Introduction
Human factors is a broad field that examines the interaction
between people, machines, and the environment for the
purpose of improving performance and reducing errors. As
aircraft became more reliable and less prone to mechanical
failure, the percentage of accidents related to human factors
increased. Some aspect of human factors now accounts for
over 80 percent of all accidents. Pilots, who have a good
understanding of human factors, are better equipped to plan
and execute a safe and uneventful flight.
Flying in instrument meteorological conditions (IMC) can
result in sensations that are misleading to the body’s sensory
system. A safe pilot needs to understand these sensations and
effectively counteract them. Instrument flying requires a pilot
to make decisions using all available resources.
The elements of human factors covered in this chapter include
sensory systems used for orientation and illusions in flight.
For more information about physiological and psychological
factors, medical factors, aeronautical decision-making
(ADM), and crew resource management (CRM), refer to the
Pilot’s Handbook of Aeronautical Knowledge.
Human
Factors
Chapter 3
Figure 3-1. Rubik’s cube graphic depicting the visual processing of information.
Sensory Systems for Orientation
Orientation is the awareness of the position of the aircraft
and of oneself in relation to a specific reference point.
Disorientation is the lack of orientation, and spatial
disorientation specifically refers to the lack of orientation
with regard to position in space and to other objects.
Orientation is maintained through the body’s sensory organs
in three areas: visual, vestibular, and postural. The eyes
maintain visual orientation. The motion sensing system in
the inner ear maintains vestibular orientation. The nerves in
the skin, joints, and muscles of the body maintain postural
orientation. When healthy human beings are in their natural
environment, these three systems work well. When the
human body is subjected to the forces of flight, these senses
can provide misleading information. It is this misleading
information that causes pilots to become disoriented.
Eyes
Of all the senses, vision is most important in providing
information to maintain safe flight. Even though the human
eye is optimized for day vision, it is also capable of vision
in very low light environments. During the day, the eye uses
receptors called cones, while at night, vision is facilitated
by the use of rods. Both of
these provide a level of vision
optimized for the lighting
conditions that they were
intended. That is, cones are
ineffective at night and rods
are ineffective during the day.
Rods, which contain rhodopsin
(called visual purple), are
especially sensitive to light
and increased light washes out
the rhodopsin compromising
the night vision. Hence, when
strong light is momentarily
introduced at night, vision
may be totally ineffective as
the rods take time to become
effective again in darkness.
Smoking, alcohol, oxygen
deprivation, and age affect
vision, especially at night. It
should be noted that at night,
oxygen deprivation, such
as one caused from a climb
to a high altitude, causes a
significant reduction in vision.
A return back to the lower
altitude does not restore a pilot’s vision in the same transitory
period used at the climb altitude.
The eye also has two blind spots. The day blind spot is the
location on the light sensitive retina where the optic nerve
fiber bundle (which carries messages from the eye to the
brain) passes through. This location has no light receptors,
and a message cannot be created there to be sent to the brain.
The night blind spot is due to a concentration of cones in an
area surrounding the fovea on the retina. Because there are
no rods in this area, direct vision on an object at night will
disappear. As a result, off-center viewing and scanning at
night is best for both obstacle avoidance and to maximize
situational awareness (SA). (See the Pilot’s Handbook of
Aeronautical Knowledge and the Aeronautical Information
Manual (AIM) for detailed reading.)
The brain also processes visual information based upon color,
relationship of colors, and vision from objects around us.
Figure 3-1 demonstrates the visual processing of information.
The brain assigns color based on many items, to include an
object’s surroundings. In the figure below, the orange square
on the shaded side of the cube is actually the same color
as the brown square in the center of the cube’s top face.
Figure 3-2. Shepard’s tables illustrating problems with perception as both tables are the same length.
Isolating the orange square from surrounding influences
will reveal that it is actually brown. The application to a real
environment is evident when processing visual information
that is influenced by surroundings. The ability to pick out an
airport in varied terrain or another aircraft in a light haze are
examples of problems with interpretation that make vigilance
all the more necessary.
Figure 3-2 illustrates problems with perception. Both tables
are the same lengths. Objects are easily misinterpreted in
size to include both length and width. Being accustomed to
a 75-foot-wide runway on flat terrain is most likely going to
influence a pilot’s perception of a wider runway on uneven
terrain simply because of the inherent processing experience.
Vision Under Dim and Bright Illumination
Under conditions of dim illumination, aeronautical charts and
aircraft instruments can become unreadable unless adequate
flight deck lighting is available. In darkness, vision becomes
more sensitive to light. This process is called dark adaptation.
Although exposure to total darkness for at least 30 minutes is
required for complete dark adaptation, a pilot can achieve a
moderate degree of dark adaptation within 20 minutes under
dim red flight deck lighting.
Red light distorts colors (filters the red spectrum), especially
on aeronautical charts, and makes it very difficult for the
eyes to focus on objects inside the aircraft. Pilots should
use it only where optimum outside night vision capability is
necessary. White flight deck lighting (dim lighting) should
be available when needed for map and instrument reading,
especially under IMC conditions.
Since any degree of dark adaptation is lost within a few
seconds of viewing a bright light, pilots should close one eye
when using a light to preserve some degree of night vision.
During night flights in the vicinity of lightning, flight deck
lights should be turned up to help prevent loss of night vision
due to the bright flashes. Dark adaptation is also impaired by
exposure to cabin pressure altitudes above 5,000 feet, carbon
monoxide inhaled through smoking, deficiency of Vitamin
A in the diet, and prolonged exposure to bright sunlight.
During flight in visual meteorological conditions (VMC),
the eyes are the major orientation source and usually
provide accurate and reliable information. Visual cues
usually prevail over false sensations from other sensory
systems. When these visual cues are taken away, as they
are in IMC, false sensations can cause the pilot to quickly
become disoriented.
An effective way to counter these false sensations is to
recognize the problem, disregard the false sensations, rely
on the flight instruments, and use the eyes to determine the
aircraft attitude. The pilot must have an understanding of
the problem and the skill to control the aircraft using only
instrument indications.
YAW
ROLL
YAW
PITCH
PITCH
ROLL
Eardrum
Eustachian tube
Bone
Ear canal
Bone
Ear canal
Eardrum
Eustachian tube
The semicircular tubes are
arranged at approximately
right angles to each other, in
the roll, pitch, and yaw axes.
Figure 3-4. Angular acceleration and the semicircular tubes.
Saccule
Semicircular canals
Utricle
Cochlea
Tubular ducts
containing endolymph
Ampullae
Semicircular canals
Endolymph fluid
Cupola
Filaments of hair cells
Hair cells
Vestibular nerve
Ampulla of a
semicircular
canal
Cupola
Sensory hairs
Otolith organ
Vestibular nerve
The motion sensing system is
located in each inner ear in the
approximate position shown.
Figure 3-3. Inner ear orientation.
Ears
The inner ear has two major parts concerned with orientation:
the semicircular canals and the otolith organs. [Figure 3-3] The
semicircular canals detect angular acceleration of the body,
while the otolith organs detect linear acceleration and gravity.
The semicircular canals consist of three tubes at approximate
right angles to each other, each located on one of three axes:
pitch, roll, or yaw as illustrated in Figure 3-4. Each canal is
filled with a fluid called endolymph fluid. In the center of
the canal is the cupola, a gelatinous structure that rests upon
sensory hairs located at the end of the vestibular nerves. It
is the movement of these hairs within the fluid that causes
sensations of motion.
Because of the friction between the fluid and the canal, it
may take about 15–20 seconds for the fluid in the ear canal
to reach the same speed as the canal’s motion.
To illustrate what happens during a turn, visualize the
aircraft in straight-and-level flight. With no acceleration of
the aircraft, the hair cells are upright, and the body senses
that no turn has occurred. Therefore, the position of the hair
cells and the actual sensation correspond.
Placing the aircraft into a turn puts the semicircular canal and
its fluid into motion, with the fluid within the semicircular
canal lagging behind the accelerated canal walls. [Figure 3-5]
This lag creates a relative movement of the fluid within the
canal. The canal wall and the cupula move in the opposite
direction from the motion of the fluid.
The brain interprets the movement of the hairs to be a turn in
the same direction as the canal wall. The body correctly senses
that a turn is being made. If the turn continues at a constant
rate for several seconds or longer, the motion of the fluid in
Normal
Accelerating
Head tilted back
Figure 3-6. Linear acceleration.
Endolymph
Tube
Cupola
No turning
No sensation.
Start of turn
Sensation of turning
as moving fluid deflects
hairs.
Constant rate turn
No sensation after fluid
accelerates to same
speed as tube wall.
Turn stopped
Sensation of turning in
opposite direction as moving
fluid deflects hairs in opposite
direction.
Figure 3-5. Angular acceleration.
the canals catches up with the canal walls. The hairs are no
longer bent, and the brain receives the false impression that
turning has stopped. Thus, the position of the hair cells and
the resulting sensation during a prolonged, constant turn in
either direction results in the false sensation of no turn.
When the aircraft returns to straight-and-level flight, the fluid
in the canal moves briefly in the opposite direction. This sends
a signal to the brain that is falsely interpreted as movement
in the opposite direction. In an attempt to correct the falsely
perceived turn, the pilot may reenter the turn placing the
aircraft in an out-of-control situation.
The otolith organs detect linear acceleration and gravity in a
similar way. Instead of being filled with a fluid, a gelatinous
membrane containing chalk-like crystals covers the sensory
hairs. When the pilot tilts his or her head, the weight of these
crystals causes this membrane to shift due to gravity, and
the sensory hairs detect this shift. The brain orients this new
position to what it perceives as vertical. Acceleration and
deceleration also cause the membrane to shift in a similar
manner. Forward acceleration gives the illusion of the head
tilting backward. [Figure 3-6] As a result, during takeoff and
while accelerating, the pilot may sense a steeper than normal
climb resulting in a tendency to nose-down.
Nerves
Nerves in the body’s skin, muscles, and joints constantly
send signals to the brain, which signals the body’s relation to
gravity. These signals tell the pilot his or her current position.
Acceleration is felt as the pilot is pushed back into the seat.
Forces, created in turns, can lead to false sensations of the
true direction of gravity and may give the pilot a false sense
of which way is up.
Uncoordinated turns, especially climbing turns, can cause
misleading signals to be sent to the brain. Skids and slips
give the sensation of banking or tilting. Turbulence can create
motions that confuse the brain as well. Pilots need to be aware
that fatigue or illness can exacerbate these sensations and
ultimately lead to subtle incapacitation.
Illusions Leading to Spatial
Disorientation
The sensory system responsible for most of the illusions
leading to spatial disorientation is the vestibular system.
Visual illusions can also cause spatial disorientation.
Vestibular Illusions
The Leans
A condition called “the leans” can result when a banked
attitude, to the left for example, may be entered too slowly
to set in motion the fluid in the “roll” semicircular tubes.
[Figure 3-5] An abrupt correction of this attitude sets the
fluid in motion, creating the illusion of a banked attitude to
the right. The disoriented pilot may make the error of rolling
the aircraft into the original left banked attitude, or if level
flight is maintained, feel compelled to lean in the perceived
vertical plane until this illusion subsides.