Figure 17-20. Self-imposed stress.
HYPOGLYCEMIA
HYPOGLYCEMIA
HYPOGLYCEMIA
HYPOGLYCEMIA
ALCOHOL
ALCOHOL
ALCOHOL
ALCOHOL
TOBACCO
TOBACCO
TOBACCO
TOBACCO
POOR PHYSICAL CONDITION
POOR PHYSICAL CONDITION
POOL PHYSICAL CONDITION
EXHAUSTION
EXHAUSTION
EXHAUSTION
EXHAUSTION
DRUGS
DRUGS
DRUGS
DRUGS
NUTRITIONAL
DEFICIENCY
NUTRITIONAL
DEFICIENCY
NUTRITIONAL
DEFICIENCY
NUTRITIONAL
DEFICIENCY
• Select approach and departure routes that avoid
highways and residential areas where illumination
can impair night vision.
Self-Imposed Stress
Night flight can be more fatiguing and stressful than day
flight, and many self- imposed stressors can limit night vision.
Pilots can control this type of stress by knowing the factors
that can cause self-imposed stressors. Some of these factors
are listed in the following paragraphs. [Figure 17-20]
Drugs
Drugs can seriously degrade visual acuity during the day and
especially at night. Pilots who become ill should consult an
aviation medical examiner (AME) or flight surgeon as to
which drugs are appropriate to take while flying.
Exhaustion
Pilots who become fatigued during a night flight will not be
mentally alert and will respond more slowly to situations
requiring immediate action. Exhausted pilots tend to
concentrate on one aspect of a situation without considering
the total requirement. Their performance may become a
safety hazard depending on the degree of fatigue and instead
of using proper scanning techniques may get fixated on the
instruments or stare off rather than multitask.
Poor Physical Conditioning
To overcome poor physical conditioning, pilots should
participate in regular exercise programs. People who are
physically fit become less fatigued during flight and have
better night scanning efficiency. However, too much exercise
in a given day may leave crew members too fatigued for
night flying.
Alcohol
Alcohol is a sedative and its use impairs both coordination
and judgment. As a result, pilots who are impaired by alcohol
fail to apply the proper techniques of night vision. They are
likely to stare at objects and to neglect scanning techniques.
The amount of alcohol consumed determines the degree to
which night vision is affected. The effects of alcohol are long
lasting and the residual effects of alcohol can also impair
visual scanning efficiency.
Tobacco
Of all the self-imposed stressors, cigarette smoking most
decreases visual sensitivity at night. Smoking significantly
increases the amount of carbon monoxide carried by the
hemoglobin in red blood cells. This reduces the blood’s
capacity to combine with oxygen, so less oxygen is carried
in the blood. Hypoxia caused by carbon monoxide poisoning
affects peripheral vision and dark adaptation. The results
are the same as those for hypoxia caused by high altitude.
Smoking 3 cigarettes in rapid succession or 20 to 30 cigarettes
within a 24-hour period may saturate from 8 to 10 percent
of the capacity of hemoglobin. Smokers lose 20 percent of
their night vision capability at sea level, which is equal to a
physiological altitude of 5,000 feet.
Hypoglycemia and Nutritional Deficiency
Missing or postponing meals can cause low blood sugar,
which impairs night flight performance. Low blood sugar
levels may result in stomach contractions, distraction,
breakdown in habit pattern, and a shortened attention span.
Likewise, an insufficient consumption of vitamin A may
also impair night vision. Foods high in vitamin A include
eggs, butter, cheese, liver, apricots, peaches, carrots, squash,
spinach, peas, and most types of greens. High quantities of
vitamin A do not increase night vision but a lack of vitamin
A certainly impairs it.
Distance Estimation and Depth Perception
Knowledge of the mechanisms and cues affecting distance
estimation and depth perception assist pilots in judging
distances at night. These cues may be monocular or binocular.
The monocular cues that aid in distance estimation and depth
perception include motion parallax, geometric perspective,
retinal image size, and aerial perspective.
Motion Parallax
Motion parallax refers to the apparent motion of stationary
objects as viewed by an observer moving across the
landscape. When the pilot or crewmember looks outside the
aircraft perpendicular to the direction of travel, near objects
appear to move backward, past, or opposite the path of
motion; far objects seem to move in the direction of motion
or remain fixed. The rate of apparent movement depends on
the distance the observer is from the object.
Figure 17-21. Geometric perspective.
A B C
Geometric Perspective
An object may appear to have a different shape when viewed
at varying distances and from different angles. Geometric
perspective cues include linear perspective, apparent
foreshortening, and vertical position in the field.
• Linear perspective—parallel lines, such as runway
lights, power lines and railroad tracks, tend to
converge as distance from the observer increases.
[Figure 17-21A]
• Apparent foreshortening—the true shape of an object
or a terrain feature appears elliptical when viewed
from a distance. [Figure 17-21B]
• Vertical position in the field—objects or terrain
features farther away from the observer appear higher
on the horizon than those closer to the observer.
[Figure 17-21C]
Aerial Perspective
The clarity of an object and the shadow cast by it are
perceived by the brain and are cues for estimating distance.
Subtle variations in color or shade are clearer the closer the
observer is to an object. However, as distance increases,
these distinctions may become blurry. The same applies to
an object detail or texture. As a person gets farther from an
object, its discrete details become less apparent. Another
important fact to remember while flying at night is that every
object casts a shadow from a light source. The direction in
which the shadow is cast depends on the position of the light
source. If the shadow of an object is cast toward the observer,
the object is closer than the light source is to the observer.
Binocular Cues
Binocular cues of an object are dependent upon the slightly
different viewing angle of each eye of an object. Binocular
perception is useful only when the object is close enough to
make an obvious difference in the viewing angle of both eyes.
In the flight environment, most distances outside the cockpit
are so great that binocular cues are of little, if any, value. In
addition, binocular cues operate on a more subconscious
level than monocular cues and are performed automatically.
Night Vision Illusions
There are many different types of visual illusions that
commonly occur at night. Anticipating and maintaining
awareness of them is usually the best way to avoid them.
Autokinesis
Autokinesis is caused by staring at a single point of light
against a dark background for more than a few seconds.
After a few moments, the light appears to move on its own.
Apparent movement of the light source will begin in about
8 to 10 seconds. To prevent this illusion, focus the eyes on
objects at varying distances and avoid fixating on one source
of light. This illusion can be eliminated or reduced by visual
scanning, by increasing the number of lights, or by varying
the light intensity. The most important of the three solutions
is visual scanning. A light or lights should not be stared at
for more than 10 seconds.
False Horizon
A false horizon can occur when the natural horizon is
obscured or not readily apparent. It can be generated by
confusing bright stars and city lights. It can also occur while
flying toward the shore of an ocean or a large lake. Because
of the relative darkness of the water, the lights along the
shoreline can be mistaken for stars in the sky. [Figure 17-22]
Reversible Perspective Illusion
At night, an aircraft may appear to be moving away from
a second aircraft when it is, in fact, approaching a second
aircraft. This illusion often occurs when an aircraft is flying
Figure 17-22. At night, the horizon may be hard to discern due to dark terrain and misleading light patterns on the ground.
Apparent horizon Actual horizon
parallel to another’s course. To determine the direction of
flight, pilots should observe aircraft lights and their relative
position to the horizon. If the intensity of the lights increases,
the aircraft is approaching; if the lights dim, the aircraft is
moving away.
Size-Distance Illusion
This illusion results from viewing a source of light that is
increasing or decreasing in luminance (brightness). Pilots
may interpret the light as approaching or retreating.
Fascination (Fixation)
This illusion occurs when pilots ignore orientation cues and
fix their attention on a goal or an object. Student pilots tend to
have this happen when they are concentrating on the aircraft
instruments or attempting to land. They become fixated on
one task and forget to look at what is going on around them.
At night, this can be especially dangerous because aircraft
ground-closure rates are difficult to determine, and there may
be minimal time to correct the situation.
Flicker Vertigo
A light flickering at a rate between 4 and 20 cycles per
second can produce unpleasant and dangerous reactions. Such
conditions as nausea, vomiting, and vertigo may occur. On
rare occasions, convulsions and unconsciousness may also
occur. Proper scanning techniques at night can prevent pilots
from getting flicker vertigo.
Night Landing Illusions
Landing illusions occur in many forms. Above featureless
terrain at night, there is a natural tendency to fly a lower-
than-normal approach. Elements that cause any type of
visual obscurities, such as rain, haze, or a dark runway
environment, can also cause low approaches. Bright lights,
steep surrounding terrain, and a wide runway can produce the
illusion of being too low with a tendency to fly a higher-than-
normal approach. A set of regularly spaced lights along a road
or highway can appear to be runway lights. Pilots have even
mistaken the lights on moving trains as runway or approach
lights. Bright runway or approach lighting systems can create
the illusion that the aircraft is closer to the runway, especially
where few lights illuminate the surrounding terrain.
Prior to flying at night, it is best to learn and know the
challenges of the area in which you are flying in. Study the
area and know how to navigate your way through areas that
may pose a problem at night. For example, many areas near
water may be obscured by low lying clouds or fog. To help
deal with this type of situation, it is important to have a plan
before you leave the ground. In the daytime, fly the routes
and passes that you will be flying at night and determine the
minimum altitude you are willing to use at night. If weather
prevents you from maintaining the altitude that you planned,
make a decision early to turn 180° and land at an alternate
airport with better weather conditions. Always consider
safer alternatives rather than hope things will work out by
taking a chance.
Pilots who fly at night should strongly consider oxygen
supplementation at altitudes and times not required by the
FAA, especially at night when critical judgment and hand-eye
coordination is necessary (e.g., IFR) or if he/she is a smoker
or not perfectly healthy.
Enhanced Night Vision Systems
Synthetic Vision Systems (SVS) and Enhanced Flight Vision
Systems (EFVS) are two systems that can improve the safety
of flight at night. The technology of both is evolving rapidly
and being used more and more. [Figure 17-23]
Figure 17-25. Night time SVS system.Figure 17-24. SVS system.
Figure 17-23. Synthetic and enhanced vision systems.
Synthetic vision system Enhanced vision system
Synthetic Vision System
A Synthetic Vision System (SVS) is an electronic means
to display a synthetic vision image of the external scene
topography to the flight crew. [Figure 17-24] It is not a
real-time image like that produced by an EFVS. Unlike
EFVS, SVS requires a terrain and obstacle database, a
precise navigation solution, and a display. The terrain
image is based on the use of data from a Digital Elevation
Model (DEM) that is stored within the SVS. With SVS, the
synthetic terrain/vision image is intended to enhance pilot
awareness of spatial position relative to important features
in all visibility conditions. This is particularly useful during
critical phases of flight, such as takeoff, approach, and
landing, where important features, such as terrain, obstacles,
runways, and landmarks, may be depicted on the SVS
display. [Figure 17-25] During approach operations, the
obvious advantages of SVS are that the digital terrain image
remains on the pilot’s display regardless of how poor the
visibility is outside.
An SVS image can be displayed on either a head-down
display or head-up display (HUD); however, to date, SVS
has only been certified on head-down displays. Development
efforts to display a synthetic image on a HUD are currently
underway as are efforts that would combine SVS with a real-
time sensor image produced by an EFVS. These systems are
known as Combined Vision Systems. While SVS is currently
certified as an aid to situation awareness only, the FAA
and aviation industry are working on defining operational
concepts and airworthiness criteria that would enable SVS
to be used for operational credit in certain low visibility
conditions. Other future enhancements to SVS displays could
include integrating ADS-B to display traffic information.
Enhanced Flight Vision System
Enhanced Vision (EV) or Enhanced Flight Vision System
(EFVS) is an electronic means to provide a display of
the external scene by use of an imaging sensor, such as
a Forward-Looking InfraRed (FLIR) or millimeter wave
radar (MMWR). In 2004, 14 CFR part 91, section 91.175
was amended to reflect that operators conducting straight-
in instrument approach procedures (in other than Category
II or Category III operations) may now operate below the
published decision height (DH) or minimum descent altitude
(MDA) when using an approved EFVS shown on the pilot’s
HUD. This rule change provides “operational credit” for EV
equipage. No such credit exists for SV.
Chapter Summary
This chapter provides an introduction to aeromedical factors
relating to flight activities. More detailed information on
the subjects discussed in this chapter is available in the
Aeronautical Information Manual (AIM) and online at www.
faa.gov.