Calvert Cross Bar Approach Lighting System
In the late 1940s the problems of transition from the new precision radio instrument approach aids to the final, visual approach at night or in reduced visibiltiy much exercised civil aviation. Various high-intensity approach lighting systems were developed to ease the transition from instrument to visual flight. In 1949 three competing systems - a British one (the Calvert system), a French one (a form of displaced Calvert system) and an American one (the Slope Line system) were presented to ICAO. Because each system had its supporters among the ICAO members, ICAO ended up recommending that any of the three systems could be adopted. Independently, and before the ICAO decision, DCA engineers had also been sudying the problem of approach lighting and had concluded that the British 'Calvert' system was superior. A trial installation was installed on Essendon's Runway 08 toward the end of 1951. This trial proved successful and the system was commissioned for operational use in 1953.
The following information about the Calvert cross bar lighting system is based on an article High Intensity Approach Lighting by S W Hart (DCA Sectional Airways Engineer) which appeared in the Civil Aviation Jounal, the DCA's house publication, Vol 1, No 3, March 1951.
In 1946 Mr E S Calvert of the Royal Aircraft Establishment, Farnborough, was requested by a UK Ministry of Civil Aviation Airfield Lighting Committee to investigate the problem of approach lighting and establish the general principles involved. Calvert tackled the problem by attempting to ascertain the visual and mental processes by which a pilot lands an aircraft. He then developed a theoretical model by which different lighting systems could be compared, and tested his theoretical results using simulation.
Calvert's line of reasoning led him to the conclusion that to provide smooth transition from instrument to visual flying without optical illusions, and to provide sensitive and natural indications which could easily be interpreted by the average pilot, the approach lighting pattern should consist of a centre line of light with horizontal bars of light running transversely across it at even intervals. This pattern consists of two basic elements - a line of lights leading to the runway threshold, and horizontal lights to define the attitude of the aircraft. Calvert placed much stress on roll guidance compared with the Americans who, up to that time, had completely neglected it. He was the first to realise that it was easy to confuse lateral displacement with angle of bank.
The Calvert system does not indicate a defined glide path, but the widths of the horizon bars are such that, if a pilot maintains a glide that will take him to the correct touch down point, each bar will appear to be the same width as the previous one as it disappears under the nose of the aircraft. Distance is indicated by using single lights in the centre line to indicate 1000 ft or less from the threshold, double lights for 1000-2000 ft and triple lights for 2000-3000 ft.
It is interesting to note that the basic form of the Calvert cross bar lighting system still forms the basis for high-intensity approach lighting systems today.
Diagrams above: With horizon bars the pilot can see if he is on an even keel (left) or banked right wing down to turn on to the centre line (right).
Below: A modified form of the basic High Intensity Approach Lighting system is used on some capital city precision approach (ILS) runways, in this case on Melbourne/Tullamarine's Runway 27. The main difference is the larger array of lights just prior to the threshold.
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