AGL-CS-067 — Individual Light Fails but Average Passes
The AGL Knowledge Hub | Engineering Case Study Series. A photometric acceptance testing case where the average intensity passed the acceptance criterion, but one individual fixture failed — from measurement verification through statistical analysis, optical inspection, and corrective retesting.
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 acceptance test, twenty runway edge lights were measured using approved AGL photometric equipment.
The overall test results showed that the average measured intensity exceeded the required acceptance value, and the contractor initially claimed that the lighting system had passed.
However, one runway edge light produced only 58% of the required minimum intensity, while the remaining fixtures produced substantially higher values.
The average intensity remained acceptable because several lights significantly exceeded the minimum requirement.
The engineering question was:
Can a runway lighting system be considered compliant when the average photometric result passes but an individual light fails the minimum photometric requirement?
This case demonstrates one of the most important principles in ICAO photometric acceptance testing: average compliance does not eliminate individual non-compliance.
2Learning Objectives
After completing this case study, the learner should be able to:
- Interpret individual and average photometric results.
- Understand ICAO acceptance philosophy.
- Identify non-compliant individual fixtures.
- Evaluate statistical masking effects.
- Assess operational safety implications.
- Make correct acceptance and maintenance decisions.
- Document photometric non-conformities correctly.
3System Background
Photometric testing evaluates:
- Peak intensity.
- Beam distribution.
- Chromaticity.
- Beam alignment.
- Individual fixture performance.
- System uniformity.
Acceptance testing may involve:
- Individual fixture measurements.
- Sampling plans.
- Average intensity calculations.
- Minimum intensity verification.
- Compliance evaluation.
A common misunderstanding is that high-performing fixtures can compensate for a weak fixture.
4Problem Statement
The measured runway edge light intensities were:
| Fixture | Measured Intensity |
| 1 | 118% |
| 2 | 110% |
| 3 | 106% |
| 4 | 102% |
| 5 | 58% |
| 6 | 114% |
| 7 | 109% |
| 8 | 111% |
| 9 | 116% |
| 10 | 108% |
| \... | \... |
Average intensity:
106%
One fixture:
58%
The average passed, but one fixture failed.
5Initial Observations
| Average intensity | Pass |
| Individual fixture #5 | Fail |
| CCR operation | Normal |
| Beam distribution | Generally acceptable |
| Chromaticity | Acceptable |
The statistical result appeared acceptable, but one fixture was clearly deficient.
6Initial Engineering Hypotheses
The maintenance team considered:
The average result is sufficient for acceptance.
Individual failure is acceptable because neighboring lights are brighter.
The failed fixture is a measurement error.
The individual fixture is genuinely non-compliant.
The system should be accepted with a maintenance observation.
7Acceptance Testing Principle
Each runway light provides an independent visual reference.
8Test Equipment
The investigation used:
- Approved AGL photometer.
- Goniophotometric measurement equipment.
- Chromaticity meter.
- Optical inspection tools.
- Alignment verification equipment.
- ICAO Annex 14 photometric reference data.
- Airport-approved PPE.
9Investigation
Step 1: Verify Measurement Accuracy
The failed fixture was measured repeatedly.
Results:
| Measurement | Intensity |
| Test 1 | 57% |
| Test 2 | 59% |
| Test 3 | 58% |
The failure was repeatable.
Step 2: Compare Adjacent Fixtures
Adjacent fixtures measured:
- 110%
- 114%
The neighboring fixtures were healthy.
Step 3: Beam Distribution Verification
The failed fixture showed:
- Reduced peak intensity.
- Beam narrowing.
- Vertical beam depression.
The optical performance was degraded.
Step 4: Optical Inspection
Inspection revealed:
- Lens contamination.
- Reflector deterioration.
- LED optical aging.
- Minor alignment error.
The photometric failure was genuine.
Step 5: Statistical Analysis
Average intensity:
106%
Median intensity:
109%
Lowest intensity:
58%
The high average masked the failed fixture.
10Root Cause
The runway edge lighting system contained one photometrically non-compliant fixture, whose performance was masked by several higher-than-average fixtures.
The average intensity passed because:
- Most fixtures exceeded the minimum requirement.
- The statistical mean remained high.
- One severely deficient fixture had limited effect on the average.
The failed fixture remained individually non-compliant.
11Why the Average Was Misleading
Consider:
Nine fixtures = 110%
One fixture = 58%
Average:
(9 × 110 + 58) / 10 = 104.8%
The average passes comfortably.
The failed fixture remains unacceptable.
This is a classic example of statistical masking.
12ICAO Compliance Principle
Photometric acceptance normally requires evaluation of:
- Individual fixture performance.
- Beam distribution.
- Chromaticity.
- System uniformity.
A deficient individual fixture cannot automatically be accepted because:
- It creates a weak visual reference.
- It reduces pattern continuity.
- It may affect pilot visual acquisition.
- It may indicate progressive optical deterioration.
13Corrective Action
The maintenance team:
- Removed the failed fixture.
- Replaced the optical module.
- Restored beam alignment.
- Performed photometric testing.
- Verified individual compliance.
- Recalculated system uniformity.
- Updated photometric records.
- Closed the non-conformity.
14Post-Repair Verification
The system became fully compliant.
15Root-Cause Analysis
Average intensity acceptable
One fixture below minimum
Failure confirmed
Degradation found
Individual photometric degradation
Replace and retest the failed fixture
16Why This Case Was Important
Statistical masking can lead to:
- Incorrect acceptance decisions.
- Hidden maintenance problems.
- Reduced visual guidance quality.
- Progressive system deterioration.
- ICAO non-compliance.
Photometric evaluation requires both statistical and engineering judgment.
17Engineering Lessons
Average compliance does not guarantee individual compliance.
Average compliance does not guarantee individual compliance.
Every runway light is an independent visual aid.
Every runway light is an independent visual aid.
Statistical masking can hide serious photometric defects.
Statistical masking can hide serious photometric defects.
Individual failures should be investigated, not averaged away.
Individual failures should be investigated, not averaged away.
Acceptance testing requires evaluation of both individual and overall system performance.
Acceptance testing requires evaluation of both individual and overall system performance.
18Engineer's Decision Point
You observe: Average runway edge light intensity = 106%. One fixture = 58%. All measurements verified. What should you do?
19Knowledge Check
20Interview Questions
21Frequently Asked Questions
Quick answers about average vs individual photometric compliance in acceptance testing.
Q1What is AGL-CS-067?
AGL-CS-067 is an engineering training case study. It covers a runway edge light acceptance test where the average intensity passed but one individual light failed the minimum requirement. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Can a runway lighting system be compliant if the average passes but one light fails?
No. The case demonstrates that average compliance does not eliminate individual non-compliance. Each runway light is an independent visual reference, so a deficient fixture cannot simply be accepted.
Q3What were the results in AGL-CS-067?
Twenty runway edge lights were measured. The average intensity was 106%, but one fixture produced only 58% of the required minimum while the others were substantially higher.
Q4Why did the average hide the failed fixture?
This is statistical masking. Several lights well above the minimum pulled the average up, so one severely deficient fixture had little effect on it. Nine fixtures at 110% and one at 58% average about 104.8%.
Q5How was the failure confirmed as genuine?
The fixture was measured repeatedly (57%, 59%, 58%), neighbouring fixtures measured 110% and 114%, and inspection found lens contamination, reflector deterioration, LED optical aging and minor alignment error.
Q6Why must individual fixture performance be evaluated?
A deficient light creates a weak visual reference, reduces pattern continuity, may affect pilot visual acquisition, and may indicate progressive optical deterioration.
Q7What caused the failed fixture in AGL-CS-067?
Optical degradation: lens contamination, reflector deterioration, LED optical aging and minor alignment error, which gave reduced peak intensity, beam narrowing and vertical beam depression.
Q8How was the non-conformity corrected?
The failed fixture was removed, its optical module replaced and beam alignment restored. The team then retested, verified individual compliance, recalculated system uniformity, updated records and closed the non-conformity.
Q9What is the key lesson from this case?
Never accept a lighting system on the average alone. Evaluate each fixture against the minimum requirement, along with beam distribution, chromaticity and system uniformity.
Engineering & Expert Notes
Assess Each Fixture Against Its Own Requirement
PrincipleAn average hides outliers. Judge individual fixtures, not the group mean.
Investigate the Outlier, Do Not Average It Away
Follow-UpFind why the low unit is low. It may reveal a wider issue.
Report Individual Results in the Test Record
ReportingA table of each fixture shows compliance clearly and supports decisions.
Retest After Rectification
VerificationConfirm the corrected unit meets its requirement and record the result.
22Case Study Conclusion
A runway lighting system cannot be considered fully compliant when an individual light fails the minimum photometric requirement, even if the average intensity exceeds the acceptance value, because each fixture provides a critical independent visual reference for flight operations.
Systematic individual photometric verification, beam-distribution assessment, optical inspection, statistical analysis, and corrective retesting can prevent statistical masking from concealing operationally significant photometric defects.
In AGL engineering, pilots do not land on an average light; they land using every individual light that forms the runway visual pattern.
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.