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a) The nature of the flying operation — airline transport, charter, agriculture, private, recreational,
air traffic control.
b) The type of aircraft — fixed or rotary wing, cockpit layout including seating position of the pilot,
single or multi-crew arrangement.
c) The applicant — which eye is affected, what is the status of the other eye, and does the
applicant have full range of head, neck and eye movements.
d) Special tasks — helicopter slung-load operations, hoisting, search and rescue, supply drops,
nap-of-the-earth flying, crop-spraying, power-line inspection, multiple aircraft aerobatics and
display flying. Operations involving close proximity to the ground, other aircraft, ships or
people constitute high-risk flying activities.
11.6.5 In general, monocularity does not pose a significant problem for air traffic controllers. For those
working at electronic display terminals, care must be taken to ensure that fixed secondary displays such as
map boards and weather radar screens are located comfortably inside the operator’s monocular field of vision.
11.6.6 Monocular individuals can perform many flying tasks safely, particularly in multi-crew situations
where visual tasks can be shared. For single-seat operations it is sometimes possible to adjust seating or
provide aids such as rear-view or downward-looking mirrors to compensate for the loss of peripheral vision.
11.6.7 In monocular individuals it is obviously important to provide optimum vision for the normal eye
(correcting spectacles, sunglasses) and to minimize the risk of injury to that eye during high-risk flying activities,
e.g. by use of helmet with visor to minimize injury from bird strike.
11.6.8 Substandard vision in one eye has been dealt with in an earlier section of this chapter.
In many applicants with a small visual field defect in the central 50 degrees of the visual field in one eye, the
extent of the binocular visual field will be normal and medical certification may be considered.
11.7 OCULAR MUSCLE BALANCE
11.7.1 With the evolutionary migration of the eyes from the sides of the head to the front, there came the
need for accurate alignment of the two eyes so as to achieve single, binocular vision throughout the entire
visual field. Binocularity or binocular vision results from the coordinated movement of the two eyes in a way that
produces a single mental impression. The blending of the visual information gathered from each eye into a
single, unified perception is called fusion. Fusion has two components: 1) a motor component which steers the
eyes in the proper direction; and 2) a sensory component which serves the integration of the electrical data
arriving at the two halves of the occipital visual cortex.
11.7.2 In the normal individual who looks at an object in space, the images of this object in each eye will
fall on what are called corresponding retinal points. These are points in each eye which have the same “visual
direction”. For example, each of the foveae have the “straight ahead direction”. An object in the left half of the
Part III. Medical Assessment
Chapter 11. Ophthalmology III-11-41
visual field will form its image somewhere on the nasal half of the left retina and somewhere on the temporal
half of the right retina. These will therefore be corresponding retinal points.
11.7.3 For any given position of the eyes, i.e. with the eyes focused at any given distance, the locus of
those points in space the images of which fall on corresponding retinal points form an imaginary curved plane
in space which is called the horopter (from Gr. horos = limit). Objects located on the horopter will be seen as
single. Objects which are not on or close to the horopter will be seen as double. This is the physiological
diplopia (“double vision”) which we all have but which is usually unnoticed. There is an infinite number of
horopters in space depending on where the eyes are focused. At the centre of the horopter, that is at the
projection of the two foveae, even slight displacement of an object from the plane of the horopter will result in
diplopia. As one moves further away from the fovea the amount by which an object can be displaced behind
or in front of the horopter before inducing diplopia, increases. The boundary of the space in which single vision
is maintained is called Panum’s fusional area.
11.7.4 Thus rather than corresponding retinal points there is for every point in one retina a corresponding
area in the other retina. The further into the periphery of the retina, the larger the corresponding area in the
fellow eye. This explains the shape of Panum’s fusional area8.
11.7.5 Measuring ocular muscle balance in applicants is important to detect conditions which might cause
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