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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 1-5]
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, will feel compelled to lean in the perceived
vertical plane until this illusion subsides.
1-6
Figure 1-7. Graveyard Spiral.
Coriolis Illusion
The coriolis illusion occurs when a pilot has been in a turn
long enough for the fluid in the ear canal to move at the same
speed as the canal. A movement of the head in a different
plane, such as looking at something in a different part of the
flight deck, may set the fluid moving and create the illusion
of turning or accelerating on an entirely different axis.
This action causes the pilot to think the aircraft is doing a
maneuver that it is not. The disoriented pilot may maneuver
the aircraft into a dangerous attitude in an attempt to correct
the aircraft’s perceived attitude.
For this reason, it is important that pilots develop an instrument
cross-check or scan that involves minimal head movement.
Take care when retrieving charts and other objects in the flight
deck—if something is dropped, retrieve it with minimal head
movement and be alert for the coriolis illusion.
Graveyard Spiral
As in other illusions, a pilot in a prolonged coordinated,
constant rate turn, will have the illusion of not turning.
During the recovery to level flight, the pilot will experience
the sensation of turning in the opposite direction. The
disoriented pilot may return the aircraft to its original turn.
Because an aircraft tends to lose altitude in turns unless the
pilot compensates for the loss in lift, the pilot may notice
a loss of altitude. The absence of any sensation of turning
creates the illusion of being in a level descent. The pilot may
pull back on the controls in an attempt to climb or stop the
descent. This action tightens the spiral and increases the loss
of altitude; hence, this illusion is referred to as a graveyard
spiral. [Figure 1-7] At some point, this could lead to a loss
of control by the pilot.
Somatogravic Illusion
A rapid acceleration, such as experienced during takeoff,
stimulates the otolith organs in the same way as tilting the
head backwards. This action creates the somatogravic illusion
of being in a nose-up attitude, especially in situations without
good visual references. The disoriented pilot may push the
aircraft into a nose-low or dive attitude. A rapid deceleration
by quick reduction of the throttle(s) can have the opposite
effect, with the disoriented pilot pulling the aircraft into a
nose-up or stall attitude.
Inversion Illusion
An abrupt change from climb to straight-and-level flight can
stimulate the otolith organs enough to create the illusion of
tumbling backwards, or inversion illusion. The disoriented
pilot may push the aircraft abruptly into a nose-low attitude,
possibly intensifying this illusion.
Elevator Illusion
An abrupt upward vertical acceleration, as can occur in
an updraft, can stimulate the otolith organs to create the
illusion of being in a climb. This is called elevator illusion.
The disoriented pilot may push the aircraft into a nose-low
attitude. An abrupt downward vertical acceleration, usually
1-7
Figure 1-8. Sensations From Centrifugal Force.
in a downdraft, has the opposite effect, with the disoriented
pilot pulling the aircraft into a nose-up attitude.
Visual Illusions
Visual illusions are especially hazardous because pilots rely
on their eyes for correct information. Two illusions that lead
to spatial disorientation, false horizon and autokinesis, are
concerned with only the visual system.
　
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