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exposure to cabin pressure altitudes above 5,000 feet, carbon
monoxide inhaled through smoking, deficiency of Vitamin A
in the diet, and by 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.
1-4
Figure 1-4. Angular Acceleration and the Semicircular Tubes.
Figure 1-3. Inner Ear Orientation.
Ears
The inner ear has two major parts concerned with orientation,
the semicircular canals and the otolith organs. [Figure 1-3] 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 right angles
to each other, each located on one of three axes: pitch, roll,
or yaw as illustrated in Figure 1-4. 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 which 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 1-5]
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
1-5
Figure 1-6. Linear Acceleration.
Figure 1-5. 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 will result 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 1-6] 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 will be 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
　
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