What is a bidirectional fixture?
One housing, two beams: a light that faces both ways along the runway so aircraft landing from either end see it, each in the colour meant for them.
A bidirectional fixture carries two optical units back to back in one body, aimed 180° apart along the runway or taxiway axis. Each unit throws its own beam toward its own observer, so a single fixture does the work of two one-way lights.
The two sides are independent in colour. Over most of a runway the centreline fixture shows white both ways. Toward each runway end the face that serves aircraft rolling toward that end changes: alternating red and white from 900 m to 300 m from the end, then red for the last 300 m. A threshold/end fixture shows green toward the approach and red the other way, into the runway. On runway edge lights, one side shows yellow in the caution zone at the far end, and between the runway start and a displaced threshold the side facing approaching aircraft shows red. Because both beams share one axis, a fixture that is mis-rotated misaims both.
Two beams, 180° apart. Serves both directions from one body.
One beam, one direction. Used where only one approach needs the light.
Frequently Asked Questions
Q1What is a bidirectional fixture in airfield ground lighting?
A single light fixture with two optical units that send beams in opposite directions, 180 degrees apart, along the runway or taxiway axis. One fixture therefore serves aircraft approaching from either end.
Q2Which AGL lights are normally bidirectional?
Runway edge lights and runway centreline lights are commonly bidirectional, as are threshold/end fixtures that show green toward the approach and red toward the runway. Taxiway centreline lights are also often bidirectional. Touchdown zone lights, stop bars and guard lights are unidirectional because they serve one approach.
Q3Does a bidirectional fixture show the same colour both ways?
Not always. Each side has its own filter or LED colour, so the two beams can match (white and white) or differ (green and red at a threshold/end fixture, or white and yellow in a runway edge caution zone).
Q4Why use a bidirectional fixture instead of two unidirectional ones?
One housing, one base and one transformer (rated for the combined load of both lamps or LED units) can serve both landing directions. That cuts the number of fixtures to install, power and maintain, and keeps both beams locked to the same axis.
Q5What happens if a bidirectional fixture is rotated?
Both beams swing off the runway axis, so pilots see reduced intensity or the wrong colour. A colour-split fixture turned 180 degrees would show the colours to the wrong observers. Orientation is checked during installation and commissioning.
Q6What do the two beams have in common?
They share one axis and one housing, and each beam is designed to meet the photometric requirements of the standard for its light type. Always confirm against ICAO Annex 14 or FAA AC 150/5345-46 and the manufacturer's data.
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