What is the runway centreline beam?
How a runway centreline light aims and shapes its beam so that pilots travelling along the runway can see it.
The runway centreline beam is the beam pattern of the inset lights along the runway centreline. Pilots keep the aircraft on or close to the centreline during the approach, landing roll and take-off, so they look at each light from almost directly along the runway axis and from a low angle. The beam is shaped for that view: intensity is concentrated near the axis and at low vertical angles, rather than being spread widely to the sides as it is for lights that are also seen from off to one side.
The fixtures are inset in the pavement and are normally bidirectional, sending a beam each way along the runway axis so one fixture serves aircraft in either direction. In the colour-coded final 900 m (see Runway Centreline Lights), each direction shows the colour that applies to aircraft travelling that way.
This is a worked example of three earlier entries: the beam direction is along the runway axis, the spread is kept fairly narrow, and the complete distribution is what the standard's isocandela diagram for centreline lights describes. Because the beam is concentrated, alignment and lens condition matter: a misaligned or contaminated inset fixture shows reduced light to the pilot it is meant to guide.
Frequently Asked Questions
Q1What is the runway centreline beam?
The beam pattern of a runway centreline light: how it is aimed and shaped so that it is seen by pilots travelling along the runway centreline, on the approach, landing roll and take-off.
Q2Why is the centreline beam shaped differently from an edge light beam?
Because the observer is in a different place. A pilot on the centreline views each light from almost directly along the runway axis, so the pattern concentrates intensity near the axis and at the low vertical angles of an aircraft looking along the runway. Edge lights are viewed from the side as well, so their pattern differs. The exact shapes are set by the standard.
Q3Are runway centreline lights bidirectional?
Normally yes. A fixture sends a beam each way along the runway axis so it serves aircraft in either direction. In sections where colour coding is used, each direction shows the colour that applies to aircraft travelling that way.
Q4What can change the beam of a centreline light in service?
Misalignment after fixture replacement or pavement works, dirt and rubber deposits on the lens, and degraded optics or light sources can all reduce or reshape the output. This is why inset lights are checked photometrically as well as visually.
Q5Where is the centreline beam defined?
In the isocandela diagrams of ICAO Annex 14, Appendix 2 for runway centreline lights, with separate diagrams for the permitted longitudinal spacings, and in FAA AC 150/5345-46 and the fixture manufacturer's photometric data. Always confirm against the current edition.
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