What does a complete approach lighting configuration look like?
No single approach light does the whole job. White centreline and crossbars give alignment and distance, red side rows fill the inner approach, sequenced flashers pull the eye in from far out, and the green threshold ends the pattern — all fed by series circuits that one control system sets together.
A complete approach lighting configuration is every part of the pattern on the extended runway centreline, read together. Each part has its own colour, position and job. Taken one at a time they are simple; taken together they tell the pilot where the runway is, whether the aircraft is lined up and level, and how far there is still to go.
1Give the line
The white centreline runs out from the threshold, 900 m wherever possible in a precision system, with lights every 30 m and the first light 30 m from the threshold. In CAT II/III the first 300 m are barrettes. It shows the pilot where the runway is and whether the aircraft is lined up with it.
2Give the bank and the distance
Crossbars cross the centreline at fixed distances from the threshold: one at 300 m in CAT I, and two, at 150 m and 300 m, in CAT II/III. They give a roll and lateral reference, and because each sits at a known distance, they also let the pilot judge how far remains.
3Fill the inner approach
In CAT II/III, two red side rows of barrettes run along the inner 270 m, one each side of the centreline. They make the inner pattern denser and add roll and distance cues for the lowest-visibility approaches.
4Pull the eye in
Sequenced flashing lights are white flashers on the centreline, spaced 30 m like the steady lights: along the whole 900 m in CAT I, and from 300 m outward in CAT II/III. They flash one after another toward the threshold, with the sequence repeating about twice a second. They supplement the steady lights and never replace them, and they make the centreline easier to find early in fog, haze or rain.
5End at the threshold
The green threshold row marks where the runway starts, at the point the white pattern ends. Where provided, wing bars carry the green line outward beyond both runway edges.
6One system behind the lights
The steady-burning lights are fed from series circuits by constant current regulators, with an isolating transformer at each light, so a failed lamp darkens only its own light while an open loop darkens the whole circuit. Sequenced flashers normally run from their own timer and control units, and how they are powered varies by system. The control and monitoring system selects the brightness step, keeps the approach, threshold and runway lights coordinated, and returns status and faults to the operator.
| Part | Precision CAT I | Precision CAT II / III |
|---|---|---|
| Length from threshold | 900 m wherever possible | 900 m wherever possible |
| White centreline | 30 m spacing; 1, 2 then 3 light sources per position (inner, central, outer 300 m), or barrettes | 30 m spacing; barrettes over the inner 300 m |
| Crossbars | One 30 m crossbar at 300 m | Two crossbars: 150 m fills the gap between the centreline and the side rows; 300 m is 30 m wide |
| Red side rows | None | Red barrettes over the inner 270 m |
| Sequenced flashers | Supplement the steady lights along the whole length (30 flashers at 30 m) | Supplement the steady lights from 300 m outward (21 flashers at 30 m) |
| Threshold | Green row; wing bars where extra conspicuity is wanted | Green row; wing bars where extra conspicuity is wanted |
Frequently Asked Questions
Q1What is a complete approach lighting configuration?
It is the whole pattern of lights on the extended runway centreline working together: the white centreline and crossbars, red side rows in CAT II/III, sequenced flashing lights supplementing the steady lights toward the outer end, and the green threshold row with wing bars where provided. They are powered by series circuits and set to brightness steps by one control system.
Q2Which lights make up a CAT II/III approach lighting system?
A white centreline of 900 m wherever possible with lights every 30 m and barrettes over the inner 300 m, crossbars at 150 m and 300 m, and two red side rows of barrettes over the inner 270 m. Sequenced flashing lights supplement the steady lights from 300 m outward, and the green threshold row marks the start of the runway.
Q3How does a CAT I configuration differ from CAT II/III?
A CAT I system also runs 900 m wherever possible with lights every 30 m, but it has a single 30 m crossbar at 300 m and no red side rows. The centreline can use a single light source per position in the innermost 300 m, two in the central 300 m and three in the outer 300 m, or barrettes. CAT II/III adds the red side rows, a second crossbar at 150 m and barrettes over the inner 300 m for the lowest visibilities.
Q4Why are some approach lights white, some red and some green?
Colour tells the pilot which part of the pattern they are looking at. White forms the centreline and crossbars that carry alignment and distance, red marks the side rows in the inner part of a CAT II/III approach, and green marks the threshold where the runway starts.
Q5Do sequenced flashing lights replace the steady approach lights?
No. They supplement the steady-burning lights. They flash one after another toward the threshold, with the sequence repeating about twice a second, which makes the centreline easier to pick out early in fog, haze or rain, while the steady lights keep giving the pattern.
Q6How is a complete approach lighting system powered and controlled?
The steady-burning lights are fed from series circuits by constant current regulators, with an isolating transformer at each light so a failed lamp darkens only that light. The sequenced flashers have their own timer and control units. The airfield lighting control and monitoring system selects the brightness step, keeps the approach, threshold and runway lights coordinated, and returns status and faults to the operator.
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