Recognition of the aircraft navigation light color code

Navigation lights are a set of color-coded signals intended to indicate the presence, orientation, and relative direction of aircraft at night, and thereby reduce the possibility of midair collisions. It is known that some people with defective color vision have difficulty with quite simple codes. Accordingly, the International Civil Aviation Organization (ICAO) has recommended -- and most countries apply -- that applicants for pilot's licences demonstrate the ability to recognise colored light signals. Pilots who fail to meet this requirement are restricted from flying at night. But is the navigation light signal system effective? This paper concludes that the navigation light system at night can serve as a crude screening method to categorize intruder aircraft into "potential threat" and "no threat" categories. An experiment is described which shows that observers with normal color vision can determine intruder aircraft orientation and relative direction from the navigation light code with a moderately high degree of reliability. The reliability of judgement is, however, decreased by the higher-intensity presence lights also displayed by aircraft.