AGL-CS-062 — Runway Centreline Light Photometry Failure
The AGL Knowledge Hub | Engineering Case Study Series. A runway centreline lighting photometric investigation where optical degradation caused reduced intensity, asymmetric beam distribution, and chromaticity shift — from electrical verification through photometric measurement, optical inspection, and leveling 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 scheduled CAT II/III runway lighting photometric inspection, several runway centreline inset lights were found to have significantly reduced intensity and distorted beam distribution, despite operating electrically and showing no CCR alarms.
Pilots had reported that the centreline guidance became difficult to follow during low-visibility rollout, particularly in wet runway conditions.
The Constant Current Regulator (CCR) was maintaining the selected 6.6 A current, output voltage was normal, and all affected centreline lights were illuminated.
A detailed photometric investigation was initiated covering light intensity, beam symmetry, longitudinal distribution, chromaticity, optical contamination, and LED optical performance.
The investigation identified optical degradation caused by lens contamination, internal moisture staining, LED optical aging, and beam-axis misalignment, resulting in failure to meet ICAO Annex 14 runway centreline photometric requirements.
This case demonstrates that runway centreline lights can be electrically operational while failing photometric compliance and compromising precision runway guidance.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose runway centreline light photometric failures.
- Differentiate electrical and photometric defects.
- Evaluate centreline light intensity.
- Assess beam symmetry and distribution.
- Verify chromaticity compliance.
- Investigate optical degradation in inset fixtures.
- Restore ICAO-compliant runway centreline photometric performance.
3System Background
A runway centreline inset light consists of:
- LED optical module.
- Precision optical lens.
- Beam-control optics.
- Reflector assembly.
- Housing.
- Mounting and leveling system.
- Electrical driver.
Photometric performance depends on:
- Peak intensity.
- Beam symmetry.
- Vertical beam distribution.
- Horizontal beam distribution.
- White/red chromaticity.
- Optical cleanliness.
- Fixture leveling and alignment.
Common photometric failure causes include:
- Lens contamination.
- Internal condensation.
- Optical scratching.
- LED lumen depreciation.
- Reflector deterioration.
- Seal failure.
- Fixture misalignment.
- Incorrect optical module replacement.
4Problem Statement
The reported condition was:
- Centreline lights illuminated.
- Reduced apparent brightness.
- Poor low-visibility guidance.
- CCR healthy.
- No electrical alarms.
- CAT II/III photometric inspection failed.
Are the runway centreline lights electrically healthy but photometrically non-compliant with ICAO Annex 14 requirements?
5Initial Observations
| CCR status | Healthy |
| Circuit operation | Normal |
| Light operation | ON |
| Visual brightness | Reduced |
| Low-visibility guidance | Poor |
| Beam symmetry | Distorted |
The lights were electrically operational, but their photometric performance was degraded.
6Initial Engineering Hypotheses
The maintenance team considered:
CCR current reduction.
Transformer secondary voltage reduction.
Lens contamination.
LED optical degradation.
Inset fixture misalignment.
7Centreline Photometric Failure Principle
Centreline lighting requires highly controlled optical performance.
8Test Equipment
The investigation used:
- Portable AGL photometer.
- Goniophotometric measurement equipment.
- Chromaticity meter.
- Optical inspection tools.
- Digital level.
- True-RMS multimeter.
- Thermal imaging camera.
- ICAO Annex 14 photometric reference data.
- Airport-approved PPE.
9Investigation
Step 1: Verify Electrical Operation
Results:
| CCR operation | Normal |
| Current regulation | Normal |
| Secondary voltage | Normal |
| Fixture operation | Normal |
No electrical fault was identified.
Step 2: Photometric Measurement
Measured peak intensity:
18,000 cd
Required intensity:
≥ 30,000 cd (representative training value)
The fixture failed the photometric requirement.
Step 3: Beam Distribution Measurement
Testing showed:
- Reduced peak intensity.
- Vertical beam depression.
- Horizontal beam asymmetry.
- Uneven longitudinal intensity distribution.
The beam no longer matched the required centreline photometric envelope.
Step 4: Chromaticity Measurement
Measured chromaticity showed:
- Slight shift from the specified white region.
- Reduced color consistency.
- Optical aging effects.
Color performance had degraded measurably.
Step 5: Optical Inspection
Inspection revealed:
- Dirt accumulation.
- Rubber deposits.
- Surface abrasion.
- Internal moisture staining.
- Lens hazing.
Optical transmission had deteriorated significantly.
Step 6: Internal Optical Inspection
The optical assembly showed:
- LED lens discoloration.
- Reflector degradation.
- Moisture contamination.
- Optical surface deterioration.
Optical efficiency had decreased substantially.
Step 7: Fixture Leveling Verification
Measurements showed:
- Minor tilt.
- Beam-axis displacement.
- Reduced longitudinal coverage.
- Inconsistent overlap with adjacent centreline lights.
Leveling errors further reduced guidance effectiveness.
10Root Cause
The runway centreline light photometric failure was caused by combined optical degradation, including:
- Lens contamination.
- Internal moisture staining.
- LED optical aging.
- Reflector deterioration.
- Fixture leveling and alignment errors.
The electrical system remained healthy, but the fixture no longer produced ICAO-compliant intensity, beam symmetry, and longitudinal distribution.
11Corrective Action
The maintenance team:
- Removed the affected inset fixture.
- Replaced the optical lens assembly.
- Replaced the degraded optical module.
- Restored reflector condition.
- Cleaned the optical compartment.
- Re-leveled the fixture.
- Performed photometric testing.
- Verified beam symmetry.
- Confirmed chromaticity compliance.
- Updated photometric maintenance records.
12Post-Repair Verification
The runway centreline light met ICAO photometric requirements after repair.
13Root-Cause Analysis
Dim centreline light
Healthy
Below requirement
Asymmetric
Contaminated
Degraded
Optical degradation and alignment error
Replace optical components and restore leveling
14Why This Case Was Important
Centreline photometric failures can cause:
- Reduced runway directional guidance.
- Poor low-visibility tracking.
- Wet-runway visual difficulties.
- CAT II/III operational degradation.
- ICAO non-compliance.
- Reduced centreline conspicuity.
Centreline photometric performance is critical for precision runway operations.
15Engineering Lessons
An illuminated centreline light is not necessarily ICAO compliant.
An illuminated centreline light is not necessarily ICAO compliant.
Beam symmetry is critical for runway centreline guidance.
Beam symmetry is critical for runway centreline guidance.
Inset-fixture leveling affects photometric performance.
Inset-fixture leveling affects photometric performance.
Internal moisture can significantly reduce optical efficiency.
Internal moisture can significantly reduce optical efficiency.
CAT II/III systems require stricter photometric control than general airfield lighting.
CAT II/III systems require stricter photometric control than general airfield lighting.
16Engineer's Decision Point
You observe: Runway centreline lights ON. Pilots report poor low-visibility guidance. CCR healthy. No electrical faults. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about runway centreline light photometric failure and CAT II/III compliance.
Q1What is AGL-CS-062?
AGL-CS-062 is an engineering training case study. It covers runway centreline inset lights that were electrically healthy but failed photometric inspection because of optical degradation. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Can runway centreline lights be electrically healthy but still fail ICAO Annex 14 requirements?
Yes. The CCR held 6.6 A, voltage was normal and every affected light was on, yet intensity and beam distribution were below the required photometric values. Electrical health and photometric compliance are separate checks.
Q3How do you tell an electrical fault from a photometric failure on centreline lights?
Confirm the electrical side first: CCR operation, current regulation, secondary voltage and fixture operation. If these are normal but pilots report weak guidance, measure intensity, beam symmetry and chromaticity, then inspect the optics and fixture leveling.
Q4What did the photometric measurements show in AGL-CS-062?
Peak intensity was 18,000 cd against a representative training requirement of at least 30,000 cd. The beam also showed vertical depression, horizontal asymmetry, uneven longitudinal distribution and a slight chromaticity shift from the specified white region.
Q5Why is beam symmetry critical for runway centreline lighting?
Centreline lights guide the pilot's directional tracking during rollout, especially in low visibility. An asymmetric or displaced beam weakens that guidance and reduces overlap with adjacent centreline lights.
Q6What caused the failure in AGL-CS-062?
Combined optical degradation: lens contamination, internal moisture staining, LED optical aging, reflector deterioration, and fixture leveling and alignment errors. The electrical system stayed healthy.
Q7Why does inset-fixture leveling affect photometric performance?
A small tilt displaces the beam axis and reduces longitudinal coverage. In this case, minor tilt caused beam-axis displacement and inconsistent overlap with neighbouring lights, on top of the optical losses.
Q8How was compliance restored?
The team replaced the optical lens assembly and degraded optical module, restored the reflector, cleaned the optical compartment and re-leveled the fixture. They then repeated photometric testing, verified beam symmetry and chromaticity, and updated the records.
Q9Why do CAT II/III systems need stricter photometric control?
Precision runway operations in low visibility depend heavily on centreline guidance, so optical performance must be tightly controlled. The case treats the light the crew sees, not the current in the circuit, as the true measure.
Engineering & Expert Notes
Inspect Internally for Moisture Staining
OpticsInternal staining reduces output. Check seals and drainage as the likely entry route.
Level Inset Fixtures Carefully
LevellingTilt skews the beam. Check level during installation and after pavement work.
Test Centreline Lights in Positions That Matter
SamplingInclude fixtures from high-use areas and those with maintenance history.
Record Readings With Fixture Identity
RecordsLink every reading to a fixture and date so changes can be tracked.
20Case Study Conclusion
Runway centreline light photometric failure is an optical-performance problem caused by lens contamination, moisture-related optical degradation, reflector deterioration, LED aging, and beam misalignment, and a centreline light can be electrically operational while failing ICAO Annex 14 photometric requirements and compromising CAT II/III runway guidance.
Systematic photometric measurement, beam-symmetry verification, chromaticity testing, optical inspection, LED assessment, and fixture-leveling verification can identify photometric failures and restore compliant runway centreline lighting performance.
In AGL engineering, precision runway operations depend not only on whether the centreline lights are ON, but on whether every beam is exactly where the pilot expects it to be.
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