What is the edge-light beam?
How a runway edge light shapes its beam so it can be seen from the approach, along the runway and from across its width.
The edge-light beam is the beam pattern of the runway edge lights, the two rows of lights that mark the sides of the runway. Unlike a centreline light, an edge light is seen from many positions: from the approach, by an aircraft rolling anywhere across the width of the runway, and by aircraft taxiing or circling nearby. So the beam is wide in azimuth, with its intensity kept in a band of low vertical angles, instead of being concentrated along one axis.
The standard requires edge lights to show at all the azimuth angles needed to guide a pilot landing or taking off in either direction, so fixtures are normally bidirectional. Where the lights also give circling guidance, they show at all angles in azimuth. Each direction can have its own colour: white in general, yellow in the caution zone at the far end of the runway, and red before a displaced threshold. Intensity is variable in steps to suit visibility and daylight.
This is the edge-light counterpart of the previous entry. The beam direction runs along the runway, the spread is wide, and the complete distribution is what the isocandela diagram for runway edge lights describes. Compared with the runway centreline beam (Entry 35), the edge-light beam trades concentration for coverage. Because the lights are the lateral reference, tilt, rotation or a dirty lens can make the runway edge look wrong to the pilot.
Frequently Asked Questions
Q1What is the edge-light beam?
The beam pattern of a runway edge light: how it is aimed and shaped so that the two rows of edge lights can be seen from the approach, along the runway and from across its width.
Q2Why is the edge-light beam wider than the centreline beam?
Because the observers are in more places. A pilot on the centreline looks along the runway axis, but edge lights are the lateral reference for an aircraft anywhere across the runway width, so they must show over a wide range of azimuth angles. The exact shape is set by the standard.
Q3Do runway edge lights show different colours in different directions?
Yes. Edge lights are normally white. Under ICAO Annex 14 a section of the lights 600 m or one-third of the runway length (whichever is less) at the remote end, from where the take-off run starts, may show yellow as the caution zone. Lights before a displaced threshold show red in the approach direction. The FAA applies yellow to the last 2,000 ft or half the runway length (whichever is less) on instrument runways. A bidirectional fixture can therefore show white one way and yellow the other.
Q4What can change the beam of an edge light in service?
Tilt or rotation after jet blast, vehicle strikes or mowing damage, a dirty, cracked or discoloured lens, vegetation or snow blocking the view, and degraded light sources or LED modules. Elevated fixtures are therefore checked for aiming and cleanliness as well as for lighting.
Q5Where is the edge-light beam defined?
In the isocandela diagrams of ICAO Annex 14, Appendix 2 for runway edge lights, which take the runway width into account, and in FAA AC 150/5345-46 and the fixture manufacturer's photometric data. Always confirm against the current edition.
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