Hazards of Colour Coding in Visual Approach Slope Indicators

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Visual approach slope indicators (VASIs) provide visual guidance to the nominal glideslope for aircraft on approach to landing. Prior to 1981, two VASIs were approved by the International Civil Aviation Organization (ICAO), the UK Red-White VASIS and the Australian T-VASIS. A New UK development, PAPI, was recently accepted by the Visual Aids Panel of ICAO as an additional type. Red-White VASIS and PAPI both use colour differences between red and white lights as the primary means of coding the guidance signals. T-VASIS uses colour coding only as a secondary separate warning of extreme undershoot. The historical development of signal coding in VASIs is described in this report. Colour coding is examined with reference to human colour discrimination ability and the degradation and falsification of VASI signals by atmospheric and other influences. There are several ways (not rare) in which the perceived colour differences between red and white VASI signals can become small enough for the coding to be regarded as failed. The signals may then be misinterpreted, resulting in a 'too low' signal being taken as 'too high' with consequent great danger of an undershoot accident. Colour coding of VASI primary signals therefore fails unsafe and VASIs which use it, such as Red-White VASIS and PAPI, should not be used.

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