AGL-CS-044 — Runway Centreline Light Failure
The AGL Knowledge Hub | Engineering Case Study Series. A localized inset-fixture investigation where one runway centreline light failed while the rest of the circuit remained operational — from transformer secondary testing through fixture inspection, water-ingress assessment, and seal 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 a routine night inspection before low-visibility operations, one runway centreline inset light was found completely extinguished, while the remaining centreline lights operated normally.
The Constant Current Regulator (CCR) was maintaining the selected 6.6 A current, output voltage was normal, and no CCR alarms were present.
Because the fault was limited to a single inset fitting, the investigation focused on the individual transformer, secondary circuit, connector assembly, and fixture condition.
A systematic investigation identified water ingress into the inset fixture housing, which caused corrosion of the internal terminal assembly and failure of the secondary connection.
This case demonstrates that runway centreline light failures are often localized inset-fixture or secondary-circuit faults, with water ingress and sealing deterioration being major contributing factors.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose single runway centreline light failures.
- Understand the construction of inset runway centreline fittings.
- Differentiate primary and secondary circuit faults.
- Investigate water ingress in inset fixtures.
- Test transformer secondary circuits.
- Inspect connectors, seals, and fixture terminals.
- Restore centreline light operation safely.
3System Background
A runway centreline light is supplied through:
Unlike elevated edge lights, centreline fittings are:
- Installed in pavement.
- Subjected to aircraft loading.
- Exposed to water accumulation.
- Dependent on effective sealing.
- More vulnerable to moisture-related failures.
Common failure causes include:
- LED module failure.
- Water ingress.
- Terminal corrosion.
- Secondary cable failure.
- Connector deterioration.
- Transformer secondary failure.
- Mechanical damage.
4Problem Statement
The reported condition was:
- One runway centreline light OFF.
- Adjacent centreline lights operational.
- CCR healthy.
- No circuit alarms.
- Low-visibility inspection in progress.
Has the inset fixture failed, or has the secondary power supply to the fixture been interrupted?
5Initial Observations
| CCR status | Healthy |
| Selected current | 6.6 A |
| Actual current | 6.6 A |
| Output voltage | Normal |
| Adjacent centreline lights | Operational |
| Failed fixture | Completely OFF |
The healthy CCR and adjacent lights indicated a localized field fault.
6Initial Engineering Hypotheses
The maintenance team considered:
LED module failure.
Inset fixture water ingress.
Transformer secondary failure.
Connector corrosion.
Secondary cable open circuit.
7Centreline Fixture Diagnostic Principle
If adjacent centreline lights remain operational, the fault is usually localized to the transformer secondary circuit or the inset fixture itself.
8Test Equipment
The investigation used:
- True-RMS multimeter.
- Secondary voltage test equipment.
- Continuity tester.
- Low-resistance ohmmeter.
- Insulation resistance tester.
- Thermal imaging camera.
- Approved AGL circuit test equipment.
- Airport-approved PPE.
9Investigation
Step 1: Verify CCR Operation
Results:
| Current regulation | Normal |
| Voltage regulation | Normal |
| CCR alarms | None |
The CCR was healthy.
Step 2: Verify Transformer Primary
The associated transformer showed:
- Primary current present.
- Normal primary continuity.
- Normal operating temperature.
Primary power was available.
Step 3: Measure Transformer Secondary Voltage
Results:
| Primary current | Present |
| Secondary voltage | Present |
The transformer was supplying secondary power.
Step 4: Secondary Continuity Testing
Testing showed:
- Continuity up to the fixture connector.
- Open continuity inside the fixture.
- Elevated resistance at the fixture terminals.
The fault was inside the inset fitting.
Step 5: Fixture Removal and Inspection
The inset fixture was removed from the pavement.
Inspection revealed:
- Water accumulation inside the housing.
- Corroded terminal assembly.
- Oxidized electrical contacts.
- Deteriorated sealing gasket.
- Moisture damage to the internal connection.
The fixture had suffered progressive water-ingress deterioration.
Step 6: Optical and Mechanical Inspection
Additional findings:
- Lens contamination.
- Internal condensation.
- Seal compression failure.
- Mechanical wear of the cable-entry assembly.
The fixture sealing system had deteriorated significantly.
10Root Cause
The runway centreline light failed because water ingress entered the inset fixture housing through a deteriorated sealing gasket, causing corrosion of the internal terminal assembly and interruption of the secondary electrical connection.
The transformer and primary circuit remained healthy.
The fault was localized to the inset fixture assembly.
11Corrective Action
The maintenance team:
- Isolated the circuit.
- Removed the damaged inset fixture.
- Replaced the terminal assembly.
- Installed a new approved sealing gasket.
- Dried and cleaned the fixture housing.
- Verified secondary continuity.
- Tested optical performance.
- Reinstalled the fixture.
- Conducted operational verification.
- Updated maintenance records.
12Post-Repair Verification
The runway centreline light returned to full operation.
13Root-Cause Analysis
Single centreline light OFF
Healthy
Healthy
Present
Open
Water ingress found
Inset fixture water ingress and terminal corrosion
Replace terminal assembly and sealing system
14Why This Case Was Important
Runway centreline lights are critical for:
- CAT II operations.
- CAT III operations.
- Low-visibility guidance.
- Runway directional control.
- Aircraft alignment.
Inset-fixture reliability directly affects low-visibility operational capability.
15Engineering Lessons
Single centreline light failures are usually localized secondary-side faults.
Single centreline light failures are usually localized secondary-side faults.
Water ingress is a major failure mechanism for inset fittings.
Water ingress is a major failure mechanism for inset fittings.
Fixture sealing integrity is critical for long-term reliability.
Fixture sealing integrity is critical for long-term reliability.
Transformer secondary testing should precede fixture replacement.
Transformer secondary testing should precede fixture replacement.
Terminal corrosion can interrupt power even when the transformer is healthy.
Terminal corrosion can interrupt power even when the transformer is healthy.
16Engineer's Decision Point
You observe: One runway centreline light OFF. Adjacent centreline lights ON. CCR healthy. No circuit alarms. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about runway centreline light failure, based on this case study.
Q1What is AGL-CS-044?
AGL-CS-044 is an engineering training case study on a single runway centreline inset light that was OFF during a pre-low-visibility inspection. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Was the fixture or the secondary supply at fault?
The case starts with that question. The CCR was healthy at 6.6 A and adjacent centreline lights worked, which indicated a localized field fault, and the transformer and primary circuit remained healthy.
Q3What was the root cause?
Water entered the inset fixture housing through a deteriorated sealing gasket, corroding the internal terminal assembly and interrupting the secondary electrical connection. The fault was localized to the inset fixture assembly.
Q4What should you investigate first?
Verify transformer secondary voltage and inspect the inset fixture for water ingress, terminal corrosion and sealing failure. Replacing the CCR or all centreline fixtures, or increasing the current setting, is not the right first step.
Q5Which test equipment was used?
The team used a true-RMS multimeter, secondary voltage test equipment, continuity tester, low-resistance ohmmeter, insulation resistance tester and thermal imaging camera, with approved AGL circuit test equipment.
Q6How was the light repaired?
The team isolated the circuit, removed the damaged inset fixture and replaced the terminal assembly. They installed a new approved sealing gasket, dried and cleaned the housing, verified secondary continuity, tested optical performance, reinstalled the fixture and conducted operational verification.
Q7How was the repair verified?
Transformer secondary and fixture operation were normal, optical output was normal and seal integrity was verified. The centreline light returned to full operation.
Q8Why does sealing matter for inset centreline lights?
A deteriorated gasket let water reach the terminals, and the resulting corrosion cut the secondary connection. Seal integrity was specifically verified after repair.
Engineering & Expert Notes
Inspect the Gasket at Every Fixture Removal
SealingReusing a worn gasket invites a repeat failure. Replace it and check the seating surface.
Test the Secondary Before Changing the Fixture
MethodConfirm what is failing before swapping parts.
Check Base-Can Drainage While the Fitting Is Out
DrainageA blocked drain feeds water to the fitting. Clear it and note the condition.
Photograph the Corrosion for the Record
RecordsImages help judge whether the problem is local or widespread.
20Case Study Conclusion
A single runway centreline light failure is usually a localized inset-fixture or secondary-circuit fault, with water ingress, sealing deterioration, and terminal corrosion being among the most common failure mechanisms in pavement-mounted centreline lighting systems.
Systematic CCR verification, transformer secondary testing, fixture continuity testing, water-ingress inspection, seal assessment, and terminal examination can rapidly identify the failed component and restore reliable runway centreline lighting operation.
In AGL engineering, a dark centreline light often begins with a failed seal long before it ends with a failed terminal.
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