AGL-CS-080 — PAPI Photometry and Alignment Investigation
The AGL Knowledge Hub | Engineering Case Study Series. An integrated PAPI investigation where photometric degradation and geometric misalignment combined to produce unstable glide-path indication — from electrical verification through photometric measurement, beam-angle calibration, survey alignment, foundation inspection, and operational glide-path validation.
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 routine ICAO visual aid inspection, flight crews reported that the PAPI indication appeared inconsistent during final approach, with unstable red/white transitions and uneven brightness between units.
Ground inspection confirmed that all four PAPI units were operational, but maintenance records showed that Unit 2 had undergone recent optical maintenance, Unit 3 had experienced foundation settlement, and Unit 4 had accumulated significant optical contamination.
Because the problem involved both photometric and geometric symptoms, a comprehensive investigation was initiated covering:
- Beam-angle calibration.
- Unit intensity balance.
- Chromaticity.
- Optical transmission.
- Mechanical alignment.
- Foundation stability.
- Survey geometry.
- Operational glide-path verification.
The investigation identified combined photometric and alignment defects affecting multiple PAPI units, resulting in degraded visual guidance and failure to meet ICAO Annex 14 performance requirements.
This case demonstrates that PAPI performance must be evaluated as a complete integrated visual guidance system rather than as four independent light units.
2Learning Objectives
After completing this case study, the learner should be able to:
- Conduct a complete PAPI investigation.
- Evaluate photometric performance.
- Measure beam-angle accuracy.
- Assess installation geometry.
- Investigate unit intensity imbalance.
- Verify survey alignment.
- Restore ICAO-compliant PAPI performance.
3System Background
A PAPI system requires simultaneous compliance of:
- Beam angles.
- Unit intensities.
- Chromaticity.
- Optical transmission.
- Mechanical alignment.
- Foundation elevation.
- Runway reference geometry.
- Transition sharpness.
- Unit spacing.
Failure of any one parameter can degrade operational guidance.
4Problem Statement
The reported condition was:
- All four units illuminated.
- Uneven brightness.
- Inconsistent red/white transitions.
- Pilots reporting unstable glide-path indications.
- Recent maintenance completed.
- Runway rehabilitation completed six months earlier.
What combination of photometric and alignment defects is affecting the PAPI system?
5Initial Observations
| Unit operation | Normal |
| Electrical supply | Healthy |
| Brightness | Uneven |
| Transition quality | Poor |
| Pilot reports | Consistent |
| Recent maintenance | Yes |
The problem appeared multifactorial.
6Investigation Strategy
The investigation was divided into five areas:
- Electrical verification.
- Photometric investigation.
- Beam-angle calibration.
- Survey alignment verification.
- Operational validation.
7Test Equipment
The investigation used:
- PAPI photometer.
- Beam-angle measurement system.
- Total station.
- Digital level.
- GNSS survey equipment.
- Optical alignment telescope.
- Chromaticity meter.
- ICAO Annex 14 reference data.
8Electrical Verification
Measurements showed:
| Supply voltage | Normal |
| Control system | Normal |
| Driver outputs | Normal |
| Unit operation | Normal |
No electrical fault was identified.
9Photometric Investigation
Measured peak intensities:
| Unit | Measured Intensity |
| 1 | 25,000 cd |
| 2 | 24,600 cd |
| 3 | 24,800 cd |
| 4 | 17,200 cd |
Unit 4 was approximately 31% below the other units.
Optical inspection revealed:
- Internal lens haze.
- Reflector contamination.
- Reduced optical transmission.
10Beam-Angle Calibration
Measured beam angles:
| Unit | Measured | Required |
| 1 | Correct | Correct |
| 2 | Correct | Correct |
| 3 | 3.26° | 3.17° |
| 4 | Correct | Correct |
Unit 3 had significant beam-angle drift.
11Survey Alignment Verification
Measured installation geometry:
| Parameter | Required | Measured |
| Lateral offset | 15.0 m | 15.8 m |
| Longitudinal position | Correct | Correct |
| Foundation elevation | Correct | +96 mm |
The installation had shifted laterally and vertically.
12Foundation Inspection
Inspection revealed:
- Differential settlement beneath Unit 3.
- Base movement.
- Adjustment mechanism stress.
- Mounting instability.
Foundation movement had altered beam geometry.
13Operational Simulation
Simulation showed:
- 3W/1R indication on the published glide path.
- Reduced transition sharpness.
- Uneven brightness.
- Reduced night conspicuity.
The simulation matched pilot reports.
14Root Cause
The PAPI investigation identified multiple interacting defects:
Unit 3
- Beam-angle misalignment.
- Foundation settlement.
- Mechanical displacement.
Unit 4
- Optical contamination.
- Reduced transmission.
- Intensity degradation.
System
- Lateral alignment error.
- Reduced photometric balance.
The electrical system remained healthy.
15Corrective Action
The engineering team:
- Re-established survey control points.
- Corrected installation geometry.
- Re-leveled Unit 3.
- Recalibrated beam angles.
- Replaced Unit 4 optical assembly.
- Restored intensity balance.
- Verified chromaticity.
- Conducted photometric testing.
- Performed operational glide-path validation.
- Updated calibration documentation.
16Post-Repair Verification
Measured intensities:
| Unit | After Repair |
| 1 | 24,900 cd |
| 2 | 24,700 cd |
| 3 | 24,800 cd |
| 4 | 24,600 cd |
Intensity variation:
Less than 2%
Beam angles:
All units within tolerance
Operational verification:
2 White / 2 Red
Glide-path guidance restored.
17Compliance Verification
The PAPI system was restored to ICAO-compliant performance.
18Root-Cause Analysis
Pilot reports received
Healthy
Unit 4 low
Unit 3 incorrect
Alignment error found
Settlement confirmed
Photometry and alignment retested
Successful
ICAO compliance restored
19Why This Case Was Important
Combined photometric and alignment defects can cause:
- Incorrect glide-path guidance.
- Poor transition visibility.
- Uneven visual presentation.
- Increased pilot workload.
- ICAO non-compliance.
- Reduced operational confidence.
PAPI performance requires both optical and geometric integrity.
20Engineering Lessons
PAPI investigation must include photometry and alignment.
PAPI investigation must include photometry and alignment.
Beam-angle and intensity problems can occur simultaneously.
Beam-angle and intensity problems can occur simultaneously.
Foundation movement can alter calibration.
Foundation movement can alter calibration.
Optical degradation can affect visual guidance without electrical failure.
Optical degradation can affect visual guidance without electrical failure.
Operational validation is essential after corrective action.
Operational validation is essential after corrective action.
21Engineer's Decision Point
You observe: All PAPI units ON. Uneven brightness. Incorrect transition sequence. Recent maintenance and construction activity. What should you investigate first?
22Knowledge Check
23Interview Questions
24Frequently Asked Questions
Quick answers about integrated PAPI photometric and alignment investigation.
Q1What is AGL-CS-080?
AGL-CS-080 is an engineering training case study. It covers a PAPI with several interacting photometric and alignment defects across multiple units, investigated as one integrated system. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Why should a PAPI be evaluated as a system and not four separate lights?
The pilot sees one combined pattern. In AGL-CS-080, defects in different units and in the overall geometry combined to degrade the whole indication.
Q3What defects were found in AGL-CS-080?
Unit 3 had beam-angle drift (3.26° against 3.17°) from foundation settlement. Unit 4 was about 31% low in intensity (17,200 cd against roughly 25,000 cd) from optical contamination. The installation also had a lateral offset of 15.8 m against 15.0 m and a +96 mm elevation error.
Q4Was the electrical system involved?
No. Supply voltage, control system, driver outputs and unit operation were all normal.
Q5What did operational simulation show?
A 3W/1R indication on the published glide path, reduced transition sharpness, uneven brightness and reduced night conspicuity, which matched the pilot reports.
Q6What did the foundation inspection find?
Differential settlement beneath Unit 3, base movement, adjustment mechanism stress and mounting instability. The foundation movement had altered beam geometry.
Q7What optical problem affected Unit 4?
Internal lens haze, reflector contamination and reduced optical transmission, which caused the intensity degradation and unbalanced the units.
Q8How was the PAPI corrected?
The team re-established survey control points, corrected installation geometry, re-leveled Unit 3 and recalibrated beam angles. They replaced Unit 4's optical assembly, restored intensity balance and verified chromaticity, performed photometric testing and operational glide-path validation, then updated calibration documentation.
Q9What was verified before returning the PAPI to service?
Beam angles, intensity balance, chromaticity, transition sharpness, survey alignment and operational guidance all passed, restoring ICAO-compliant performance.
Engineering & Expert Notes
Expect More Than One Defect in a System Check
ApproachWhen one problem is found, keep looking. Misalignment and contamination often coexist.
Verify Alignment and Photometry as Separate Checks
MethodDo not let a good result in one hide a problem in the other.
Look at the Foundation as Well as the Unit
FoundationSettlement alters calibration. Include levels in the check.
Record Each Finding and Its Fix
RecordsA full list helps justify planned improvements.
25Case Study Conclusion
PAPI performance is an integrated photometric and geometric system, and correct visual glide-path guidance requires simultaneous compliance of beam-angle calibration, unit intensity balance, optical transmission, survey alignment, foundation stability, and operational validation, because a PAPI can be electrically healthy while still providing incorrect approach guidance if either the photometric or alignment components are degraded.
Systematic photometric measurement, beam-angle verification, survey alignment inspection, foundation assessment, intensity balancing, optical restoration, and operational glide-path validation provide a complete engineering methodology for restoring ICAO-compliant PAPI performance.
In AGL engineering, a PAPI is not merely four lights beside a runway; it is a precision optical surveying system whose purpose is to place every pilot on the correct glide path with confidence, consistency, and compliance.
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