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coordinated action plus the mental integration of all their messages establish a reference which keeps
human beings upright and oriented in relation to the direction of the gravitational force.
The eye is a very reliable orientation mechanism provided adequate reference points are available.
When flying, however, there are disadvantages in trying to interpret visual clues. Objects seen from the
air often look quite different from objects seen from the ground. In the air, there is also a lack of visual
clues that a continuous background provides for recognition of objects and assessment of their size and
distance.
Visual illusions in flight may be caused by any of the following factors:
a) Optical characteristics of windshields
b) Rain on windshields
c) Fog, haze, dust and their effects on depth perception
d) Glide slope
e) Width and length of runway
f) Runway lighting systems
g) Runway slope
h) Terrain slope
i) Landing at night over water or other unlit terrain
j) Auto-kinetic illusion
k) White-out, specifically in high-latitude areas.
The semicircular canals are associated with equilibrium. Angular movement or rotation of the body
moves the fluid of the semicircular canal, thereby causing displacement of the cupulae covering the hair
cells in the ampullae. Impulses are transmitted to the brain and interpreted as motion. Since each one of
the three semicircular canals lies in a different plane, they can report rotation in three planes. The normal
mode of stimulation for these organs is an abrupt, short-duration acceleration followed immediately by a
short deceleration.
It must be remembered that the semicircular canals provide information only about angular
movements of the head. Sensations of relative motion and relative position of body parts are supplied by
perceptors in the skin, joints and muscles. Otoliths provide information about position.
Humans normally depend on the complex integration of the three above-mentioned sensory inputs,
i.e. eyes, inner ear and proprioceptors, for the perception of the body’s relationships to terrestrial
references.
The following are common examples of disorientation in flight:
a) In a horizontal turn, the illusion of continued straight flight may be experienced if the rate of turn
is too low to stimulate the semicircular canals.
b) The subjective impression of angle of bank during instrument flying is false when the angular
change is introduced gradually and below the thresholds of stimulation of the semicircular canals
and proprioceptors.
c) The “graveyard spiral” results when, in a prolonged (> 20 seconds), coordinated banked turn the
cupulae come to rest and the sensation of turning is lost. When leveling the wings, the pilot may
experience a sensation of now turning to the opposite side. To counter-act this sensation of
ICAO Preliminary Unedited Version — October 2008 II-1-17
turning, the pilot may re-enter the original turn. Because the instruments indicate loss of altitude,
the pilot may pull back on the stick and add power, thus making the turn tighter (increasing the
bank) and inducing the spiral.
d) The somatogravic illusion is caused by the effect of acceleration on the otolith organ. When
deprived of visual input from the surrounding world (for example taking-off in IMC6), a pilot
may interpret accelerative forces (+GX
7) as a nose high attitude of his aircraft, correct this false
sensation by pushing the stick forward and may thus fly his aircraft into the ground.
A further elaboration on disorientation in flight, as well as vertigo, is contained in Part III,
Chapters 10 and 12.
COMMUNICATIONS
The importance of the communication system in present-day civil aviation operations cannot be
overemphasized. Speech intelligibility and communication are vital elements in the safety of civil
aviation. In order to start the engine, taxi the aircraft, line up for take-off, get clearance for take- off, start
climbing procedures, reach cruising level, or to initiate the sequence of events that will lead to the safe
approach and landing of the aircraft at the destination, a licence holder must be able to transmit and
receive verbal instructions to and from the air traffic control system as well as from the crew complement.
In this particular respect, account should be taken not only of the physiological speech intelligibility in
noisy surroundings, but also of the aspect of hearing under operational conditions, when the attention is
required to encompass a multiplicity of stimuli which are of paramount importance.
Interference with intelligibility and speech communication is a potentially serious problem which can
be brought about by higher levels of noise at certain frequencies. This problem can prevent crew members
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Manual of Civil Aviation Medicine 1(65)
