AGL-CS-086 — Illuminated Taxiway Sign Failure
The AGL Knowledge Hub | Engineering Case Study Series. An illuminated taxiway sign investigation where internal LED driver failure caused complete loss of illumination — from CCR verification through transformer testing, secondary-cable continuity, input voltage measurement, driver output analysis, internal inspection, and LED module verification.
Important: This is an engineering simulation created for training. It is not presented as an actual airport incident. All readings, values, dates and outcomes are illustrative, and no airport, organisation, person or manufacturer is identified or implied.
1Case Study Overview
During routine night operations, air traffic control reported that one illuminated taxiway guidance sign was completely dark.
The failed sign was a mandatory instruction sign located at a runway holding position, and pilots reported difficulty identifying the holding point during taxi operations.
Adjacent taxiway edge lights and centerline lights were operating normally, and the associated Constant Current Regulator (CCR) was healthy.
A detailed investigation was initiated covering:
- Isolating transformer operation.
- Sign input voltage.
- Internal LED module condition.
- Sign driver electronics.
- Secondary cable continuity.
- L-823 connectors.
- Water ingress.
- Internal sign wiring.
The investigation identified failure of the internal LED driver module, resulting in complete loss of illumination of the taxiway sign.
This case demonstrates that illuminated taxiway sign failures require systematic diagnosis of the sign power system, internal electronics, and field connections rather than replacement of the entire sign assembly.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose illuminated taxiway sign failures.
- Verify sign power supply.
- Test isolating transformers.
- Investigate LED driver failures.
- Assess secondary cable integrity.
- Restore taxiway sign illumination.
- Perform post-repair operational verification.
3System Background
An illuminated taxiway sign system includes:
- Primary series circuit.
- Isolating transformer.
- Secondary cable.
- L-823 connectors.
- Sign power input.
- LED driver module.
- Internal LED arrays.
- Sign control electronics.
- Optical diffuser panels.
Sign failures may result from:
- LED driver failure.
- LED module failure.
- Transformer failure.
- Secondary cable damage.
- Connector corrosion.
- Water ingress.
- Internal wiring failure.
- Power-supply abnormalities.
Taxiway sign reliability is critical for ground movement safety.
4Problem Statement
The reported condition was:
- Sign completely dark.
- Adjacent AGL lights operating.
- CCR healthy.
- No visible external damage.
- Mandatory instruction sign affected.
Has the taxiway sign lost power, or has an internal sign component failed?
5Initial Observations
| Sign illumination | OFF |
| Adjacent lights | ON |
| CCR operation | Normal |
| External damage | None |
| Sign housing | Intact |
The failure appeared localized to the sign.
6Initial Engineering Hypotheses
The investigation considered:
CCR output failure.
Isolating-transformer failure.
Secondary-cable failure.
LED driver failure.
LED module failure.
7Taxiway Sign Failure Principle
The sign is an electrically isolated load within the AGL circuit.
8Test Equipment
The investigation used:
- Digital multimeter.
- Insulation resistance tester.
- Transformer test equipment.
- LED driver test equipment.
- Continuity tester.
- Thermal imaging camera.
- Portable photometer.
- ICAO Annex 14 taxiway sign reference requirements.
9Investigation
Step 1: Verify CCR Operation
Results:
| Output current | Normal |
| Output voltage | Normal |
| Regulation | Stable |
| CCR alarms | None |
The CCR was healthy.
Step 2: Verify Transformer Output
Measurements:
- Primary current: Normal
- Secondary voltage: Present
The isolating transformer was operating correctly.
Step 3: Secondary Cable Continuity
Results:
| Phase continuity | Good |
| Return continuity | Good |
| Connector continuity | Good |
The secondary cable was healthy.
Step 4: Sign Input Voltage
Measured voltage at the sign input terminals:
Correct input voltage present
Power was reaching the sign assembly.
Step 5: Internal Driver Test
Measured LED driver output:
0 V DC
Expected output:
36 V DC
The LED driver was not functioning.
Step 6: Internal Inspection
Inspection revealed:
- Burned driver components.
- Capacitor failure.
- Thermal damage.
- No significant LED damage.
The failure was confined to the driver electronics.
Step 7: LED Module Verification
Using a temporary regulated DC supply:
The LED arrays illuminated normally.
The LED modules were healthy.
10Root Cause
The illuminated taxiway sign failure was caused by failure of the internal LED driver module, resulting from:
- Internal electronic failure.
- Thermal stress.
- Capacitor degradation.
- Driver output collapse.
The transformer, secondary cable, connectors, and LED arrays remained healthy.
The primary failure mechanism was loss of regulated DC output from the sign driver.
11Corrective Action
The maintenance team:
- Isolated the sign.
- Replaced the LED driver module.
- Verified transformer output.
- Tested LED arrays.
- Inspected internal wiring.
- Verified sign brightness.
- Checked color compliance.
- Conducted operational observation.
- Updated maintenance records.
- Recommended periodic driver-health inspections.
12Post-Repair Verification
Driver replacement restored full sign operation.
13Root-Cause Analysis
Taxiway sign dark
Healthy
Healthy
Healthy
Present
Absent
Sign retested
LED driver failure
Replace driver and verify illumination
14Why This Case Was Important
Taxiway sign failures can cause:
- Reduced taxiway identification.
- Holding-position recognition problems.
- Increased pilot workload.
- Low-visibility operational degradation.
- Runway-incursion risk.
- ICAO maintenance concerns.
Illuminated signs are critical visual guidance aids.
15Engineering Lessons
A dark sign does not necessarily indicate a transformer failure.
A dark sign does not necessarily indicate a transformer failure.
Input voltage and driver output must be tested separately.
Input voltage and driver output must be tested separately.
LED driver modules are common sign failure points.
LED driver modules are common sign failure points.
LED arrays should be tested independently.
LED arrays should be tested independently.
Post-repair brightness verification is essential.
Post-repair brightness verification is essential.
16Engineer's Decision Point
You observe: Taxiway sign completely dark. Adjacent taxiway lights operating. CCR healthy. Sign input voltage present. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about illuminated taxiway sign failure and LED driver diagnosis.
Q1What is AGL-CS-086?
AGL-CS-086 is an engineering training case study. It covers a completely dark illuminated taxiway sign at a runway holding position, with the CCR and adjacent lights healthy. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2How did the team decide whether the sign had lost power or an internal part had failed?
They worked from supply to sign. The CCR, transformer output, secondary cable and connectors were healthy, and correct input voltage was present at the sign terminals. That pointed inside the sign.
Q3What was found inside the sign?
The LED driver output was 0 V DC against an expected 36 V DC. Inspection found burned driver components, capacitor failure and thermal damage, with no significant LED damage.
Q4Were the LED arrays healthy?
Yes. On a temporary regulated DC supply the LED arrays lit normally, so the failure was confined to the driver electronics.
Q5What was the root cause of AGL-CS-086?
Failure of the internal LED driver module from internal electronic failure, thermal stress and capacitor degradation, causing loss of regulated DC output.
Q6Why was this sign failure significant?
It was a mandatory instruction sign at a runway holding position, and pilots reported difficulty identifying the holding point during taxi.
Q7Why not replace the whole sign?
Only the driver had failed. Systematic diagnosis of the power system, internal electronics and field connections avoids unnecessary replacement.
Q8What corrective actions were taken?
The team isolated the sign and replaced the LED driver module. They also verified transformer output, tested the LED arrays, inspected internal wiring, checked brightness and colour compliance, conducted operational observation and updated records.
Q9What preventive step was recommended?
Periodic driver-health inspections.
Engineering & Expert Notes
Check Input at the Sign Before Opening the Electronics
MethodConfirm supply reaches the sign, then test the driver output.
Test the LED Array Separately From the Driver
IsolationThis tells you which part has failed.
Consider Thermal Stress on Driver Components
ReliabilityHeat ages capacitors. Look at enclosure temperature.
Verify Legibility After Repair
VerificationCheck brightness and uniformity of the sign face.
20Case Study Conclusion
Illuminated taxiway sign failures are often internal electronic failures caused by LED driver degradation, and systematic verification of transformer output, sign input voltage, driver performance, LED module operation, and post-repair brightness compliance provides the most effective method for restoring safe taxiway guidance.
Systematic electrical diagnosis, driver testing, transformer verification, LED-module testing, optical inspection, and operational validation can rapidly identify taxiway sign failures and restore ICAO-compliant illuminated sign performance.
In AGL engineering, a dark taxiway sign is not merely a failed display; it is a missing instruction that can affect every aircraft approaching a runway holding position.
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