurs because the
rods reach a photochemical equilibrium that prevents any
further response until the scene changes. This produces
a potentially unsafe operating condition. To overcome
this night vision limitation, pilots must be aware of the
phenomenon and avoid viewing an object for longer than 2
or 3 seconds. The peripheral field of vision will continue to
pick up the object when the eyes are shifted from one off-
center point to another.
Night Vision Protection
Several things can be done to help with the dark adaptation
process and to keep the eyes adapted to darkness. Some of
the steps pilots and flight crews can take to protect their night
vision are described in the following paragraphs.
Figure 17-19. Off-center viewing.
Focal points
X
X XX
X
Once an object is detected in the peripheral field of dark-
adapted vision, continued surveillance is maintained by
use of “off-center” vision. Looking 10° right or left and
above and below the object, viewing no longer than 2 to
3 seconds at each position.
OBSERVER
10 degrees
10 degrees 10 degrees
10 degrees
Sunglasses
If a night flight is scheduled, pilots and crew members should
wear neutral density (N-15) sunglasses or equivalent filter
lenses when exposed to bright sunlight. This precaution
increases the rate of dark adaptation at night and improves
night visual sensitivity.
Oxygen Supply
Unaided night vision depends on optimum function and
sensitivity of the rods of the retina. Lack of oxygen to the rods
(hypoxia) significantly reduces their sensitivity. Sharp clear
vision (with the best being equal to 20–20 vision) requires
significant oxygen especially at night. Without supplemental
oxygen, an individual’s night vision declines measurably at
pressure altitudes above 4,000 feet. As altitude increases,
the available oxygen decreases, degrading night vision.
Compounding the problem is fatigue, which minimizes
physiological well being. Adding fatigue to high altitude
exposure is a recipe for disaster. In fact, if flying at night at
an altitude of 12,000 feet, the pilot may actually see elements
of his or her normal vision missing or not in focus. Missing
visual elements resemble the missing pixels in a digital image
while unfocused vision is dim and washed out.
For the pilot suffering the effects of hypoxic hypoxia, a simple
descent to a lower altitude may not be sufficient to reestablish
vision. For example, a climb from 8,000 feet to 12,000 feet for
30 minutes does not mean a descent to 8,000 feet will rectify
the problem. Visual acuity may not be regained for over an
hour. Thus, it is important to remember, altitude and fatigue
have a profound effect on a pilot’s ability to see.
High Intensity Lighting
If, during the flight, any high intensity lighting areas are
encountered, attempt to turn the aircraft away and fly in the
periphery of the lighted area. This will not expose the eyes to
such a large amount of light all at once. If possible, plan your
route to avoid direct over flight of built-up, brightly lit areas.
Flightdeck Lighting
Flightdeck lighting should be kept as low as possible so that
the light does not monopolize night vision. After reaching
the desired flight altitude, pilots should allow time to
adjust to the flight conditions. This includes readjustment
of instrument lights and orientation to outside references.
During the adjustment period, night vision should continue
to improve until optimum night adaptation is achieved. When
it is necessary to read maps, charts, and checklists, use a dim
white light flashlight and avoid shining it in your or any other
crewmember’s eyes.
Airfield Precautions
Often time, pilots have no say in how airfield operations are
handled, but listed below are some precautions that can be
taken to make night flying safer and help protect night vision.
• Airfield lighting should be reduced to the lowest
usable intensity.
• Maintenance personnel should practice light discipline
with headlights and flashlights.
• Position the aircraft at a part of the airfield where the
least amount of lighting exists.