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Instrument Flying Handbook Menu>Human
Factors>Sensory
Systems for Orientation >Ears
The inner ear has two
major parts concerned with orientation, the semicircular canals
and the otolith organs. [Figure 1-1] 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 the three axes: pitch, roll, or yaw. 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.
Figure 1-1. Inner ear orientation.
Figure 1-2 illustrates what happens during
a turn. When the ear canal is moved in its plane, the relative
motion of the fluid moves the cupola, which, in turn, stimulates
the sensory hairs to provide the sensation of turning. This
effect can be demonstrated by taking a glass filled with water
and turning it slowly. The wall of the glass is moving, yet
the water is not. If these sensory hairs were attached to the
glass, they would be moving in relation to the water, which
is still standing still.
Figure 1-2. Angular acceleration.
The ear was designed to detect turns of a rather
short duration. After a short period of time (approximately
20 seconds), the fluid accelerates due to friction between the
fluid and the canal wall. Eventually, the fluid will move at
the same speed as the ear canal. Since both are moving at the
same speed, the sensory hairs detect no relative movement and
the sensation of turning ceases. This can also be illustrated
with the glass of water. Initially, the glass moved and the
water did not. Yet, continually turning the glass would result
in the water accelerating and matching the speed of the wall
of the glass.
The pilot is now in a turn without any sensation
of turning. When the pilot stops turning, the ear canal stops
moving but the fluid does not. The motion of the fluid moves
the cupola and therefore, the sensory hairs in the opposite
direction. This creates the sensation of turning in the opposite
direction even though the turn has stopped.
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/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 1-3]
Figure 1-3. Linear acceleration.
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