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Spatial Orientation in Flight
Spatial orientation in flight is difficult to achieve because numerous sensory
stimuli (visual, vestibular, and proprioceptive) vary in magnitude, direction,
and frequency. Any differences or discrepancies between visual, vestibular, and
proprioceptive sensory inputs result in a sensory mismatch that can produce
illusions and lead to spatial disorientation. Good spatial orientation relies on
the effective perception, integration and interpretation of visual, vestibular
(organs of equilibrium located in the inner ear) and proprioceptive (receptors
located in the skin, muscles, tendons, and joints) sensory information.
Vestibular Aspects of Spatial Orientation
The inner ear contains the vestibular system, which is also known as the
organ of equilibrium. About the size of an pencil eraser, the vestibular system
contains two distinct structures: the semicircular canals, which detect changes
in angular acceleration, and the otolith organs (the utricule and the saccule),
which detect changes in linear acceleration and gravity. Both the semicircular
canals and the otolith organs provide information to the brain regarding our
body’s position and movement. A connection between the vestibular system
and the eyes helps to maintain balance and keep the eyes focused on an object
while the head is moving or while the body is rotating.
The Semicircular Canals
The semicircular canals are three half-circular, interconnected tubes located
inside each ear that are the equivalent of three gyroscopes located in three
planes perpendicular (at right angles) to each other. Each plane corresponds
to the rolling, pitching, or yawing motions of an aircraft.
Each canal is filled with a fluid called endolymph and contains a motion
sensor with little hairs whose ends are embedded in a gelatinous structure
called the cupula. The cupula and the hairs move as the fluid moves inside the
canal in response to an angular acceleration.
The movement of the hairs is similar to the movement of seaweed caused
by ocean currents or that of wheat fields moved by wind gusts. When the
head is still and the airplane is straight and level, the fluid in the canals does
not move and the hairs stand straight up, indicating to the brain that there is
no rotational acceleration (a turn).
If you turn either your aircraft or your head, the canal moves with your head, but
the fluid inside does not move because of its inertia. As the canal moves, the hairs
inside also move with it and are bent in the opposite direction of the acceleration by
the stationary fluid (A). This hair movement sends a signal to the brain to indicate
that the head has turned. The problem starts when you continue turning your aircraft
at a constant rate (as in a coordinated turn) for more than 20 seconds.
In this kind of turn, the fluid inside the canal starts moving initially, then
friction causes it to catch up with the walls of the rotating canal (B). When
this happens, the hairs inside the canal will return to their straight up position,
sending an erroneous signal to the brain that the turn has stopped–when, in
fact, the turn continues.
If you then start rolling out of the turn to go back to level flight, the fluid
inside the canal will continue to move (because of its inertia), and the hairs will
now move in the opposite direction (C), sending an erroneous signal to the brain
indicating that you are turning in the opposite direction, when in fact, you are
actually slowing down from the original turn.
A
B C
Vestibular Illusions
(Somatogyral - Semicircular Canals)
Illusions involving the semicircular canals of the vestibular system occur
primarily under conditions of unreliable or unavailable external visual references
and result in false sensations of rotation. These include the Leans, the
Graveyard Spin and Spiral, and the Coriolis Illusion.
The Leans. This is the most common illusion during flight and is caused
by a sudden return to level flight following a gradual and prolonged turn that
went unnoticed by the pilot.
The reason a pilot can be unaware of such
a gradual turn is that human exposure to a
rotational acceleration of 2 degrees per second
or lower is below the detection threshold
of the semicircular canals. Leveling the
wings after such a turn may cause an illusion
that the aircraft is banking in the opposite
direction. In response to such an illusion, a
pilot may lean in the direction of the original
turn in a corrective attempt to regain the
perception of a correct vertical posture.
The Graveyard Spin is an illusion that can occur to a pilot who intentionally
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