What are sign failure modes?
The different ways an airfield sign can fail — dark, dim, patchy, wet, damaged or out of place — and what each one points to, so the right part is checked first.
A failure mode is one particular way a sign stops doing its job, and what it looks like when it does. Naming the mode narrows the search: a sign that is dark, one that is patchy and one that is leaning each point to different parts, so the right one is checked first.
1The light fails or fades
The sign is dark, dim or patchy. The cause lies somewhere along the chain from circuit to face. The whole sign dark usually points upstream, to the circuit, the connectors, the sign transformer or the LED driver. Part of the face dark or uneven usually points to the LED modules or the light guide plate. A sign can fade slowly and still look normal by day, so lit signs are judged at night.
2The structure and surroundings fail
A cracked housing or failed seal lets in water, which can fog the face, corrode parts and cause insulation faults. A mount can loosen, and an impact or jet blast can tilt or turn the sign or break a leg. A frangible mount must still break away as designed, so a damaged one is repaired to specification. The view can also be lost to grass, snow or equipment.
3The circuit fails
An open circuit or earth fault can affect more than one light. A sign’s primary connectors are links in the series loop, so an open there stops every light on the circuit, while a fault on the secondary side only darkens that one sign. An earth fault is a safety matter and may trip the circuit. Record the mode with the sign’s location and symptom; repeat failures at one place point to a root cause.
| Failure mode | What is seen | Often points to |
|---|---|---|
| Whole sign dark | No light at all | Open or tripped circuit, primary connector, sign transformer, LED driver |
| Part of the face dark or dim | One letter or section lacks light | LED module or string, driver output, light guide plate |
| Uneven or patchy light | Bright and dim areas | Light guide plate, optics, ageing LED modules |
| Faded or discoloured face | Hard to read by day or night | Aged or dirty face and legend, drift in colour or brightness |
| Water or fogging inside | Misted face, corrosion | Failed seals, cracked housing, wet joints |
| Tilted, turned or broken | Out of place, leg snapped | Impact, jet blast, ground movement, frangible leg failure |
| Circuit fault | Others on the circuit out, or the circuit trips | Open loop or earth fault: connector, cable, damaged insulation |
Frequently Asked Questions
Q1What are sign failure modes?
They are the different ways an airfield sign can stop doing its job, such as going dark, dimming, lighting unevenly, fading, letting water in, being knocked out of place, or suffering an electrical fault. Naming the mode points to the likely cause.
Q2How can I tell whether the fault is in the sign or in the circuit?
If other lights or signs on the same series circuit are also out, or the circuit trips, suspect the circuit, for example an open loop or an earth fault. If only one sign is affected, suspect that sign's own transformer, connectors, LED driver or LED modules. The isolating transformer keeps the loop closed when a sign fails on its secondary side.
Q3Why can one connector put out a whole circuit?
A sign's primary connectors are links in the series loop and carry the full circuit current. If one opens or fails, the loop is broken and every light on that circuit goes out. A fault on the secondary side only affects that one sign.
Q4Can a sign fail without looking wrong by day?
Yes. A sign can dim, light unevenly or lose a section and still look normal in daylight. Lit signs are therefore judged at night, from where the pilot or driver sees them.
Q5Why does water inside a sign matter?
Failed seals or a cracked housing let in moisture, which can fog the face, corrode parts and cause insulation faults. Moisture in a joint is a common cause of insulation faults, so seals and connectors are checked during inspection and testing.
Q6Does a failed sign need action straight away?
A sign that cannot be read, or is out of place or damaged, may need prompt action. The aerodrome's procedures decide how it is reported and how quickly it is repaired. Opening a sign or touching its wiring needs the circuit isolated, locked out and proved dead first.