AGL-CS-065 — Approach Light Photometry Failure
The AGL Knowledge Hub | Engineering Case Study Series. An approach lighting photometric investigation where optical degradation caused reduced 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 approach lighting photometric inspection, a section of the approach lighting system was found to have significantly reduced intensity and non‑uniform beam distribution, despite operating electrically and showing no CCR alarms.
Flight crews reported that the approach lighting pattern became difficult to acquire during night approaches, particularly in reduced visibility and wet‑weather conditions.
The Constant Current Regulator (CCR) was maintaining the selected 6.6 A current, output voltage was normal, and all affected approach lights were illuminated.
A detailed photometric investigation was initiated covering light intensity, beam distribution, longitudinal guidance, crossbar uniformity, chromaticity, optical contamination, and LED optical performance.
The investigation identified optical degradation caused by lens contamination, UV‑related lens aging, reflector deterioration, and fixture alignment errors, resulting in failure to meet ICAO Annex 14 approach‑light photometric requirements.
This case demonstrates that approach lights can be electrically operational while failing photometric compliance and reducing runway acquisition effectiveness during precision approach operations.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose approach‑light photometric failures.
- Differentiate electrical and photometric defects.
- Evaluate approach‑light intensity.
- Assess beam distribution and pattern uniformity.
- Verify white chromaticity compliance.
- Investigate optical degradation.
- Restore ICAO‑compliant approach‑light photometric performance.
3System Background
An approach light consists of:
- High‑intensity LED optical module.
- Precision optical lens.
- Beam‑control optics.
- Reflector assembly.
- Housing.
- Mounting and alignment system.
- Electrical driver.
Photometric performance depends on:
- Peak intensity.
- Vertical beam distribution.
- Horizontal beam distribution.
- White chromaticity.
- Optical cleanliness.
- Lens transmission.
- Fixture alignment.
- Approach‑pattern 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:
- Approach lights illuminated.
- Reduced apparent brightness.
- Poor approach‑pattern visibility.
- CCR healthy.
- No electrical alarms.
- Photometric inspection failed.
Are the approach 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 |
| Approach visibility | Poor |
| Pattern 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.
7Approach Photometric Failure Principle
Approach lighting is the pilot's primary visual reference during the final approach.
8Test Equipment
The investigation used:
- Portable AGL photometer.
- Goniophotometric measurement equipment.
- 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:
28,000 cd
Required intensity:
≥ 45,000 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 narrowing.
- Non‑uniform approach‑pattern distribution.
The beam no longer matched the required approach‑light 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 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.
- Lateral beam displacement.
- Reduced longitudinal guidance coverage.
- Uneven crossbar intensity.
Alignment errors further reduced approach guidance effectiveness.
10Root Cause
The approach‑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 intensity, beam distribution, and approach‑pattern performance.
11Corrective Action
The maintenance team:
- Removed the affected approach fixture.
- Replaced the 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 chromaticity compliance.
- Updated photometric maintenance records.
12Post‑Repair Verification
The approach light met ICAO photometric requirements after repair.
13Root‑Cause Analysis
Dim approach 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
Approach photometric failures can cause:
- Reduced runway acquisition distance.
- Poor approach alignment.
- Wet‑night visibility problems.
- CAT I/II/III operational degradation.
- ICAO non‑compliance.
- Reduced approach conspicuity.
Approach‑light photometric performance is essential for precision approach operations.
15Engineering Lessons
An illuminated approach light is not necessarily ICAO compliant.
An illuminated approach light is not necessarily ICAO compliant.
Approach‑pattern uniformity is critical for runway acquisition.
Approach‑pattern uniformity is critical for runway acquisition.
Fixture alignment directly affects approach guidance.
Fixture alignment directly affects approach guidance.
Lens contamination can significantly reduce approach visibility.
Lens contamination can significantly reduce approach visibility.
Precision approach systems require strict photometric verification.
Precision approach systems require strict photometric verification.
16Engineer's Decision Point
You observe: Approach lights ON. Pilots report poor runway acquisition. CCR healthy. No electrical faults. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about approach light photometric failure and runway acquisition.
Q1What is AGL-CS-065?
AGL-CS-065 is an engineering training case study. It covers approach lights that were electrically healthy but failed photometric inspection because of reduced intensity and degraded beam distribution. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Can approach lights be electrically healthy but still non-compliant?
Yes. The CCR held 6.6 A and all affected approach lights were on, yet intensity and beam distribution fell below ICAO Annex 14 approach-light requirements.
Q3What did the measurements show in AGL-CS-065?
Peak intensity was 28,000 cd against a representative training requirement of at least 45,000 cd. The beam showed vertical depression, horizontal narrowing and a non-uniform approach pattern.
Q4Why is approach lighting so important to pilots?
It is the pilot's primary visual reference during the final approach. Reduced intensity or pattern distortion makes the runway harder to acquire, particularly in reduced visibility and wet weather.
Q5What caused the failure in AGL-CS-065?
Combined optical degradation: lens contamination, UV-related lens aging, reflector deterioration, LED optical degradation and fixture alignment errors. The electrical system remained healthy.
Q6How did alignment errors affect the approach pattern?
Measurements showed vertical aiming error and lateral beam displacement, which reduced longitudinal guidance coverage and produced uneven crossbar intensity.
Q7How was compliance restored?
The team replaced the optical lens assembly and degraded optical module, restored the reflector, cleaned the optical compartment and realigned the fixture. Photometric testing, beam distribution and chromaticity were verified and records updated.
Q8Which operations are affected by approach photometric failures?
The case notes reduced runway acquisition distance, poor approach alignment, wet-night visibility problems and degraded CAT I/II/III operations, as well as ICAO non-compliance.
Q9What should you investigate first when runway acquisition is poor but the CCR is healthy?
Measure photometric intensity, verify beam distribution, inspect the optical assembly and check fixture alignment. Replacing the CCR or all transformers would not address an optical fault.
Engineering & Expert Notes
Include Approach Lights in the Cleaning Plan
MaintenanceDust and film accumulate quickly on exposed fixtures. Clean on a schedule.
Verify Aiming and Elevation on Each Unit
AlignmentApproach lights are aimed. Check angles after impacts and maintenance.
Inspect Structures Supporting the Lights
StructuresSettlement or movement of masts alters aiming. Check them during photometric visits.
Track Output Over Time
TrendingComparison with commissioning data shows decline before it becomes a defect.
20Case Study Conclusion
Approach‑light photometric failure is an optical‑performance problem caused by lens contamination, optical aging, reflector deterioration, LED degradation, and beam misalignment, and an approach light can be electrically operational while failing ICAO Annex 14 photometric requirements and reducing runway acquisition effectiveness during precision approach operations.
Systematic photometric measurement, beam‑distribution verification, pattern‑uniformity assessment, chromaticity testing, optical inspection, LED assessment, and fixture‑alignment verification can identify photometric failures and restore compliant approach‑lighting performance.
In AGL engineering, the approach lighting system is the pilot's first visual conversation with the runway, and photometric compliance determines whether that conversation begins with clarity or confusion.
Turn Case Studies Into Engineering Capability
A case study shows you how a problem is investigated. Precision AGL Academy helps you build the knowledge to understand why it happens, diagnose it systematically, and apply the right engineering approach.
Study real-world-style engineering scenarios, identify failure patterns, understand root causes, and develop a systematic approach to AGL troubleshooting.
Study the case. Understand the system. Build the skill.
Follow the Academy
Important Notice, Disclaimer & Limitation of Liability
Please read before relying on any content on this page or on any record downloaded from it.
- Purpose and intended users. This page is an independent educational and informational reference for competent, suitably trained and authorised airfield ground lighting (AGL) and aerodrome electrical personnel. It is not professional engineering, legal or regulatory advice, and using it does not create any professional or client relationship.
- Fictional training simulation. This case study is an engineering simulation created for training. It is not a report of an actual airport incident. Scenarios, sequences, dates, readings, measurements, outcomes and numerical values are illustrative and invented for teaching. No fault, failure, cause or liability is attributed to any person, company, airport, regulator, manufacturer or product, and any resemblance to a real event is coincidental. The content is not a forensic, legal, insurance, regulatory or accident-investigation finding and must not be cited as evidence, as a record of an event, or as a basis for disciplinary, contractual, insurance or legal action.
- Safety. Work on series circuits, constant current regulators (CCRs), isolating transformers, cables, connectors and control equipment can be fatal. Always apply the aerodrome’s permit-to-work and isolation/LOTO procedures, prove dead, use correctly rated instruments and PPE, and follow the manufacturer’s instructions. Nothing here authorises work on live equipment or the bypassing of any safety device or interlock. Where a case narrates an inspection, measurement or test, that narrative is not an instruction or an endorsement of the method.
- Values, test results and acceptance. Test voltages, insulation-resistance and current values, tolerances, thresholds, photometric and angular figures and pass/fail or “acceptable” wording in a case are specific to its invented scenario. They are not acceptance criteria, limits or targets. Use the project specification, the applicable standard and the manufacturer’s data for real work. Completing or following a case produces no test certificate, calibration record, acceptance or evidence of compliance.
- Aviation operations. Nothing here authorises return to service, a change of operating category (CAT I/II/III), a NOTAM, a flight inspection outcome or any aerodrome certification decision. These rest with the aerodrome operator, the air navigation service provider and the competent regulator.
- Standards and regulations. References to ICAO, FAA, IEC, IEEE, ISO and other standards are for orientation only. They are summarised, may be incomplete or out of date, and standards are revised from time to time. Consult the current official editions. Following this content does not guarantee compliance with any law, standard or regulatory approval. Official documents, regulations and manufacturers’ instructions always prevail.
- Learning aids. Knowledge checks, decision points, interview questions, FAQs and suggested answers are teaching aids with illustrative answers. They are not an assessment, certification, accreditation or competency or authorisation to work on any aerodrome, and they do not predict or guarantee the outcome of any job interview or application.
- Generic content; no warranty. Diagnostic sequences, cause rankings, tolerances and values are generic and indicative; real installations differ in design, equipment make and model, age and condition. Content is provided “as is” and “as available”, without warranty of any kind, express or implied, including accuracy, completeness, currency or fitness for a particular purpose.
- Limitation of liability. To the fullest extent permitted by applicable law, Precision AGL Academy and its owners, authors and contributors are not liable for any injury, death, loss, damage, downtime, penalty or regulatory consequence, whether direct or indirect, arising from the use of, reliance on, or inability to use this content. Nothing in this notice excludes or limits any liability that cannot lawfully be excluded or limited.
- Independence and trademarks. This is an independent publication, not affiliated with or endorsed by ICAO, the FAA, the IEC, DGCA, any airport, regulator or equipment manufacturer. Third-party names, standard designations and trademarks belong to their respective owners and are used for identification only.
- Copyright and use. © 2026 Precision AGL Academy. All rights reserved. You may read this page and use it for personal reference and internal training, keeping this notice attached. Any other copying, republication, redistribution, scraping or commercial use requires our prior written permission.
- Corrections. Content is reviewed periodically, with no obligation to update. To report an error, safety concern or rights issue, write to precisionaglsolutions@gmail.com.