AGL-CS-045 — Runway Centreline Light Water Ingress
The AGL Knowledge Hub | Engineering Case Study Series. A moisture-investigation where multiple centreline inset lights failed after heavy rainfall due to deteriorated sealing and blocked drainage — from forensic fixture inspection through seal assessment, cable-entry examination, and drainage 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
Following several days of heavy rainfall, the maintenance team observed that multiple runway centreline inset lights exhibited reduced intensity, intermittent operation, and repeated fixture failures within the same runway section.
The Constant Current Regulator (CCR) remained healthy, and the primary circuit operated normally.
Inspection revealed that several inset fittings contained standing water inside the fixture housing, with visible corrosion, moisture contamination, and insulation deterioration.
A detailed forensic investigation identified failure of the fixture sealing system combined with inadequate drainage around the base can, allowing progressive water ingress into the optical and electrical compartments.
This case demonstrates that water ingress is one of the most destructive long-term failure mechanisms affecting runway centreline inset lighting systems.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose water ingress in runway centreline inset lights.
- Understand inset-fixture sealing systems.
- Investigate moisture-entry mechanisms.
- Assess corrosion and insulation deterioration.
- Evaluate drainage and installation conditions.
- Differentiate condensation from water ingress.
- Develop preventive maintenance strategies for inset lighting.
3System Background
A runway centreline inset light consists of:
- Optical assembly.
- LED or lamp module.
- Internal terminal assembly.
- Cable-entry system.
- Sealing gaskets.
- O-rings.
- Pressure-equalization components.
- Fixture housing.
- Base can installation.
Because the fixture is installed within the pavement surface, it is exposed to:
- Rainwater.
- Standing water.
- Hydraulic pressure from aircraft traffic.
- Temperature cycling.
- Condensation.
- Chemical contamination.
Long-term reliability depends heavily on effective environmental sealing.
4Problem Statement
The reported condition was:
- Multiple centreline lights dim.
- Some fixtures intermittent.
- Some fixtures completely failed.
- Failures concentrated after heavy rain.
- CCR healthy.
- No primary circuit alarms.
How did water enter the inset fixtures, and which component allowed the ingress?
5Initial Observations
| CCR status | Healthy |
| Primary circuit | Operational |
| Affected fixtures | Multiple |
| Weather | Heavy rainfall |
| Internal moisture | Visible |
| Corrosion | Significant |
The concentration of failures after rainfall strongly suggested environmental ingress.
6Initial Engineering Hypotheses
The maintenance team considered:
Condensation only.
Cable-entry leakage.
Gasket failure.
Base-can drainage failure.
Pressure-equalization failure.
7Water Ingress Mechanism
Normal protection:
Failure mechanism:
Water ingress is usually progressive, not sudden.
8Test Equipment
The investigation used:
- True-RMS multimeter.
- Insulation resistance tester.
- Moisture inspection equipment.
- Thermal imaging camera.
- Optical inspection tools.
- Low-resistance ohmmeter.
- Pressure and seal inspection tools.
- Approved AGL circuit test equipment.
- Airport-approved PPE.
9Investigation
Step 1: Verify CCR and Primary Circuit
Results:
| CCR operation | Normal |
| Current regulation | Normal |
| Primary circuit continuity | Normal |
The primary system was healthy.
Step 2: Fixture Removal
The affected inset fixtures were removed.
Internal inspection revealed:
- Standing water.
- Moisture droplets.
- Corroded terminals.
- Oxidized connectors.
- Discolored insulation.
- Contaminated optical surfaces.
The moisture level exceeded normal condensation.
Step 3: Seal Examination
Inspection of the sealing system showed:
- Compressed gasket deformation.
- Surface cracking.
- Loss of elasticity.
- Contamination of sealing surfaces.
- Improper compression.
The primary gasket had deteriorated.
Step 4: Cable-Entry Inspection
The cable-entry assembly showed:
- Seal shrinkage.
- Mechanical wear.
- Inadequate compression.
- Moisture migration along the cable.
The cable entry was a secondary ingress path.
Step 5: Drainage Assessment
The base can showed:
- Sediment accumulation.
- Blocked drainage path.
- Standing water around the fixture.
- Repeated water exposure.
Poor drainage significantly increased water exposure duration.
Step 6: Insulation Resistance Testing
Results:
| Dry fixture | Greater than 1000 MΩ |
| Wet fixture | 8 MΩ |
Moisture had severely reduced insulation performance.
Step 7: Corrosion Analysis
The terminal assembly exhibited:
- Copper oxidation.
- Galvanic corrosion.
- Contact resistance increase.
- Progressive conductor deterioration.
Corrosion was responsible for the intermittent electrical behavior.
10Root Cause
The runway centreline light water ingress was caused by failure of the fixture sealing gasket combined with blocked base-can drainage, allowing standing water to enter the fixture housing through both the primary seal and cable-entry system.
The resulting moisture caused:
- Terminal corrosion.
- Insulation deterioration.
- Increased contact resistance.
- Leakage current.
- Intermittent operation.
- Progressive fixture failure.
11Corrective Action
The maintenance team:
- Removed all affected fixtures.
- Replaced sealing gaskets.
- Replaced damaged cable-entry seals.
- Cleaned and dried fixture housings.
- Replaced corroded terminal assemblies.
- Restored base-can drainage.
- Performed insulation resistance testing.
- Verified optical performance.
- Reinstalled the fixtures.
- Updated preventive maintenance procedures.
12Post-Repair Verification
All affected runway centreline lights returned to normal operation.
13Root-Cause Analysis
Multiple wet fixtures
Healthy
Water present
Deteriorated
Leakage path identified
Blocked
Seal failure and inadequate drainage
Replace seals and restore drainage
14Why This Case Was Important
Water ingress causes progressive reliability degradation, including:
- Corrosion.
- Insulation failure.
- Optical degradation.
- Intermittent faults.
- Repeated maintenance.
- CAT II/III reliability reduction.
Preventive sealing maintenance is often more valuable than repeated fixture replacement.
15Engineering Lessons
Water ingress is a progressive failure mechanism.
Water ingress is a progressive failure mechanism.
Gasket condition should be inspected regularly.
Gasket condition should be inspected regularly.
Drainage is part of electrical reliability.
Drainage is part of electrical reliability.
Low insulation resistance often indicates moisture contamination.
Low insulation resistance often indicates moisture contamination.
Connector and terminal corrosion usually follows prolonged water exposure.
Connector and terminal corrosion usually follows prolonged water exposure.
16Engineer's Decision Point
You observe: Multiple centreline lights affected. Recent heavy rainfall. CCR healthy. Moisture visible inside the fixture. What should you investigate first?
17Knowledge Check
18Interview Questions
Quick Reference: Runway Centreline Light Colour & ICAO Requirements
ICAO runway centerline lights (Annex 14) are fitted on precision approach runways and certain others, along the centreline of the runway. Their colour changes towards the runway end so that a pilot can judge the remaining distance. Water ingress, as in this case, affects inset fixtures in particular, because they sit in the pavement.
19Frequently Asked Questions
Quick answers about water ingress in runway centreline lights, based on this case study.
Q1What is AGL-CS-045?
AGL-CS-045 is an engineering training case study on water ingress into multiple runway centreline inset lights after several days of heavy rain. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2What symptoms appeared?
Multiple centreline lights showed reduced intensity, intermittent operation and repeated failures within the same runway section, concentrated after heavy rainfall. Several fixtures contained standing water, visible corrosion and insulation deterioration, while the CCR and primary circuit were normal.
Q3What was the root cause?
Failure of the fixture sealing gasket combined with blocked base-can drainage let standing water enter through both the primary seal and the cable-entry system.
Q4What did the moisture cause?
It caused terminal corrosion, insulation deterioration, increased contact resistance, leakage current, intermittent operation and progressive fixture failure.
Q5What should you inspect first when several lights fail after rain with moisture visible?
Inspect fixture seals, cable-entry assemblies, internal moisture and base-can drainage. Replacing the CCR or all transformers, or increasing the current setting, is not the right first step.
Q6How was the section repaired?
The team removed the affected fixtures, replaced sealing gaskets and cable-entry seals, cleaned and dried the housings and replaced corroded terminal assemblies. They restored base-can drainage, tested insulation resistance, verified optical performance, reinstalled the fixtures and updated preventive maintenance procedures.
Q7How was the repair verified?
Fixture insulation was greater than 1000 MΩ, there was no internal moisture, optical output was normal and drainage was restored. All affected centreline lights returned to normal operation.
Q8Which test equipment was used?
The team used a true-RMS multimeter, insulation resistance tester, moisture inspection equipment, thermal imaging camera, optical inspection tools, a low-resistance ohmmeter and pressure and seal inspection tools.
Q9What color are runway centerline lights?
Runway centerline lights are white along most of the runway. Towards the runway end the colour changes, with alternating red and white and then red only for the last section, to warn the pilot of the remaining distance. Confirm the exact lengths against the current ICAO Annex 14.
Engineering & Expert Notes
Treat Drainage as an Electrical Item
DrainageA blocked base-can drain is a reliability defect. Add drain checks to inspection.
Replace Gasket and Entry Seals Together
SealingWater can enter through more than one path. Check all of them.
Test Insulation After Drying
VerificationConfirm that the circuit's insulation has recovered and is stable.
Schedule Inspection After Wet Weather
PlanningEarly checks catch standing water before corrosion begins.
20Case Study Conclusion
Runway centreline light water ingress is primarily a sealing and environmental-control failure that progressively causes corrosion, insulation deterioration, contact resistance increase, and eventual secondary-circuit failure within inset lighting systems.
Systematic CCR verification, fixture removal, seal inspection, cable-entry assessment, drainage evaluation, insulation testing, and corrosion analysis can accurately identify the ingress mechanism and significantly improve the long-term reliability of runway centreline lighting.
In AGL engineering, water inside an inset fixture is not merely a maintenance issue; it is the beginning of a reliability failure that will continue until the sealing and drainage system is restored.
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