Putting the Common Sense in Colour Vision Deficiency

By Dr Ian Perry MAE. DAvMed(London), Academician IAASM, FRAeS, Past Master Company of Air Pilots. FAMA,

Colour Vision is a feature of visual perception. It is an ability to perceive differences between light composed of different frequencies independently of light intensity. At the back of the retina of the eye are the photoreceptors, put simply the rods, which work in low light conditions and cones. The cones contain a pigment protein that has different spectral sensitivities. Human eyes have three types of this protein which result in what is known as trichromatic colour vision. These proteins are encoded on the X chromosome. Defective coding of these pigments can lead to the various forms of colour deficiency, the neurophysiology of what takes place is complicated and has been argued about for years. What we are interested here is that the majority of people are born with a mild deficiency of one of the three proteins. The most common variation is red-green deficiency called Deutan (green) and Protan (red).  The other variation of colour deficiency is yellow-blue, which is named Tritan, this is a much rarer form.

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In men worldwide, the prevalence of colour blindness(deficiency) is 5-8%. That is about 1 in 8-10 men. In women it is 0.5-1.0%. Which is about 1 in 200.

The highest incidence is in Caucasian people, Arab and Indian people. The lowest incidence is in people of a much darker skin, Africans and people of a Caribbean origin.

Very few medical conditions can affect CV. Multiple Sclerosis can in its very early stages pick off an eye. The FAA always insisted that CV was tested in each eye at the routine license examination to look for this condition. To date no case has been recorded. Obviously brain injury or optic neuritis can affect CV and vision, but these conditions are associated with other signs and symptoms.

The Aviation Story

Ever since flying began, it has been assumed that all pilots need 100% colour vision, to go with their 100% vision. Early flying standards encompassed this approach. We used maps to navigate which had elevations in different colours, but not the early airliners. They began to rely on instruments where colour was not that important. At that time most pilots had come out of the military, so they had 100% normal CV, as that was the military requirement.

A system of landing aids was introduced later on in aviation. The Precision Approach Path Indicator (PAPI) and the Visual Approach Slope Indicator (VASI). These systems originally used the colour Red as the warning light of being too low. Red of course has always been the colour of danger. It does not of course have to be red, and modern instrument systems now have not necessarily used this colour.

Modern airline flying and navigational systems now default into black, white, blue and grey colours when there is an issue in the cockpit. The original colours of most of these systems have changed in recent years, as there is no operational requirement to use the old standard colours. There are no safety implications as a result of these changes. You can fly any modern aircraft very safely anywhere and not need perfect CV. This suggestion of not needing perfect CV caused serious disturbances amongst aviation medical people when it was raised at various regulatory meetings. Most of them had been raised by the military, as I had, but very few of them had ever been asked to consider lowering the CV standard. The top standard Colour Perception One (CP1), had always been what they had experienced, and they saw no need to change it, despite there being very good and sensible arguments to the contrary. I have been in these international meetings and heard the resistance. The resistance could not now justify keeping CP 1. However the Regulators unfortunately are not going to change, unless some of us can convince them that perfect CV is not now a requisite to flying modern aircraft, civil or military. This would allow more people to fly, especially with the impending pilot shortage.

Where has this inflexibility originated from?

Going back a bit, military selection stated that if you could ride a horse and had perfect vision, you could train to be a pilot. In those days military flying was simple. I have simplified it a bit further. You took off, and visually reported back what you had seen. When radio had been invented you could do this from the air. You could also report on where your artillery was shooting and its effectiveness. This function continued to this day using helicopters. When I was flying full-time in the Army Air Corps, artillery spotting was the name of the game. This has now changed and the artillery has been put on the helicopters, in the shape of cannon and missiles.

It was taken for granted that the pilots had normal colour vision as they were required to report what they could actually see. A classic test remark was “can you see the man in red shirt at 1000yds?” Today using modern optics you can see the buttons on the man’s shirt. Using such optics having 100% CV is not really necessary, as the optics can involve night vision enhancement which is not colour sensitive, it is monochrome green. If you were a sailor you used to have to be able to see signal lights. This has also changed to using more optical equipment. The modern naval requirements are not part of this discussion, but are of interest.

Prior to the 1900’s a number of practical CV tests were developed. Most people are familiar with Professor Ishihara’s Colour Plate test. This test was in fact predated by some 70 yrs by a German, Dr Stilling in 1877. This test consisted of a number of colour plates of different intensities. It became obsolete, when Professor Shinobu Ishihara, a Japanese ophthalmic professor in Tokyo, developed his book of spotted plates which was published in 1917. Some considered this test too severe for a number of occupations and other tests such as the Farnsworth Lantern, also called the Fallant test, was found to be less exclusive. The Giles Archer Lantern was another test method. This consisted of a number of colour filters. A number of those people who had failed Ishihara’s pseudoachromatic plates, would pass the Giles Archer Lantern. Since then more sophisticated computer based test equipment has been developed, and this has been used basically to select the 100% CV normals.

This then is the background from where we are today. 

There has never been, and I have reviewed all the available data, any modern-day aircraft accidents attributed to any failure of any pilots colour vision. There are those, and I have been in international meetings where the national medical representatives will say, there have been no accidents because we have maintained the 100% CV rule. That argument still persists in many states and regulatory organisations. It is the easy way out and gives no leeway to those who may be just outside the 100% normal CV.

What is needed are more simple practical tests, using a simulator. All flying today can be simulated, from the initial training, to the more complex tasks of operating an Airbus 380 or large Boeing. The size does not matter anymore as the instruments on most modern aircraft are the same, or very similar, from taxiing to take off, to landing and return to a stand. From simple airports to the busiest, with complex runways and taxiways. All these modern-day flying and operational tasks including taxing taxiing, can be done without requiring 100% CV. These tasks could probably be quite safely undertaken by someone with complete CV loss, who can only see brown, blue and white.

Some years ago, I saw a senior aircraft engineer who had somehow learnt to fly, who was an airframes and engines person. He could only see the three colours. He was sent to see me, as he was on an upgrade course which involved electronics. He had got lost in the wiring looms. Since then, all wires now have numbers on them to avoid this problem. The systems have been adapted for the CV deficient.

Any aircraft instrument which has colour components, will revert to black, white and blue if there is a problem. 

Many modern-day aircraft instruments do not rely on colour to differentiate the information presented, so why insist that the pilots need 100% normal CV?