AGL-CS-063 — Threshold Light Photometry Failure
The AGL Knowledge Hub | Engineering Case Study Series. A runway threshold lighting photometric investigation where optical degradation caused reduced green intensity, non-uniform beam distribution, and chromaticity shift — from electrical verification through photometric measurement, optical inspection, and alignment 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 runway lighting photometric inspection, several runway threshold lights were found to have significantly reduced green intensity and non-uniform beam distribution, even though the lights were operating electrically and no CCR alarms were present.
Flight crews had reported that the runway threshold was less conspicuous during night approaches, particularly in wet-weather conditions and during reduced-visibility operations.
The Constant Current Regulator (CCR) was maintaining the selected 6.6 A current, output voltage was normal, and all affected threshold lights were illuminated.
A detailed photometric investigation was initiated covering green intensity, approach-direction beam distribution, chromaticity, threshold-bar uniformity, optical contamination, and LED optical performance.
The investigation identified optical degradation caused by lens contamination, UV-related lens aging, reflector deterioration, and incorrect fixture alignment, resulting in failure to meet ICAO Annex 14 threshold-light photometric requirements.
This case demonstrates that threshold lights can be electrically operational while failing photometric compliance and reducing runway threshold identification effectiveness.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose runway threshold light photometric failures.
- Differentiate electrical and photometric defects.
- Evaluate threshold-light intensity.
- Assess approach-direction beam distribution.
- Verify green chromaticity compliance.
- Investigate optical degradation.
- Restore ICAO-compliant threshold-light photometric performance.
3System Background
A runway threshold light consists of:
- LED optical module.
- Green optical lens.
- Precision beam-control optics.
- Reflector assembly.
- Housing.
- Mounting and alignment system.
- Electrical driver.
Photometric performance depends on:
- Green intensity.
- Vertical beam distribution.
- Horizontal beam distribution.
- Green chromaticity.
- Optical cleanliness.
- Lens transmission.
- Fixture alignment.
- Threshold-bar uniformity.
Common photometric failure causes include:
- Lens contamination.
- UV degradation.
- Optical scratching.
- LED lumen depreciation.
- Reflector deterioration.
- Internal condensation.
- Fixture misalignment.
- Incorrect optical module replacement.
4Problem Statement
The reported condition was:
- Threshold lights illuminated.
- Reduced green brightness.
- Poor threshold conspicuity.
- CCR healthy.
- No electrical alarms.
- Photometric inspection failed.
Are the runway threshold lights electrically healthy but photometrically non-compliant with ICAO Annex 14 requirements?
5Initial Observations
| CCR status | Healthy |
| Circuit operation | Normal |
| Light operation | ON |
| Green brightness | Reduced |
| Threshold visibility | Poor |
| Beam uniformity | Non-uniform |
The lights were electrically operational, but their optical performance was degraded.
6Initial Engineering Hypotheses
The maintenance team considered:
CCR current reduction.
Transformer secondary voltage reduction.
Lens contamination.
LED optical degradation.
Fixture misalignment.
7Threshold Photometric Failure Principle
Threshold lights are critical visual references during the final approach.
8Test Equipment
The investigation used:
- Portable AGL photometer.
- Goniophotometric measurement equipment.
- Green chromaticity meter.
- Optical inspection tools.
- True-RMS multimeter.
- Fixture alignment instruments.
- 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:
9,500 cd
Required intensity:
≥ 16,000 cd (representative training value)
The fixture failed the photometric requirement.
Step 3: Beam Distribution Measurement
Testing showed:
- Reduced peak intensity.
- Vertical beam depression.
- Narrow horizontal beam spread.
- Non-uniform threshold-bar distribution.
The beam no longer matched the required threshold-light photometric envelope.
Step 4: Chromaticity Measurement
Measured green chromaticity showed:
- Reduced green saturation.
- Shift toward yellow-green appearance.
- Optical aging effects.
The emitted color no longer met the desired operational appearance.
Step 5: Optical Inspection
Inspection revealed:
- Dirt accumulation.
- Rubber deposits.
- Surface scratching.
- UV-related lens hazing.
- Reduced optical transparency.
Lens transmission had deteriorated significantly.
Step 6: Internal Optical Inspection
The optical assembly showed:
- Reflector discoloration.
- LED lens degradation.
- Moisture contamination.
- Optical surface deterioration.
Optical efficiency had decreased substantially.
Step 7: Fixture Alignment Verification
Measurements showed:
- Vertical aiming error.
- Slight inward rotation.
- Reduced approach-direction coverage.
- Uneven threshold-bar intensity.
Alignment errors further reduced threshold conspicuity.
10Root Cause
The runway threshold light photometric failure was caused by combined optical degradation, including:
- Lens contamination.
- UV-related lens aging.
- Reflector deterioration.
- LED optical degradation.
- Fixture alignment errors.
The electrical system remained healthy, but the fixture no longer produced ICAO-compliant green intensity, beam distribution, and threshold-bar performance.
11Corrective Action
The maintenance team:
- Removed the affected threshold fixture.
- Replaced the green optical lens assembly.
- Replaced the degraded optical module.
- Restored reflector condition.
- Cleaned the optical compartment.
- Realigned the fixture.
- Performed photometric testing.
- Verified beam distribution.
- Confirmed green chromaticity compliance.
- Updated photometric maintenance records.
12Post-Repair Verification
The runway threshold light met ICAO photometric requirements after repair.
13Root-Cause Analysis
Dim green threshold light
Healthy
Below requirement
Non-compliant
Contaminated and aged
Degraded
Optical degradation and alignment error
Replace optical components and restore alignment
14Why This Case Was Important
Threshold photometric failures can cause:
- Reduced runway threshold identification.
- Poor approach guidance.
- Wet-night visibility problems.
- ICAO non-compliance.
- Reduced threshold conspicuity.
- Increased operational risk.
Threshold-light photometric performance is essential for safe runway approach operations.
15Engineering Lessons
An illuminated threshold light is not necessarily ICAO compliant.
An illuminated threshold light is not necessarily ICAO compliant.
Threshold-bar uniformity is critical for runway identification.
Threshold-bar uniformity is critical for runway identification.
Green chromaticity is an operational requirement.
Green chromaticity is an operational requirement.
Lens contamination can significantly reduce threshold visibility.
Lens contamination can significantly reduce threshold visibility.
Fixture alignment directly affects approach-direction beam performance.
Fixture alignment directly affects approach-direction beam performance.
16Engineer's Decision Point
You observe: Threshold lights ON. Pilots report poor threshold visibility. CCR healthy. No electrical faults. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about runway threshold light photometric failure and green chromaticity.
Q1What is AGL-CS-063?
AGL-CS-063 is an engineering training case study. It covers runway threshold lights that were electrically healthy but failed photometric inspection because of reduced green intensity and degraded optics. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Can threshold lights be electrically healthy but still fail photometric compliance?
Yes. In this case the CCR held 6.6 A, voltage was normal and all affected threshold lights were illuminated, but green intensity and beam distribution were below the required values.
Q3What did the measurements show in AGL-CS-063?
Peak intensity was 9,500 cd against a representative training requirement of at least 16,000 cd. The beam showed vertical depression, narrow horizontal spread and non-uniform threshold-bar distribution.
Q4Why is green chromaticity important for threshold lights?
Threshold lights identify the runway threshold during the final approach, and their green colour is an operational requirement. Here the green lost saturation and shifted toward a yellow-green appearance due to optical aging.
Q5Why does threshold-bar uniformity matter?
The threshold bar is read as a pattern. If individual fixtures differ in intensity or beam direction, the bar looks uneven and the threshold is less conspicuous to approaching crews.
Q6What caused the failure in AGL-CS-063?
Combined optical degradation: lens contamination, UV-related lens aging, reflector deterioration, LED optical degradation and fixture alignment errors. The electrical system remained healthy.
Q7How did fixture alignment contribute?
Measurements showed vertical aiming error and slight inward rotation, which reduced approach-direction coverage and produced uneven threshold-bar intensity.
Q8How was compliance restored?
The team replaced the green optical lens assembly and degraded optical module, restored the reflector, cleaned the optical compartment and realigned the fixture. Photometric testing, beam distribution and green chromaticity were then verified and records updated.
Q9What should you investigate first when threshold visibility is poor but the CCR is healthy?
Measure green photometric intensity, verify beam distribution, inspect the optical assembly and check fixture alignment. Replacing the CCR, raising the current setting or replacing transformers would not address an optical problem.
Engineering & Expert Notes
Verify Green Chromaticity as Well as Intensity
ColourColour compliance is part of the requirement. Measure it, do not rely on appearance.
Check Pattern Uniformity Along the Bar
UniformityOne dim unit stands out in a threshold bar. Compare units side by side.
Clean and Re-Measure After Replacement
MaintenanceCleaning may restore performance. Re-test to confirm.
Add Photometric Checks to Periodic Inspection
PlanningVisual checks alone miss gradual degradation.
20Case Study Conclusion
Runway threshold light photometric failure is an optical-performance problem caused by lens contamination, optical aging, reflector deterioration, LED degradation, and beam misalignment, and a threshold light can be electrically operational while failing ICAO Annex 14 photometric requirements and reducing runway threshold identification effectiveness.
Systematic photometric measurement, beam-distribution verification, chromaticity testing, optical inspection, LED assessment, and fixture-alignment verification can identify photometric failures and restore compliant runway threshold lighting performance.
In AGL engineering, the runway threshold is the pilot's first visual commitment point, and photometric compliance determines whether that commitment begins with confidence or uncertainty.
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