AGL-CS-059 — Blue Taxiway Light Photometric Failure
The AGL Knowledge Hub | Engineering Case Study Series. A photometric investigation where a blue taxiway edge light failed to meet ICAO intensity and chromaticity requirements despite normal electrical operation — from photometer verification through optical inspection, LED module analysis, lens contamination assessment, and corrective optical 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 photometric inspection of taxiway edge lighting, one blue taxiway edge light was found to have failed photometric performance requirements under ICAO Annex 14, Volume I, Chapter 5.
The Constant Current Regulator (CCR) was operating normally at the selected 6.6 A current, the fixture was electrically functional and appeared visually blue, but photometric measurements showed insufficient intensity and incorrect chromaticity coordinates.
A comprehensive optical investigation was initiated covering photometer calibration, fixture positioning, lens condition, LED module performance, optical-system contamination, and chromaticity verification.
The investigation identified inadvertent use of an incorrect LED module during a previous replacement, combined with progressive optical degradation of the lens assembly, which reduced both output intensity and shifted the chromaticity outside the specified blue color band.
This case demonstrates that a blue taxiway light can appear visually correct while failing photometric requirements — and electrical functionality does not guarantee optical compliance.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose photometric failures in blue taxiway edge lights.
- Understand ICAO Annex 14 chromaticity and intensity requirements for blue taxiway lights.
- Differentiate electrical functionality from optical compliance.
- Inspect optical assemblies for contamination and degradation.
- Verify LED module specifications and compatibility.
- Use photometric test equipment correctly.
- Restore photometric compliance safely.
3System Background
A blue taxiway edge light provides:
- Blue taxiway edge delineation.
- ICAO-compliant chromaticity.
- Minimum intensity requirements.
- Uniform beam pattern.
- Omnidirectional or directional guidance.
The optical system consists of:
Photometric requirements for blue taxiway edge lights under ICAO Annex 14 include:
- Chromaticity coordinates within the defined blue color band.
- Minimum intensity values at specified angles.
- Beam spread and uniformity.
- Intensity ratios between adjacent lights.
Common photometric failure causes include:
- Incorrect LED module.
- Lens contamination.
- Optical degradation.
- Incorrect fixture alignment.
- Photometer calibration error.
- Incorrect current setting.
4Problem Statement
The reported condition was:
- Taxiway edge light electrically operational.
- Blue light appears visually normal.
- Photometric test failed.
- Intensity below ICAO minimum.
- Chromaticity outside blue color band.
- Adjacent blue taxiway lights passing.
Why does a blue taxiway edge light that appears visually normal fail photometric testing, and what optical-system degradation has occurred?
5Initial Observations
| CCR status | Healthy |
| Selected current | 6.6 A |
| Actual current | 6.6 A |
| Fixture operation | Functional |
| Visual appearance | Blue |
| Photometric result | FAIL |
| Adjacent fixtures | Passing |
The fixture was operational but photometrically non-compliant.
6Initial Engineering Hypotheses
The maintenance team considered:
Photometer calibration error.
Incorrect LED module installed.
Lens contamination.
Optical degradation.
Incorrect fixture alignment.
7Photometric Diagnostic Principle
Normal photometric performance:
Photometric failure:
Visual appearance is not a reliable indicator of photometric compliance.
8Test Equipment
The investigation used:
- Calibrated photometer.
- Colorimeter / spectrometer.
- Lens inspection equipment.
- LED module test equipment.
- True-RMS multimeter.
- Current measurement equipment.
- Alignment verification tools.
- Approved AGL photometric test equipment.
- Airport-approved PPE.
9Investigation
Step 1: Verify Photometer Calibration
The photometer was:
- Calibrated within the valid period.
- Tested with a reference standard.
- Functioning correctly.
- Verified against a known-good blue taxiway light.
The photometer was accurate.
Step 2: Verify CCR Operation and Current
Results:
| Current regulation | Normal |
| Selected current | 6.6 A |
| Actual current | 6.6 A |
| Output voltage | Normal |
The CCR was healthy and delivering the correct current.
Step 3: Measure Photometric Intensity
Measured intensity at the specified test angle:
8.2 cd
ICAO minimum requirement:
≥ 10.0 cd
The fixture was below the minimum intensity.
Step 4: Measure Chromaticity
Measured chromaticity coordinates (x, y):
x = 0.210, y = 0.160
ICAO blue color band requirements:
x between 0.136 and 0.220, y between 0.070 and 0.200
The chromaticity shifted outside the acceptable blue color band.
Step 5: LED Module Inspection
Inspection revealed:
- LED module part number did not match the approved specification.
- Module was an unapproved replacement type.
- Primary wavelength was shifted toward cyan.
- Output flux was lower than the original specification.
An incorrect LED module had been installed during a previous maintenance action.
Step 6: Lens and Optical Assembly Inspection
The lens showed:
- Surface contamination.
- Fine particulate deposits.
- Minor abrasions.
- Reduced optical transmission.
- UV degradation.
The optical path was partially obstructed and degraded.
Step 7: Fixture Alignment Verification
Alignment was found to be:
- Within acceptable tolerances.
- Not contributing to the photometric failure.
The alignment was correct.
Step 8: Reference Comparison
Testing an adjacent blue taxiway light with the same photometer:
- Intensity: 12.4 cd (PASS).
- Chromaticity: within ICAO blue color band (PASS).
The adjacent fixture confirmed that the problem was localized to the failed fixture.
10Root Cause
The blue taxiway light photometric failure was caused by installation of an incorrect, unapproved LED module during a previous replacement, combined with progressive optical degradation of the lens assembly through contamination and UV exposure.
The combined effects produced:
- Reduced luminous flux.
- Wavelength shift away from ICAO blue.
- Optical transmission loss.
- Chromaticity shift outside the blue color band.
- Intensity below ICAO minimum.
The fixture was electrically functional but optically non-compliant.
11Corrective Action
The maintenance team:
- Removed the incorrect LED module.
- Cleaned the lens and optical assembly.
- Installed an approved ICAO-compliant blue LED module.
- Replaced the degraded lens assembly.
- Verified photometric intensity.
- Verified chromaticity coordinates.
- Checked optical alignment.
- Performed full photometric testing.
- Updated maintenance records and parts inventory.
- Implemented LED module verification procedure.
12Post-Repair Verification
The blue taxiway edge light met all ICAO photometric requirements.
13Root-Cause Analysis
Blue taxiway light photometric failure
Calibrated and accurate
Healthy, 6.6 A
Below ICAO minimum
Outside blue color band
Incorrect, unapproved part
Contaminated and degraded
Incorrect LED module + optical degradation
Replace LED module and lens assembly
14Why This Case Was Important
Blue taxiway light photometric failures can cause:
- Incorrect taxiway edge perception.
- Reduced pilot guidance.
- Color-confusion hazards.
- ICAO non-compliance.
- Regulatory findings.
- Operational restrictions.
- Safety audit issues.
Photometric compliance is essential for safe surface movement guidance.
15Engineering Lessons
Visual appearance does not guarantee photometric compliance.
Visual appearance does not guarantee photometric compliance.
Incorrect LED modules can shift chromaticity.
Incorrect LED modules can shift chromaticity.
Optical contamination reduces intensity.
Optical contamination reduces intensity.
Photometric testing is essential, not optional.
Photometric testing is essential, not optional.
LED module verification should be part of any replacement procedure.
LED module verification should be part of any replacement procedure.
16Engineer's Decision Point
You observe: Blue taxiway edge light operational. Visually appears blue. Photometric test shows intensity below ICAO minimum and chromaticity outside blue color band. CCR healthy. What should you investigate first?
17Knowledge Check
18Interview Questions
Quick Reference: Taxiway Lighting Photometric Test
A taxiway lighting photometric test measures the intensity and colour of a taxiway light against the required values. For blue taxiway edge lights, a lens or filter that has faded or is dirty can cause a failure even when the light is on, so check the lens condition before replacing the unit.
19Frequently Asked Questions
Quick answers about blue taxiway light photometric failure, based on this case study.
Q1What is AGL-CS-059?
AGL-CS-059 is an engineering training case study on a blue taxiway edge light that looked normal but failed photometric testing under ICAO Annex 14, Volume I, Chapter 5. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2How can a light look blue and still fail?
The fixture was electrically functional and visually blue, but measurements showed insufficient intensity and chromaticity coordinates outside the blue color band. Adjacent blue lights passed.
Q3What was the root cause?
An incorrect, unapproved LED module installed during a previous replacement, combined with progressive lens degradation from contamination and UV exposure. This reduced luminous flux, shifted the wavelength away from ICAO blue and lowered optical transmission.
Q4What should you investigate first?
Verify the LED module specification, inspect the optical assembly for contamination and degradation, and confirm photometer calibration. Replacing the CCR or the whole fixture, or increasing the current setting, is not the right first step.
Q5Which test equipment was used?
The team used a calibrated photometer, colorimeter or spectrometer, lens inspection equipment, LED module test equipment, a true-RMS multimeter, current measurement equipment and alignment verification tools.
Q6How was the light corrected?
The team removed the incorrect LED module, cleaned the optical assembly, installed an approved ICAO-compliant blue LED module and replaced the degraded lens assembly. They verified intensity, chromaticity and alignment, performed full photometric testing, updated records and implemented an LED module verification procedure.
Q7How was the correction verified?
The module specification was correct, the lens was restored, intensity was at least 10.0 cd and chromaticity was within the ICAO blue band. The light passed all photometric requirements.
Q8Why verify LED module specification during replacement?
The failure began with an unapproved module installed during a previous replacement. The case therefore added an LED module verification procedure to stop incorrect modules entering service.
Engineering & Expert Notes
Control Replacement Parts to the Approved Type
SparesAn unapproved LED module can shift colour and output. Replace only with approved parts.
Measure Colour and Intensity, Do Not Judge by Eye
PhotometryBlue looks blue under many conditions. Chromaticity needs instruments.
Clean and Inspect the Lens for UV Damage
MaintenanceDiscolouration and crazing reduce transmission. Replace lenses that no longer clean up.
Record the Module Type Installed at Each Location
RecordsWithout records the wrong part can be reused unnoticed.
20Case Study Conclusion
A blue taxiway light photometric failure is an optical-performance problem caused by incorrect LED modules, optical contamination, or lens degradation, and it can create ICAO non-compliance and operational guidance hazards even when the electrical system is fully functional and the fixture appears visually normal.
Systematic photometer calibration verification, intensity measurement, chromaticity testing, LED module specification verification, optical assembly inspection, and corrective optical restoration can identify and eliminate blue taxiway light photometric failures.
In AGL engineering, a light that looks blue to the human eye can still fail ICAO chromaticity — the photometer is the only reliable witness.
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