Aviation Colour Perception Standards


The Aviation Colour Perception Standard (ACPS) was born of an idea that was transposed into regulation some 94 years ago. The idea came from two established facts:

  1. Colour signalling was being used to convey instructions to taxiing and airborne aircraft;
  2. There existed a group of people whose colour discrimination and recognition abilities were demonstrably degraded.

The Aviation Colour Perception Standard:

The applicant shall be required to demonstrate the ability to perceive readily those colours the perception of which is necessary for the safe performance of duties (ICAO Convention Annex 1 – Chapter

The wording has changed little since that time, but the catalogue of colour usage in the aviation environment has expanded exponentially. The original justification based on coloured signals has disappeared as electronic communication became universal, reliable and diverse.

What are the fundamental facts?

  1. Almost 10% of human males have an inherited deficiency in one or other of the three colour receptor pigments rendering them Colour Vision Deficient. The type of colour vision deficiency depends directly on which of the three pigments is deficient and the severity of the colour vision deficiency may vary from mild to severe, depending on just how much the function of the pigment is missing or degraded. (For more detail see Colour Vision Deficiency Facts)
  2. The aviation environment is highly complex, potentially dangerous and safety is without doubt the most paramount consideration to which all participants must pay the greatest attention.
  3. There are extensive variations in the way the aviation colour perception standard is administered. Some jurisdictions prohibit any participation by colour vision defectives and at the other extreme, colour vision defectives of all types and grades of severity are fully involved at all levels of the aviation industry, even as captains of large modern jet aircraft.
  4. Australia is the country that has the most liberal implementation, a fact that has resulted from two successful legal challenges to the integrity of the aviation colour perception standard. Dr Arthur Pape played a pivotal role in bringing these challenges to their successful conclusion.

Analysis of the Aviation Colour Perception Standard:

A logical analysis of the ACPS can be made based on three key assumptions:

Assumption 1:

There is extensive ‘use’ of colour in the aviation environment.

Assumption 2:

The ‘safe performance of duties’ in the aviation environment requires ‘the ability to perceive readily those colours the perception of which is necessary for the safe performance of duties.’

Assumption 3:

Without ‘the ability to perceive readily those colours the perception of which is necessary for the safe performance of duties,’ these duties will be performed unsafely.

Assumption 1 is self-evident. The list is long and includes dozens of instances both inside and outside the aircraft in the aviation environment. Within the list are such items as instrument panels and controls, as well as maps and charts. In the outside environment, examples include aerodrome beacons, navigation lights on aircraft, obstruction lighting, runway and taxiway lighting, approach lighting and the parking guidance systems.

However, simply listing such instances of colour usage is not sufficient to validate Assumption 2: The ‘safe performance of duties’ in the aviation environment requires ‘the ability to perceive readily those colours the perception of which is necessary for the safe performance of duties.’

To do that requires firstly a high level of understanding of what is involved in the ‘safe performance’ of their duties by pilots and in this regard, the devil is in the detail. For each instance of colour usage, it is possible to apply a test: is the usage of colour, when isolated from other cues ‘Sufficient ‘or ‘Necessary’ to convey the information required by the pilot to perform safely?

In the Pape & Crassini Paper in the Journal of the Australasian Society of Aerospace Medicine (see Further Reading section), we have used an informal illustration of this test, which could be applied to all instances of cockpit instrumentation colour usage.

It is a simple matter to illustrate for instances of colour usage in the external aviation environment that colour is neither sufficient nor necessary to the information processing component of safe performance. The scope of analysis of these particular examples is so large that in the 1980’s Australian AAT cases, almost 40 days of evidence was presented which examined these instances in meticulous detail.

Finally, Assumption 3 is equally problematic: Without ‘the ability to perceive readily those colours the perception of which is necessary for the safe performance of duties,’ these duties will be performed unsafely.

In the latest (3rd) edition of the ICAO Manual of Civil Aviation Medicine, in section 11.8.29, it states:

The problem with colour vision standards for pilots and air traffic controllers is that there is very little information which shows the real, practical implications of colour vision defects on aviation safety.

We argue that there is now a growing population of colour vision defective pilots, particularly in the USA and in Australia, from which an evidentiary basis for the truth or falsity of Assumption 3 can be determined by appropriate empirical testing. The ‘safe performance’ of pilots is something that is regularly tested and assessed within check and training programs that all airlines are obliged to administer. As a matter of routine, professional pilots are required to demonstrate ‘safe performance’ in normal and emergency procedures in sophisticated and highly realistic simulators, where all manner of emergency and extraordinary scenarios can be invoked with no risk to life or safety.

Tallahassee B727 Accident and Co-Pilots Colour Vision Deficiency

This NTSB investigation report relating to an accident in 2002 is often quoted as a reason for requiring more stringent colour vision standards. Again, the ICAO Manual of Civil Aviation Medicine section 11.8.44 also states:

Aircraft accidents in which colour perception defects have been cited as a contributing factor are rare but have occurred. One example is the crash of FedEx flight 1478 (a Boeing 727) in Tallahassee in 2002 during a night visual approach to land, where the first officer’s colour deficiency interfered with his ability to discern the red and white lights of the PAPI*. Studies of colour perception in the aviation environment have so far been limited. Further research in this area is required to determine precisely the importance of colour perception and what defects can be allowed without affecting safety.’

(*PAPI: Precision Approach Path Indicator, a series of white and red lights that aid flight crews in determining if they are on a proper glide slope to the runway).

The details of the accident quoted in the inquiry by the NTSB raise more questions than answers. There were three crew members on the flight deck of the ill-fated FedEx Flight 1478. All three survived the crash and gave evidence to the NTSB investigation of the accident. The co-pilot flying the aircraft had a colour vision deficiency. The Captain and the Second Officer (the flight engineer) both had normal colour vision. All three had unblemished safety records. All three had visual access to the PAPI.

No crew member perceived the danger of a crash short of the runway, which must have been apparent from the PAPI indication. Fatigue was found to be a significant factor in relation to the Captain’s and the First Officer’s performance, which clearly proved to be ‘unsafe.’ There were a number of other factors such as a late and unnecessary change of landing runway which added to the crew’s workload. In spite of these, the NTSB attributed a major cause of the crash to the co-pilot’s defective colour vision and went on to make strong recommendations to the FAA regarding more stringent colour vision screening for aircrew. What is most surprising of all is that no questions were raised as to the design features of the PAPI system whereby the two colour vision normal observers failed to appreciate the critical information that the PAPI was presenting to them.

The fundamental design flaws of the PAPI system were foreshadowed many decades earlier by B.A.J. Clark and J.E. Gordon in December 1981, in a paper entitled ‘Hazards of Colour Coding in Visual Approach Slope Indicators.’ It was published for the Department of Defence Aeronautical Research Laboratories in Melbourne, Australia. In this paper, the exact circumstances of this accident were predicted by the authors and in their conclusion they state:

Colour coding as a primary cue is almost universally condemned in the ergonomics literature and an examination in this report of the numerous factors which act to degrade the reliability of colour-coded primary signals from VASI’s supports the view that neither Red-White VASIS nor PAPI should be used in air transport operations.

Apart from reasonably common circumstances which render colour-coded signals from these aids as unreliable, e.g. atmospheric conditions and windshield and projection optics scattering, there is a strong prima facie case that the signals, through combinations of physical and physiological circumstances which are not rare, can become sufficiently misleading as to be hazardous.

In spite of this significant condemnation, Australia was forced by the ICAO bureaucratic mechanism to adopt PAPI in favour of the far superior T-VASIS system which used shape coding as primary slope guidance. Cost considerations won out over safety and the FedEx Flight 1478 outcome in Tallahassee is a demonstrable casualty.

As alluded to earlier, the questions raised by the circumstances of this accident go far beyond the colour vision of one crew member. The readiness of both the FAA and the CAA UK to attribute such significance as they do to this event in support of their colour vision policies is quite astounding.