Bidirectional versus unidirectional fixtures
One body that lights both ways, or one beam for one observer. Fewer fixtures against exactly the right light in exactly the right direction.
A bidirectional fixture carries two optical units back to back, aimed 180° apart along the runway or taxiway axis, so one body serves observers from both directions. A unidirectional fixture has a single optical unit and sends its beam one way only, with nothing useful shown behind it.
The choice follows who needs to see the light. Both directions, as with runway centreline, runway edge, and combined threshold/end lights, suit bidirectional. One observer, such as touchdown zone lights, stop bars and runway guard lights, suits unidirectional. Bidirectional saves fixtures, bases and transformers. Unidirectional avoids wasted light and misleading signals to the wrong aircraft.
Two beams, 180° apart. Serves both directions from one body, one base and one transformer.
One beam, one direction. Dark behind, so nothing shows to the wrong aircraft.
| Aspect | Bidirectional | Unidirectional |
|---|---|---|
| Optical units | Two, back to back | One |
| Beams | Two, 180° apart, on the runway or taxiway axis | One, aimed at a single approach or holding aircraft |
| Rear | Second beam toward the opposite observer | Dark: no useful light behind the fixture |
| Colour | Set independently per side, same or different | One colour, toward its observer |
| Typical uses | Runway edge, runway centreline, combined threshold/end (green one way, red the other), taxiway centreline | Touchdown zone, stop bar, runway guard, threshold-only (green toward the approach), runway end-only (red) |
| Equipment | One body, base and transformer instead of two one-way lights | One per direction needed |
| Orientation | Mis-rotation misaims both beams | Fitted 180° round, it lights away from the aircraft it serves |
| Fault effect | A shared fault can remove light in both directions | Affects one direction only |
| Power | Series circuit via isolation transformer (both) | |
What decides one-way or two-way
Who needs to see it
One approach or holding aircraft calls for unidirectional. Aircraft from both ends call for bidirectional.
Colour on each side
Confirm the colour pair at each position, such as white and white, white and yellow (edge lights in the caution zone), white and red (centreline near the runway end, or a displaced threshold area), or green and red.
Orientation
Check which way each beam faces at installation and again at commissioning. A reversed fixture lights the wrong way.
Misleading signals
Make sure no beam shows to an aircraft it was not meant for, especially at stop bars and guard lights.
Shared faults
One body, base and transformer means one fault can darken both directions. Plan spares and fault response accordingly.
Standard and design
Follow ICAO Annex 14 and the aerodrome design for the required type and photometrics at each position.
Frequently Asked Questions
Q1What is the difference between a bidirectional and a unidirectional fixture?
A bidirectional fixture has two optical units back to back in one body, aimed 180 degrees apart, so it serves observers from both directions. A unidirectional fixture has one optical unit and sends its beam one way only, with no useful light behind it.
Q2Where are unidirectional fixtures used?
Where only one observer needs the light: touchdown zone lights facing the approach, stop bars and runway guard lights facing the aircraft at the holding position, and threshold lights showing green toward the approach.
Q3Where are bidirectional fixtures used?
Where aircraft use the same lights from both ends: runway centreline and edge lights, taxiway centreline lights, and threshold or end lights that show green one way and red the other. One body does the work of two one-way lights.
Q4Can the two sides of a bidirectional fixture show different colours?
Yes. Each side is set independently. A centreline fixture can show white both ways, or white one way and red near the runway end. An edge fixture can show white one way and yellow the other in the caution zone of an instrument runway. A threshold or end fixture shows green toward the approach and red the other way.
Q5What happens if a unidirectional fixture is installed the wrong way round?
Its light points away from the aircraft it is meant to serve. Orientation is critical, so it is checked at installation and again at commissioning. A bidirectional fixture that is mis-rotated misaims both beams.
Q6Why not use bidirectional fixtures everywhere?
Where only one direction needs light, a second beam wastes light and can show a misleading signal to the wrong aircraft. Bidirectional fixtures also share one body, base and transformer, so a fault in a shared element (base, transformer, secondary cable or connector) can take out both directions.
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