AGL-CS-081 — Approach Light Segment Failure
The AGL Knowledge Hub | Engineering Case Study Series. An approach lighting system investigation where a complete segment of lights became inoperative due to a cable-joint failure — from CCR verification through failure-boundary identification, continuity testing, TDR fault localization, joint excavation, and operational restoration.
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 a complete segment of the approach lighting system had become inoperative, with approximately 12 consecutive approach lights not illuminated.
Pilots reported that the approach lighting pattern appeared discontinuous, making runway acquisition more difficult during reduced-visibility approaches.
The Constant Current Regulator (CCR) remained operational, circuit current was maintained, and no major CCR trip occurred.
Because the failure affected a consecutive group of approach lights, a detailed investigation was initiated covering series-circuit continuity, isolating transformers, cable joints, L-823 connectors, segment boundaries, and field fault localization.
The investigation identified failure of a cable joint within the affected approach-light segment, resulting in loss of power to the downstream portion of the series circuit.
This case demonstrates that approach-light segment failures are typically circuit-distribution problems requiring systematic fault localization rather than replacement of individual light fittings.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose approach-light segment failures.
- Interpret segment-failure patterns.
- Perform series-circuit fault localization.
- Verify isolating-transformer continuity.
- Investigate cable-joint failures.
- Restore complete approach-light operation.
- Conduct post-repair operational verification.
3System Background
An approach-light segment includes:
- Series primary cable.
- Isolating transformers.
- Secondary cables.
- L-823 connectors.
- Approach light fittings.
- Cable joints.
- Junction interfaces.
Segment failures may result from:
- Cable-joint failure.
- Primary-cable damage.
- Connector failure.
- Transformer open circuit.
- Water ingress.
- Excavation damage.
- Mechanical stress.
- Insulation failure.
Approach-light reliability depends on series-circuit continuity.
4Problem Statement
The reported condition was:
- One approach-light segment OFF.
- Approximately 12 consecutive lights inoperative.
- CCR healthy.
- Remaining approach lights operational.
- Pilot reports of discontinuous approach pattern.
Where is the circuit interruption that has isolated the downstream approach-light segment?
5Initial Observations
| CCR operation | Normal |
| Circuit current | Stable |
| Upstream lights | ON |
| Downstream lights | OFF |
| Failure pattern | Consecutive |
The consecutive failure pattern suggested a common circuit fault.
6Initial Engineering Hypotheses
The investigation considered:
CCR output failure.
Multiple light failures.
Cable-joint failure.
Isolating-transformer failure.
Primary-cable open circuit.
7Segment Failure Principle
Consecutive failures usually indicate a common circuit interruption.
8Test Equipment
The investigation used:
- Digital multimeter.
- Insulation resistance tester.
- Time-domain reflectometer (TDR).
- Cable fault locator.
- Transformer test equipment.
- Connector inspection tools.
- CCR monitoring equipment.
- ICAO Annex 14 approach-light reference layout.
9Investigation
Step 1: Verify CCR Operation
Results:
| Output current | Normal |
| Output voltage | Normal |
| CCR alarms | None |
| Regulation | Stable |
The CCR was healthy.
Step 2: Determine Failure Boundary
Field inspection showed:
| Light Number | Status |
| A1-A18 | ON |
| A19-A30 | OFF |
The failure began between A18 and A19.
Step 3: Circuit Continuity Test
Measurements showed:
- Upstream continuity normal.
- Downstream continuity absent.
- Open circuit located near the A18-A19 transition.
The fault was localized to the segment boundary.
Step 4: Isolating Transformer Verification
Testing showed:
- A18 transformer healthy.
- A19 transformer healthy.
- No transformer open circuit detected.
Transformers were not the primary cause.
Step 5: Connector Inspection
Inspection revealed:
- Moisture present.
- Corrosion.
- Heating marks.
- Increased contact resistance.
One connector showed significant deterioration.
Step 6: Cable-Joint Investigation
Excavation exposed the cable joint between A18 and A19.
Inspection revealed:
- Water ingress.
- Insulation deterioration.
- Conductor corrosion.
- Joint separation.
The joint had failed electrically.
Step 7: TDR Verification
TDR located the fault at:
146.8 m from the reference point
The location matched the excavated cable joint.
10Root Cause
The approach-light segment failure was caused by failure of the cable joint between A18 and A19, resulting from:
- Water ingress.
- Insulation degradation.
- Conductor corrosion.
- Progressive joint deterioration.
The CCR and isolating transformers remained healthy.
The primary failure mechanism was open-circuit interruption of the series cable.
11Corrective Action
The maintenance team:
- Isolated the circuit.
- Removed the damaged cable joint.
- Replaced the affected cable section.
- Installed a new approved cable joint.
- Replaced degraded connectors.
- Performed insulation testing.
- Verified circuit continuity.
- Energized the circuit.
- Verified segment illumination.
- Updated cable-joint maintenance records.
12Post-Repair Verification
The repaired joint restored full segment operation.
13Root-Cause Analysis
Approach-light segment failed
Healthy
A18-A19
Open circuit
Healthy
Failed
Segment retested
Cable-joint failure
Replace joint and verify circuit
14Why This Case Was Important
Approach-light segment failures can cause:
- Reduced runway acquisition.
- Discontinuous visual guidance.
- Increased pilot workload.
- Low-visibility operational degradation.
- ICAO maintenance concerns.
- Potential operational restrictions.
Segment failures require rapid circuit localization and repair.
15Engineering Lessons
Consecutive light failures usually indicate a common circuit fault.
Consecutive light failures usually indicate a common circuit fault.
Failure boundaries are critical for fault localization.
Failure boundaries are critical for fault localization.
Cable joints are major reliability points in ALS systems.
Cable joints are major reliability points in ALS systems.
TDR significantly improves fault-location efficiency.
TDR significantly improves fault-location efficiency.
Post-repair continuity and operational verification are essential.
Post-repair continuity and operational verification are essential.
16Engineer's Decision Point
You observe: 12 consecutive approach lights OFF. CCR healthy. Upstream lights ON. Downstream lights OFF. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about approach-light segment failure, cable joints and fault localization.
Q1What is AGL-CS-081?
AGL-CS-081 is an engineering training case study. It covers a run of about 12 consecutive approach lights that went dark while the CCR stayed healthy. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Why do consecutive approach lights fail together?
On a series circuit, an interruption isolates everything downstream of it. Consecutive failures usually point to a common circuit interruption rather than many separate fixture failures.
Q3How was the failure boundary found in AGL-CS-081?
Field inspection showed A1–A18 ON and A19–A30 OFF. Continuity was normal upstream and absent downstream, placing the open circuit near the A18–A19 transition.
Q4Were the isolating transformers at fault?
No. The A18 and A19 transformers tested healthy and no transformer open circuit was found.
Q5What did connector and joint inspection find?
One L-823 connector showed moisture, corrosion, heating marks and higher contact resistance. The excavated cable joint between A18 and A19 showed water ingress, insulation deterioration, conductor corrosion and joint separation.
Q6How was the fault location confirmed?
TDR located the fault 146.8 m from the reference point, which matched the excavated cable joint.
Q7What was the root cause of AGL-CS-081?
Failure of the cable joint between A18 and A19 from water ingress, insulation degradation, conductor corrosion and progressive deterioration. The primary mechanism was an open-circuit interruption of the series cable.
Q8How was the circuit restored?
The team removed the damaged joint, replaced the cable section and installed a new approved joint. They also replaced degraded connectors, tested insulation, verified continuity, energized the circuit, verified segment illumination and updated cable-joint records.
Q9Why not just replace the individual dark lights?
The lights were healthy. Replacing fittings would not fix the open circuit, so systematic fault localization is needed instead.
Engineering & Expert Notes
Identify the Boundary of the Failed Group
Fault LocationThe boundary between working and dead lights points to the faulty section.
Use Fault-Location Equipment to Narrow the Dig
TechniqueCombine instrument readings with route records to minimise excavation.
Inspect Other Joints in the Same Area
MoistureIf water entered one joint, others may be affected.
Update Joint Records After Repair
RecordsAdd the new joint to the as-built so the next search starts well.
20Case Study Conclusion
Approach-light segment failures are typically series-circuit continuity problems caused by cable-joint deterioration, connector failure, or primary-cable interruption, and systematic identification of the failure boundary, continuity testing, transformer verification, and TDR-based fault localization provide the most effective method for restoring complete approach-light operation.
Systematic circuit-boundary analysis, continuity testing, cable-joint inspection, TDR verification, insulation testing, and post-repair operational validation can rapidly identify segment failures and restore ICAO-compliant approach-light performance.
In AGL engineering, a dark approach-light segment is not merely a group of failed lights; it is a broken visual pathway that must be restored with the same precision as the approach itself.
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Please read before relying on any content on this page or on any record downloaded from it.
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