"Why didn't I see that wire until too late?"
This Article first
appeared in the Australian Bureau of Air Safety magazine: “Aviation
The human eyeball
“Mark I” is a very versatile apparatus that serves us well.
however, even with “perfect” sight, physical limitations in its
One such limitation is its power of
resolution - that is,
minimal size of an object that can be registered - due to, the
construction of the sensor, the retina.
In some respects
the retina resembles the grain in black and white photographic film.
The grain in your eye is determined by the finite size of the sense
organs, the cones.
As anyone who has enlarged black and white
film knows, the size of grain limits the detail that can be
periphery of the retina is coarse grained and picks up movement but not detail
central part is fine grained and registers detail.
The usual country power line or telephone wire when viewed from a
safe (in flying terms) distance makes
too small a vision angle for
it to register on any of the cones in your eyes.
Then how then do we ever see it?
conditions, that is against a plain contrasting background such
as the sky, the eye has a compensating mechanism that relies on
In effect, we perceive the break in continuity of the
background rather than ' seeing ' the wire itself.
computer, the brain, happily translates this into seeing.
reduce the contrast and break up the plain background and we are
thrown back on to the basic visual mechanism limited by the grain
The wire literally disappears.
It is not
'camouflaged', it is simply beyond the
limits of the eye to see it and no matter how hard we
stare, squint or move our heads we will never be able to see it.
We are wasting our time looking.
physiological facts have obvious and important implication for
pilots in country areas, particularly agricultural pilots and those
who must have a 'closer look'.
is necessary to fly low in the course of a job, up to date
charts/maps of line obstructions must be obtained and supplemented
by a ground survey.
The extra power line to a shed may be new since the last time you
flew in the
For those who must
look or work closer, an adequate safety height must be maintained
and prudence observed wherever pylons/poles can be seen.
Do not, repeat
“do not, expect to spot the actual wires
from the air”.
Your visual apparatus (eyes) is not
sufficiently sensitive, and if you do not see them it will be too
The lower you are - the less chance that you will see a power pole.
you are that low (spray, locust survey) try to minimise your head
movements and scan using your eyes like a radar receiver (side to