What makes a sign visible?
A sign only guides a pilot if the pilot can read it in time. Visibility comes from legend size, a lit and contrasting face, and an unobstructed view, and it is lost when any of them fails.
A pilot taxiing at night or in poor visibility has seconds to read a sign and act on it. Annex 14 therefore specifies the size of the legend, the colours, and the luminance of a lit face, and the sign must also stay in a condition where those values are actually delivered.
1Big enough to read in time
The legend is sized so it can be read from a distance. Annex 14 sets minimum character heights of 200, 300 and 400 mm, and the sign face is at least twice the legend height, which leaves the margin around the characters that keeps them legible. A larger legend is read from further away, which is why signs for faster or more critical movements use the larger sizes.
2Lit and contrasting
The face carries the specified colours, and a lit face has its luminance set in Annex 14, Volume I, section 5.4 and Appendix 4, so it stays readable after dark and in poor visibility. FAA guidance generally has signs lit where the runway or taxiway they serve is lit. Evenness matters as much as brightness: hot spots and dark patches make a legend harder to read.
3Seen and kept seen
The pilot needs a clear line of sight to the face, set by position, height and alignment at installation. In service, visibility is lost to dirt and fading, failed light sources, vegetation, snow, parked equipment and glare. These are caught by inspection and put right by cleaning or repair, as later entries cover.
| Factor | What it needs |
|---|---|
| Legend size | Minimum character heights of 200, 300 or 400 mm, with face height at least twice the legend height |
| Luminance | Face lit to the luminance and colour set in ICAO Annex 14, Volume I, section 5.4 and Appendix 4 |
| Contrast | Legend and background in the specified colours, on a clean, unfaded face |
| Line of sight | No vegetation, snow, equipment or other sign hiding the face from the approach |
| Condition | Face clean, light sources working, no glare or reflections |
| Verification | Checked by inspection and lit test, and corrected when it falls short |
Frequently Asked Questions
Q1What decides whether a pilot can read a sign?
Four things work together: the size of the legend, the luminance and colour contrast of the face, a clear line of sight to it, and the condition of the sign. If any one fails, the sign is harder to read however good the others are.
Q2Why are legend heights fixed at set sizes?
A taller legend can be read from further away. Annex 14 sets minimum character heights of 200, 300 and 400 mm, with a face at least twice the legend height, so the legend has the margin around it that keeps it legible at a distance.
Q3How is a sign kept visible at night and in poor weather?
By lighting it. A lit sign has a face whose luminance and colour are set in ICAO Annex 14, Volume I, section 5.4 and Appendix 4. FAA guidance generally has signs lit where the runway or taxiway they serve is lit. Lighting is only useful if the face is clean and the lamps or LEDs are working.
Q4What most often reduces sign visibility in service?
Dirt and fading on the face, failed or dim light sources, vegetation, snow or equipment hiding the face, and glare or reflections. These are found by regular inspection, and the sign is cleaned or repaired. Inspection and maintenance have their own entries later.
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