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or unintentionally enters a spin. For example, a pilot who enters
a spin to the left will initially
have a sensation of spinning in
the same direction. However, if
the left spin continues the pilot
will have the sensation that the
spin is progressively decreasing.
At this point, if the pilot applies
right rudder to stop the left spin,
the pilot will suddenly sense a
spin in the opposite direction (to
the right). If the pilot believes
that the airplane is spinning to
the right, the response will be to apply left rudder to counteract the sensation
of a right spin. However, by applying left rudder the pilot will unknowingly
re-enter the original left spin. If the pilot cross checks the turn indicator, he/
she would see the turn needle indicating a left turn while he/she senses a
right turn. This creates a sensory conflict between what the pilot sees on the
instruments and what the pilot feels. If the pilot believes the body sensations
instead of trusting the instruments, the left spin will continue. If enough
􀁰 􀁱
altitude is lost before this illusion is recognized and corrective action is taken,
impact with terrain is inevitable.
The Graveyard Spiral is more
common than the Graveyard Spin,
and it is associated with a return to
level flight following an intentional
or unintentional prolonged bank
turn. For example, a pilot who enters
a banking turn to the left will
initially have a sensation of a turn in
the same direction. If the left turn
continues (~20 seconds or more),
the pilot will experience the sensation
that the airplane is no longer
turning to the left. At this point, if
the pilot attempts to level the wings
this action will produce a sensation that the airplane is turning and banking
in the opposite direction (to the right). If the pilot believes the illusion of a
right turn (which can be very compelling), he/she will reenter the original left
turn in an attempt to counteract the sensation of a right turn. Unfortunately,
while this is happening, the airplane is still turning to the left and losing altitude.
Pulling the control yoke/stick and applying power while turning would
not be a good idea–because it would only make the left turn tighter. If the
pilot fails to recognize the illusion and does not level the wings, the airplane
will continue turning left and losing altitude until it impacts the ground.
The Coriolis Illusion involves the simultaneous stimulation of two semicircular
canals and is associated
with a sudden tilting (forward
or backwards) of the pilot’s head
while the aircraft is turning. This
can occur when you tilt you head
down (to look at an approach
chart or to write a note on your
knee pad), or tilt it up (to look
at an overhead instrument or
switch) or tilt it sideways. This
produces an almost unbearable sensation that the aircraft is rolling, pitching,
and yawing all at the same time, which can be compared with the sensation
of rolling down on a hillside. This illusion can make the pilot quickly become
disoriented and lose control of the aircraft.
The Otolith Organs
Two otolith organs, the saccule and utricle, are located in each ear and are
set at right angles to each other. The utricle detects changes in linear acceleration
in the horizontal plane, while the saccule detects gravity changes in the
vertical plane. However, the inertial forces resulting from linear accelerations
cannot be distinguished from the force of gravity; therefore, gravity can also
produce stimulation of the utricle and saccule. These organs are located at
the base (vestibule) of the semicircular canals, and their structure consists of
small sacs (maculas) covered by hair cell filaments that project into an overlying
gelatinous membrane (cupula) tipped by tiny, chalk-like calcium stones
called otoconia.
Change in Gravity
When the head is tilted, the weight of the otoconia of the saccule pulls the
cupula, which in turn bends the hairs that send a signal to the brain indicating
that the head has changed position. A similar response will occur during
a vertical take-off in a helicopter or following the sudden opening of a parachute
after a free fall.
Change in Linear Acceleration
The inertial forces resulting from a forward linear acceleration (take-off,
increased acceleration during level flight, vertical climb) produce a backward
displacement of the otoconia of the utricle that pulls the cupula, which in
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turn bends the haircell filaments that send a signal to the brain, indicating
that the head and body have suddenly been moved forward. Exposure to a
　
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