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descent, this manoeuvre is accompanied by a fall in the temperature of the cabin pressurization system
which could permit a potential buildup of ozone. During descent, levels of 400-1 000 μg/m3 (0.2-0.5 ppm)
may be experienced for about ten minutes within the pressurized section of the aircraft. The existing data
on the health effects of ozone, considered in conjunction with its high natural background level, lead to
the recommendation of a 1-hour guideline in the range of 150-200 μg/m3 (0.076-0.1 ppm). To lessen the
potential for adverse acute and chronic effects and to provide an additional margin of protection, an 8-
hour guideline for exposure to ozone of 100-120 μg/m3 (0.05-0.06 ppm) is recommended by the World
Health Organization (WHO). Tests, based on the exposure concentrations and time intervals calculated
for SST aircraft, have been conducted by the Medical Research Council of the United Kingdom and
showed no significant functional impairment. Although the original research concerning ozone and
aviation was undertaken for SST operations, catalytic converters were recommended by the UK House of
Lords Select Committee on Science and Technology to be fitted to subsonic aircraft when they could be
expected to fly through higher concentrations of ozone. Such equipment is now standard on many modern
aircraft.
ACCELERATION EFFECTS
Short-term accelerations
Speed itself in straight and level flight has no effect on the human body; accelerations due to changing
speed and/or direction of flight may, on the other hand, produce very considerable physiological effects
upon the occupants of an aircraft depending on the following factors:
a) magnitude, rate and direction of acceleration;
b) duration;
c) area of application; and
d) protection.
Accelerations of relatively short duration, usually less than a second, are associated with situations
such as flying in turbulence or emergencies such as crash landings. The critical protective factor for
short-term accelerations and rapid decelerations is the availability of restraint systems. The desirability of
shoulder harnesses for flight crew has been documented, taking into account not only crash protection but
also the possibility of on-duty incapacitation of a kind that might interfere with the operation of flight
controls.
The reader is referred to other texts for information relating to long-duration accelerations and other
aspects relevant to in-flight acceleration. Acceleration effects may result in sensory illusions (see below).
ICAO Preliminary Unedited Version — October 2008 II-1-16
SENSORY ILLUSIONS
The sensory perceptors of the human body associated primarily with maintaining equilibrium and
orientation are the eyes, the inner ears and proprioceptors in muscles, tendons and joint capsules. Their
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.
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