AGL-CS-051 — Taxiway Edge Light Circuit Failure
The AGL Knowledge Hub | Engineering Case Study Series. A complete taxiway edge lighting circuit outage investigation — from CCR verification through continuity testing, TDR localization, excavation, and joint replacement.
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 evening airport operations, the Air Traffic Control (ATC) tower reported that an entire taxiway edge lighting circuit became unavailable, leaving a major taxiway segment without blue edge guidance lights.
The Constant Current Regulator (CCR) was operational, but the taxiway edge lighting circuit could not be energized successfully.
Because the failure affected the entire taxiway circuit, a systematic investigation was initiated covering the CCR, primary series cable, cable joints, isolating transformers, and taxiway field distribution network.
The investigation identified failure of a major underground primary cable joint located between two taxiway junction pits, which interrupted the complete taxiway edge lighting series circuit.
This case demonstrates that complete taxiway edge light circuit failure requires structured system-level diagnosis, rapid fault localization, and safe restoration of taxiway guidance lighting.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose complete taxiway edge lighting circuit failures.
- Differentiate CCR faults from primary circuit failures.
- Perform systematic taxiway circuit isolation.
- Locate complete series-circuit interruptions.
- Investigate cable joints and underground taxiway primary cables.
- Restore taxiway edge lighting safely.
- Document complete taxiway circuit failure investigations.
3System Background
A taxiway edge lighting circuit consists of:
Taxiway edge lights provide:
- Blue taxiway boundary guidance.
- Surface movement guidance.
- Taxi route definition.
- Apron access guidance.
- Low-visibility taxi guidance.
Complete circuit failure may result from:
- Primary cable open circuit.
- Cable joint failure.
- Excavation damage.
- Water ingress.
- Earth fault.
- Mechanical damage.
- Incorrect field modifications.
Because taxiway edge lights operate in a series circuit, one major interruption can disable the entire taxiway lighting circuit.
4Problem Statement
The reported condition was:
- Entire taxiway edge lighting circuit OFF.
- CCR operational.
- Circuit could not be energized.
- Adjacent taxiway circuits operational.
- No scheduled maintenance in progress.
Has the CCR failed, or has the taxiway edge lighting primary circuit become completely interrupted?
5Initial Observations
| CCR status | Healthy |
| Selected current | 6.6 A |
| Actual current | 0 A |
| Output voltage | Rising rapidly |
| Taxiway edge lights | Entire circuit OFF |
| Circuit energization | Unsuccessful |
The absence of output current suggested a complete circuit interruption.
6Initial Engineering Hypotheses
The maintenance team considered:
CCR output failure.
Primary taxiway cable open circuit.
Major cable joint failure.
Complete transformer circuit interruption.
Taxiway switching or isolation error.
7Taxiway Circuit Failure Principle
A taxiway AGL circuit requires:
If the primary loop is interrupted:
The CCR may remain electrically healthy.
8Test Equipment
The investigation used:
- 5 kV insulation resistance tester.
- Continuity tester.
- Cable fault location equipment.
- Time-domain reflectometer (TDR).
- Low-resistance ohmmeter.
- Thermal imaging camera.
- Approved AGL circuit test equipment.
- Taxiway circuit drawings.
- Airport-approved PPE.
9Investigation
Step 1: Verify CCR Operation
Testing confirmed:
| CCR startup | Normal |
| Internal diagnostics | Normal |
| Output capability | Normal |
| Protection system | Normal |
The CCR was healthy.
Step 2: Measure Circuit Continuity
The taxiway primary circuit showed:
- No continuity.
- Infinite resistance.
- No measurable loop current.
The circuit was open.
Step 3: Sectional Isolation
The taxiway circuit was divided into major sections.
Results:
| CCR–T1 | Continuous |
| T1–T2 | Continuous |
| T2–T3 | Open circuit |
| T3–End | Not tested |
The interruption was localized to the T2–T3 section.
Step 4: Cable Fault Location
TDR measurements indicated:
- Reflection characteristic of an open circuit.
- Estimated distance matching taxiway joint pit T3.
- No evidence of a short circuit.
The fault was localized accurately.
Step 5: Cable Joint Excavation
Excavation revealed:
- Water ingress.
- Corroded conductor connection.
- Carbonized insulation.
- Mechanical separation of the primary conductor.
- Complete joint failure.
The joint had interrupted the taxiway edge lighting series circuit.
Step 6: Transformer Verification
Transformers downstream of the joint showed:
- No primary current.
- No secondary voltage.
- No transformer defects.
The transformers were healthy but de-energized.
10Root Cause
The complete taxiway edge lighting circuit failure was caused by catastrophic failure of a major underground primary cable joint, which completely interrupted the taxiway edge lighting series circuit.
The joint failed because of:
- Long-term moisture ingress.
- Corrosion.
- Insulation deterioration.
- Mechanical conductor separation.
The CCR was healthy and correctly indicated loss of circuit current.
11Corrective Action
The maintenance team:
- Isolated the taxiway circuit.
- Excavated the failed cable joint.
- Removed the damaged cable section.
- Installed a new approved primary cable joint.
- Restored conductor continuity.
- Performed insulation resistance testing.
- Verified complete circuit continuity.
- Energized the CCR.
- Confirmed taxiway edge light operation.
- Restored taxiway availability.
12Post-Repair Verification
The complete taxiway edge lighting circuit was restored successfully.
13Root-Cause Analysis
Complete taxiway circuit outage
Healthy
Open circuit
Fault localized
Joint location identified
Cable joint failure confirmed
Primary cable joint open circuit
Replace joint and restore continuity
14Why This Case Was Important
Complete taxiway circuit failure can cause:
- Taxiway closure.
- Surface movement restrictions.
- Increased ATC workload.
- Reduced airport operational efficiency.
- Low-visibility operational limitations.
Rapid fault localization is operationally critical.
15Engineering Lessons
A healthy CCR can exist with a completely failed taxiway lighting circuit.
A healthy CCR can exist with a completely failed taxiway lighting circuit.
Zero output current usually indicates a complete primary interruption.
Zero output current usually indicates a complete primary interruption.
Sectional isolation significantly reduces taxiway fault-location time.
Sectional isolation significantly reduces taxiway fault-location time.
TDR is highly effective for taxiway cable fault localization.
TDR is highly effective for taxiway cable fault localization.
Taxiway cable joints are critical reliability components.
Taxiway cable joints are critical reliability components.
16Engineer's Decision Point
You observe: Entire taxiway edge lighting circuit OFF. CCR healthy. Output current 0 A. Output voltage rising rapidly. What should you investigate first?
17Knowledge Check
18Interview Questions
Quick Reference: Taxiway Edge Light Colour, ICAO Requirements, Circuit & Failure Causes
Taxiway edge light color is blue. ICAO taxiway edge lights are used to mark the edges of taxiways, and are normally supplied from their own series circuit. A taxiway lighting circuit diagram normally runs: CCR, then the primary series cable, then an isolating transformer at each light, then the taxiway light.
What causes taxiway light failure? The causes are similar to runway lights, with more damage from vehicles and wheel loads. Typical causes are moisture ingress, lamp or LED failure, damaged connectors or base cans, failed isolating transformers and excavation damage to cable. When a whole circuit fails, as in this case, check the CCR, the main cable and any open circuit first.
19Frequently Asked Questions
Quick answers about taxiway edge lighting circuit failure, based on this case study.
Q1What is AGL-CS-051?
AGL-CS-051 is an engineering training case study on a complete taxiway edge lighting circuit failure, reported by the ATC tower, that left a major taxiway segment without blue edge lights. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2What did the CCR readings show?
The CCR was operational, but actual current was 0 A against the selected 6.6 A and output voltage was rising rapidly. Adjacent taxiway circuits were operational, and the absence of output current suggested a complete circuit interruption.
Q3Does zero current mean the CCR has failed?
Not necessarily. Zero current with a healthy CCR points to a complete series-circuit interruption, and the CCR correctly indicated the loss of circuit current.
Q4What was the root cause?
Catastrophic failure of a major underground primary cable joint completely interrupted the taxiway edge lighting series circuit, through long-term moisture ingress, corrosion, insulation deterioration and mechanical conductor separation.
Q5Which equipment helped locate the open circuit?
The team used a 5 kV insulation resistance tester, continuity tester, cable fault location equipment, a time-domain reflectometer (TDR), a low-resistance ohmmeter and a thermal imaging camera, with taxiway circuit drawings.
Q6How was the circuit restored?
The team isolated the taxiway circuit, excavated the failed joint and removed the damaged cable section. They installed a new approved joint, restored conductor continuity, tested insulation resistance, verified continuity, energized the CCR and restored taxiway availability.
Q7How was the repair verified?
Output current returned to 6.6 A with normal output voltage, circuit continuity was restored and the taxiway edge lights were operational.
Q8What should you investigate first if the whole taxiway circuit is OFF with 0 A and rising voltage?
Verify primary circuit continuity and localize the open circuit. Replacing the CCR, increasing the current setting or replacing all taxiway transformers is not the right first step.
Q9What causes taxiway light failure?
Taxiway light failure is usually caused by water in the fixture or connector, lamp or LED failure, damaged base cans or connectors, failed isolating transformers, and cable damage from excavation or heavy loads. If the whole circuit is out, check the CCR output and the primary series circuit for an open circuit or earth fault.
Engineering & Expert Notes
Match the Locator to the Fault Type
TechniqueTime-domain reflectometry suits opens and some shorts. Combine it with route records and insulation tests to confirm the position.
Split the Taxiway Circuit at Accessible Points
MethodUse pits and junctions to halve the search. Record each result before moving on.
Prove Isolation Before Excavating Near the Joint
SafetyA failed joint may still carry hazardous voltage. Apply the permit and test procedure before digging.
Mark the New Joint on the Drawing
RecordsAdd its position, method and test results so the next search starts from facts.
20Case Study Conclusion
Complete taxiway edge lighting circuit failure is usually a primary series-circuit problem that requires systematic continuity verification, sectional isolation, cable fault localization, and field-joint investigation rather than immediate CCR replacement.
Systematic CCR verification, continuity testing, TDR analysis, cable-joint excavation, insulation testing, and post-repair circuit verification can rapidly restore taxiway edge lighting operation and minimize airport operational disruption.
In AGL engineering, when an entire taxiway edge circuit goes dark, the fastest restoration begins with proving where the primary series loop has been interrupted.
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