How do markings and lights stay visible at night and in low visibility?
Paint carries the picture by day, lights carry it after dark, and in fog the lights are made brighter and more numerous. Each form of guidance covers where the other is weakest.
Visual guidance has to work in daylight, in darkness and in fog, and no single form of it does all three well. Markings are strongest by day, lights by night, and a set of extra lights and brightness steps is added when visibility is low.
1By day: paint
White markings on dark pavement, and yellow on taxiways, are high in contrast and show the whole pattern at once: threshold, centreline, aiming point and edges. Lights are on in daylight too, but they have to compete with sunlight, so they are used at a higher brightness step.
2By night: lights
Paint only returns the light that falls on it, mostly from the aircraft's own lights. Markings that are retroreflective return more of it, which helps nearby, but they are seen at short range. Lights make their own light, so the edge, threshold and centreline lights show the layout and the colours show where on the runway the aircraft is. Signs are internally lit or retroreflective for the same reason.
3Brightness is stepped to the conditions
The lights are set to one of several brightness steps, chosen from the visibility and the ambient light. A high step in fog or daylight would dazzle on a clear night, so it is turned down when the conditions allow. The step is set by air traffic control or the lighting control system.
4In low visibility: more lights
Runways used in low visibility add centreline and touchdown zone lights and brighter approach lights, so the pilot has cues even when the far end is hidden. At very low runway visual range, taxiways have centreline lights and holding positions have stop bars, so movement is guided and held by lights rather than by paint and signs that cannot be seen.
5Keeping them readable
Rubber deposits and fading dull the paint, and dirt, snow or damaged lenses cut the light output. Cleaning, repainting and photometric checks matter most in the conditions where the guidance is hardest to read. Where one form is degraded, the other has to cover for it.
| Condition | What the pilot relies on | What supports it |
|---|---|---|
| Day, clear | Paint, then lights | High-contrast white and yellow markings, lights at a higher step |
| Night, clear | Lights | Edge, threshold and centreline lights at a lower step, retroreflective paint near the aircraft |
| Reduced visibility | Lights at a higher step | Brightness stepped up by control, approach lights, lit or retroreflective signs |
| Low visibility, precision runway | Lights first, paint only close to the aircraft | Centreline and touchdown zone lights, brighter approach lights |
| Very low runway visual range | Lights on taxiways and runways | Taxiway centreline lights, stop bars at holding positions, low visibility procedures |
Frequently Asked Questions
Q1Why can paint not do the job at night?
Paint only reflects light that falls on it. At night that is mostly the aircraft's own landing lights, so even retroreflective paint is seen only at short range and at a shallow angle. Lights make their own light, can be seen from far away, and can change colour and brightness.
Q2What does retroreflective marking do?
It returns light toward its source, so a marking containing glass beads or similar material shows up better under an aircraft's lights. It improves the paint at night and in poor visibility, but it does not replace lights.
Q3How is the brightness of the lights chosen?
The lights have several brightness steps. Air traffic control, or the control system, selects a step to suit the visibility and the ambient light, so a bright setting is used in fog or daylight and a low one on a clear night to avoid glare.
Q4Why is too much brightness a problem?
On a clear dark night a high step is dazzling and hides the pattern of the lights. Brightness is therefore matched to the conditions rather than left at maximum.
Q5What extra lights are used in low visibility?
Runways used for precision approaches in low visibility add centreline lights and touchdown zone lights, and the approach lights are brighter. For very low visibility, taxiways have centreline lights and holding positions have stop bars so that aircraft and vehicles can be guided and held without relying on paint or signs.
Q6Do dirt and wear affect visibility?
Yes. Rubber deposits and fading dull the paint, and dirt, snow or damaged lenses reduce light output. Markings are kept clean and repainted, and lights are cleaned, replaced and checked against their photometric performance, because a degraded pattern is hardest to read exactly when visibility is worst.
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