"Why didn't I see that wire until too late?"

 

 

This Article first appeared in the Australian Bureau of Air Safety magazine: “Aviation Safety Digest”

It has been amended slightly from the original and has some small additions added such as “Tip”

 

 

The human eyeball “Mark I” is a very versatile apparatus that serves us well.

 It has, however, even with “perfect” sight, physical limitations in its performance.

 

One such limitation is its power of resolution - that is, the 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 obtained.

·         The periphery of the retina is coarse grained and picks up movement but not detail

·         The 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?

 

Under specific conditions, that is against a plain contrasting background such as the sky, the eye has a compensating mechanism that relies on this contrast.

In effect, we perceive the break in continuity of the background rather than ' seeing ' the wire itself.

Our mobile computer, the brain, happily translates this into seeing.

However, reduce the contrast and break up the plain background and we are thrown back on to the basic visual mechanism limited by the grain (cone) size.

 

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.

 

These physiological facts have obvious and important implication for pilots in country areas, particularly agricultural pilots and those who must have a 'closer look'.

 

Where it 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.

 

Tip: The extra power line to a shed may be new since the last time you flew in the area.

 

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 late'.

 

Tip:  The lower you are - the less chance that you will see a power pole.

If you are that low (spray, locust survey) try to minimise your head movements and scan using your eyes like a radar receiver (side to side)

 

Fly Safe

TC