AGL-CS-068 — Average Passes but Individual Points Fail
The AGL Knowledge Hub | Engineering Case Study Series. A goniophotometric type-test case where the average intensity passed the acceptance criterion but mandatory beam-distribution points failed — from measurement verification through optical inspection, beam-envelope analysis, 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 laboratory goniophotometric type test, a runway edge light produced an average intensity that exceeded the required ICAO acceptance value.
The manufacturer initially considered the fixture compliant because the calculated average intensity was satisfactory.
However, detailed analysis of the photometric grid revealed that several mandatory measurement points within the required beam distribution envelope were below the specified minimum intensity.
The engineering question was:
Can a fixture pass photometric compliance when the average intensity is acceptable but individual beam-distribution points fail the required ICAO values?
This case demonstrates one of the most important principles in AGL photometric certification: compliance depends on required photometric points, not merely on average intensity.
2Learning Objectives
After completing this case study, the learner should be able to:
- Interpret point-by-point photometric measurements.
- Understand beam-distribution compliance.
- Identify critical photometric-point failures.
- Evaluate average versus point compliance.
- Assess ICAO beam-envelope requirements.
- Make correct certification decisions.
- Document photometric non-conformities accurately.
3System Background
A photometric test evaluates:
- Peak intensity.
- Vertical beam distribution.
- Horizontal beam distribution.
- Chromaticity.
- Intensity at specified test points.
- Beam-envelope compliance.
Testing is performed at defined vertical and horizontal angles.
Compliance is generally determined by:
- Minimum intensity requirements.
- Maximum intensity limitations.
- Beam-distribution requirements.
- Individual measurement-point compliance.
Average intensity is only one statistical indicator.
4Problem Statement
Measured intensities:
| Test Point | Required | Measured |
| P1 | 100% | 112% |
| P2 | 100% | 109% |
| P3 | 100% | 72% |
| P4 | 100% | 76% |
| P5 | 100% | 118% |
| P6 | 100% | 115% |
| P7 | 100% | 111% |
Average intensity:
101.9%
Two mandatory points:
72% and 76%
The average passed, but mandatory points failed.
5Initial Observations
| Average intensity | Pass |
| Peak intensity | Pass |
| Beam center | Pass |
| Beam edges | Fail |
| Chromaticity | Pass |
The fixture appeared bright overall, but parts of the required beam were deficient.
6Initial Engineering Hypotheses
The photometry team considered:
The average result is sufficient.
Edge points are not operationally important.
The beam-distribution points are mandatory.
The low points are measurement errors.
The fixture is fully compliant.
7Beam Distribution Principle
Pilots use the entire beam envelope, not only the beam center.
8Test Equipment
The investigation used:
- Goniophotometer.
- Precision photometer.
- Angular positioning system.
- Chromaticity measurement equipment.
- Optical inspection tools.
- Beam-distribution analysis software.
- ICAO Annex 14 photometric reference data.
9Investigation
Step 1: Repeat Measurements
Repeated measurements confirmed:
| Point | Measurement 1 | Measurement 2 | Measurement 3 |
| P3 | 72% | 73% | 72% |
| P4 | 76% | 75% | 76% |
The failures were repeatable.
Step 2: Beam Distribution Analysis
The beam showed:
- Strong central intensity.
- Weak edge intensity.
- Narrow horizontal spread.
- Reduced vertical coverage.
The beam envelope was distorted.
Step 3: Optical Inspection
Inspection revealed:
- Lens contamination.
- Reflector deformation.
- LED optical-module displacement.
- Partial optical obstruction.
The optical system was not producing the designed beam.
Step 4: Alignment Verification
The fixture showed:
- Minor optical-axis shift.
- Beam-center displacement.
- Reduced beam-edge coverage.
Alignment contributed to the point failures.
10Why the Average Passed
High values:
112% 109% 118% 115% 111%
Low values:
72% 76%
Average:
101.9%
The strong beam center statistically compensated for the weak beam edges.
Operationally, the beam remained deficient.
11ICAO Compliance Principle
Photometric compliance requires:
- Required test points.
- Required beam distribution.
- Required intensity values.
- Required chromaticity.
A fixture may fail because:
- Edge intensity is insufficient.
- Vertical coverage is inadequate.
- Horizontal coverage is incomplete.
- Mandatory points are below minimum.
The average cannot replace mandatory point compliance.
12Root Cause
The fixture failed ICAO photometric compliance because the required beam-distribution envelope contained multiple non-compliant measurement points, even though the average intensity exceeded the acceptance value.
The failure was caused by:
- Optical-module displacement.
- Reflector deformation.
- Beam-axis shift.
- Reduced beam-edge intensity.
13Corrective Action
The maintenance team:
- Replaced the optical module.
- Corrected reflector alignment.
- Restored beam-axis alignment.
- Cleaned the optical assembly.
- Repeated goniophotometric testing.
- Verified all mandatory points.
- Confirmed beam-envelope compliance.
- Updated photometric certification records.
14Post-Repair Verification
All mandatory photometric points met the required values.
15Root-Cause Analysis
Average intensity acceptable
Mandatory points below minimum
Failure confirmed
Beam distortion found
Beam-distribution distortion
Restore optical alignment and beam distribution
16Why This Case Was Important
Beam-distribution failures can cause:
- Reduced visual coverage.
- Poor off-axis visibility.
- Uneven runway guidance.
- ICAO non-compliance.
- Certification failure.
Photometric compliance is a spatial distribution requirement, not merely a brightness requirement.
17Engineering Lessons
Average intensity is not beam-distribution compliance.
Average intensity is not beam-distribution compliance.
Mandatory photometric points must individually comply.
Mandatory photometric points must individually comply.
Beam-edge performance is operationally important.
Beam-edge performance is operationally important.
Optical alignment strongly influences beam-envelope compliance.
Optical alignment strongly influences beam-envelope compliance.
Photometric certification requires point-by-point verification.
Photometric certification requires point-by-point verification.
18Engineer's Decision Point
You observe: Average intensity = 101.9%. Two mandatory beam points below minimum. Measurements verified. What should you do?
19Knowledge Check
20Interview Questions
21Frequently Asked Questions
Quick answers about beam-distribution points and why an average intensity can mislead.
Q1What is AGL-CS-068?
AGL-CS-068 is an engineering training case study. It covers a laboratory goniophotometric type test where the average intensity passed but mandatory beam-distribution points failed. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Can a fixture pass ICAO photometric compliance on average intensity alone?
No. Compliance depends on the required photometric points, beam distribution, intensity values and chromaticity. The average cannot replace mandatory point compliance.
Q3What were the results in AGL-CS-068?
The average intensity was 101.9%, but two mandatory points measured 72% and 76% against a 100% requirement. Repeat measurements confirmed both failures (72–73% and 75–76%).
Q4Why did the average still pass?
Strong readings of 109–118% at the beam centre compensated statistically for the weak beam edges. Operationally, the beam remained deficient.
Q5Why do pilots need the whole beam envelope and not just the centre?
Pilots view the light from different positions, so they use the entire beam envelope. Weak edge intensity, narrow horizontal spread or reduced vertical coverage leaves gaps in visual guidance.
Q6What did the beam distribution analysis show?
Strong central intensity, weak edge intensity, narrow horizontal spread and reduced vertical coverage, so the beam envelope was distorted.
Q7What caused the failure in AGL-CS-068?
Optical-module displacement, reflector deformation, beam-axis shift and reduced beam-edge intensity. Inspection also found lens contamination and partial optical obstruction.
Q8How was compliance restored?
The team replaced the optical module, corrected reflector alignment, restored beam-axis alignment and cleaned the optical assembly. They then repeated goniophotometric testing, verified all mandatory points and updated the certification records.
Q9What is the key lesson from this case?
Check every required test point against its minimum. An acceptable average does not prove the beam envelope is compliant.
Engineering & Expert Notes
Look at the Full Distribution, Not Just the Peak
BeamCompliance depends on the pattern at specified points. Review all of them.
Inspect the Optical Module for Displacement
MechanicalMovement inside the fixture can distort the beam. Check mountings and reflectors.
Verify the Fixture Alignment Before Measuring
AlignmentMisalignment can look like a distribution problem.
Keep Measurement Grids With the Report
RecordsRaw data lets others verify the conclusion.
22Case Study Conclusion
A fixture cannot be considered ICAO compliant when mandatory photometric measurement points fail the required intensity values, even if the average intensity exceeds the acceptance criterion, because beam-distribution compliance depends on the required photometric envelope rather than statistical averaging.
Systematic point-by-point photometric analysis, beam-distribution verification, optical inspection, alignment assessment, and certification retesting can identify beam-envelope defects that average intensity calculations may conceal.
In AGL engineering, pilots do not see an average beam; they see the beam exactly where the photometric points are measured.
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.