AGL-CS-082 — Multiple Approach Lights Not Working
The AGL Knowledge Hub | Engineering Case Study Series. A multi-fault approach lighting investigation where scattered individual light failures revealed independent field defects — from CCR verification through transformer testing, connector inspection, secondary-circuit analysis, insulation assessment, and systematic field repair.
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 a routine night inspection, air traffic control reported that multiple approach lights were not working, but the failure pattern was unusual.
Instead of one complete dark segment, individual approach lights were inoperative at several different locations across the approach lighting system.
Pilots reported that the approach pattern appeared irregular, with dark gaps between operating lights.
The Constant Current Regulator (CCR) remained healthy, circuit current was maintained, and no major circuit trip occurred.
A detailed investigation was initiated covering isolating transformers, secondary cables, connectors, water ingress, insulation resistance, maintenance history, and failure-pattern analysis.
The investigation identified multiple independent field faults involving connectors, transformer secondary circuits, and water-related cable deterioration, rather than a single primary-circuit failure.
This case demonstrates that multiple scattered approach-light failures require systematic individual fault diagnosis and cannot be treated as a single segment failure.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose multiple non-consecutive approach-light failures.
- Interpret scattered failure patterns.
- Investigate multiple independent field faults.
- Verify transformer secondary circuits.
- Assess connector reliability.
- Evaluate water-ingress-related failures.
- Develop maintenance prioritization strategies.
3System Background
An approach-light circuit includes:
- Primary series cable.
- Isolating transformers.
- Secondary cables.
- L-823 connectors.
- Light fittings.
- Cable joints.
- Junction interfaces.
Multiple scattered failures may result from:
- Connector failures.
- Transformer secondary failures.
- Water ingress.
- Secondary-cable damage.
- Optical-module failures.
- Seal deterioration.
- Mechanical damage.
- Aging infrastructure.
Scattered failures usually indicate multiple localized defects.
4Problem Statement
The reported condition was:
- Multiple approach lights OFF.
- Non-consecutive failure pattern.
- CCR healthy.
- Circuit current normal.
- Remaining lights operating.
Are the multiple failed lights caused by one common fault or by several independent field defects?
5Initial Observations
| Light Number | Status |
| A3 | OFF |
| A7 | ON |
| A11 | OFF |
| A18 | ON |
| A22 | OFF |
| A29 | ON |
| A34 | OFF |
The failure pattern was distributed across the circuit.
6Initial Engineering Hypotheses
The investigation considered:
CCR output instability.
Primary-cable failure.
Multiple transformer failures.
Multiple connector failures.
Water-related secondary-circuit deterioration.
7Multiple Failure Principle
Scattered failures usually require individual field investigation.
8Test Equipment
The investigation used:
- Digital multimeter.
- Insulation resistance tester.
- Transformer test equipment.
- Secondary-circuit continuity tester.
- Connector inspection tools.
- Moisture detection equipment.
- Thermal imaging camera.
- ICAO Annex 14 approach-light reference layout.
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 Primary-Circuit Continuity
Measurements showed:
- Continuous primary circuit.
- No major voltage interruption.
- No segment isolation.
The primary circuit was healthy.
Step 3: Test Isolating Transformers
Results:
| Light | Transformer |
| A3 | Healthy |
| A11 | Healthy |
| A22 | Secondary fault |
| A34 | Healthy |
Only one transformer had a secondary defect.
Step 4: Connector Inspection
Inspection revealed:
A3
- Corrosion.
- Moisture.
- High contact resistance.
A11
- Loose connector.
- Carbon tracking.
A34
- Water ingress.
- Insulation damage.
Connectors showed different failure mechanisms.
Step 5: Secondary-Cable Testing
Testing revealed:
- A22: Open secondary cable.
- A34: Low insulation resistance.
Secondary-circuit failures were confirmed.
Step 6: Insulation Resistance Measurement
Measured values:
| Location | IR |
| A3 | 18 MOhm |
| A11 | 42 MOhm |
| A22 | Open |
| A34 | 0.8 MOhm |
A34 showed severe moisture-related deterioration.
Step 7: Maintenance History Review
Records showed:
- Connector replacements at A3 and A11.
- Water-ingress history at A34.
- Repeated transformer maintenance at A22.
The failures were consistent with known local reliability problems.
10Root Cause
The multiple approach-light failures were caused by several independent field defects, including:
- Connector corrosion.
- Loose connector termination.
- Transformer secondary failure.
- Secondary-cable open circuit.
- Water-ingress-related insulation deterioration.
The CCR and primary circuit remained healthy.
The primary failure mechanism was multiple localized component failures.
11Corrective Action
The maintenance team:
- Replaced failed connectors.
- Repaired secondary cables.
- Replaced the defective transformer.
- Removed water-damaged cable sections.
- Improved connector sealing.
- Performed insulation testing.
- Verified continuity.
- Energized the circuit.
- Verified all approach lights.
- Updated reliability records.
12Post-Repair Verification
Independent fault repairs restored full system operation.
13Root-Cause Analysis
Multiple scattered failures
Healthy
Healthy
One defective
Multiple defective
Multiple faults
Independent localized field defects
Repair each failed field component
14Why This Case Was Important
Multiple scattered failures can cause:
- Irregular approach guidance.
- Reduced runway acquisition quality.
- Increased maintenance complexity.
- Misdiagnosis of circuit problems.
- Progressive reliability deterioration.
- Operational degradation.
Failure-pattern analysis is essential for efficient troubleshooting.
15Engineering Lessons
Scattered failures usually indicate multiple independent defects.
Scattered failures usually indicate multiple independent defects.
A healthy CCR does not eliminate field failures.
A healthy CCR does not eliminate field failures.
Connector reliability is critical in ALS systems.
Connector reliability is critical in ALS systems.
Water ingress can create multiple localized failures.
Water ingress can create multiple localized failures.
Maintenance history is a valuable diagnostic tool.
Maintenance history is a valuable diagnostic tool.
16Engineer's Decision Point
You observe: Four approach lights OFF. Failures at widely separated locations. CCR healthy. Primary circuit healthy. What should you investigate first?
17Knowledge Check
18Interview Questions
Quick Reference: Approach Lighting Working Principle
The approach lighting working principle is to give the pilot a pattern of lights along the extended runway centreline that can be seen in poor visibility. The pilot uses the pattern to line up with the runway and judge the roll and distance. Because the lights work as a pattern, several lights being out at once, as in this case, can change what the pilot sees.
19Frequently Asked Questions
Quick answers about scattered approach-light failures and independent field faults.
Q1What is AGL-CS-082?
AGL-CS-082 is an engineering training case study. It covers multiple approach lights out at scattered, non-consecutive locations while the CCR and primary circuit stayed healthy. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2How does a scattered failure pattern differ from a segment failure?
A segment failure is one dark run caused by a common circuit interruption. Scattered dark lights with a healthy primary circuit usually mean several independent field defects, each needing its own diagnosis.
Q3What did the checks on the primary circuit show?
The CCR was healthy, the primary circuit was continuous with no major voltage interruption, and no segment was isolated.
Q4What faults were found at each location?
A3 had connector corrosion and moisture. A11 had a loose connector with carbon tracking. A22 had a transformer secondary fault and an open secondary cable. A34 had water ingress and insulation damage.
Q5What insulation resistance values were measured?
A3 measured 18 MΩ, A11 measured 42 MΩ, A22 was open and A34 measured 0.8 MΩ, which showed severe moisture-related deterioration.
Q6Did maintenance history help?
Yes. Records showed connector replacements at A3 and A11, water-ingress history at A34 and repeated transformer maintenance at A22, consistent with known local reliability problems.
Q7What was the root cause of AGL-CS-082?
Several independent defects: connector corrosion, a loose termination, a transformer secondary failure, an open secondary cable and water-related insulation deterioration.
Q8How were the faults corrected?
The team replaced failed connectors and the defective transformer, repaired secondary cables and removed water-damaged cable sections. They also improved connector sealing, tested insulation, verified continuity, verified all approach lights and updated reliability records.
Q9What is the key lesson from this case?
Do not treat scattered failures as one segment fault. Confirm the primary circuit is healthy, then test each failed location individually.
Engineering & Expert Notes
Work Through the Failures One by One
MethodSeparate faults need separate fixes. Keep a list and tick off each repair.
Inspect Connectors and Secondaries as a Group
TerminationsScattered failures often share the same causes. Look for a theme.
Look at the Common Environmental Factor
MoistureWater is often the shared cause. Check drainage and sealing.
Verify the Whole Approach After the Last Repair
VerificationWalk the pattern at night to confirm nothing remains out.
20Case Study Conclusion
Multiple approach lights not working at scattered locations usually indicate independent localized field defects involving connectors, transformer secondary circuits, secondary cables, and water-related deterioration, and systematic investigation of each failed location is essential for accurate diagnosis and complete restoration of approach-light reliability.
Systematic failure-pattern analysis, transformer testing, connector inspection, secondary-circuit continuity testing, insulation assessment, and maintenance-history evaluation can efficiently identify multiple independent faults and restore ICAO-compliant approach-light performance.
In AGL engineering, a scattered pattern of dark lights is usually not one failure repeated; it is several failures revealing the reliability of the field system itself.
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