AGL-CS-066 — PAPI Photometry Failure
The AGL Knowledge Hub | Engineering Case Study Series. A PAPI photometric investigation where optical degradation and beam‑angle misalignment caused unstable glide‑path indication — from electrical verification through photometric measurement, beam‑angle calibration, and 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 PAPI calibration and photometric inspection, pilots reported that the PAPI indication appeared inconsistent during final approach, with unstable red/white transitions and reduced brightness in wet‑weather conditions.
The PAPI electrical system was fully operational, all four units were illuminated, and no power‑supply abnormalities were detected.
Because the PAPI units were operating electrically but the glide‑path indication was unreliable, a detailed photometric investigation was initiated covering intensity, beam angle, transition sharpness, chromaticity, optical contamination, and alignment accuracy.
The investigation identified optical degradation combined with beam‑angle misalignment and lens contamination, resulting in failure to meet ICAO Annex 14 PAPI photometric and angular transition requirements.
This case demonstrates that PAPI systems can remain electrically operational while providing incorrect visual glide‑path information to approaching aircraft.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose PAPI photometric failures.
- Differentiate electrical and optical defects.
- Evaluate PAPI intensity and beam performance.
- Verify red/white transition accuracy.
- Assess beam‑angle calibration.
- Investigate optical degradation.
- Restore ICAO‑compliant PAPI performance.
3System Background
A PAPI system consists of:
- Four precision optical units.
- High‑intensity light source.
- Optical lens assembly.
- Red filter system.
- White optical path.
- Precision beam‑angle adjustment mechanism.
- Power supply and control system.
PAPI performance depends on:
- Light intensity.
- Beam angle.
- Transition sharpness.
- Red/white chromaticity.
- Optical cleanliness.
- Lens transmission.
- Mechanical alignment.
- Unit‑to‑unit consistency.
Common photometric failure causes include:
- Lens contamination.
- Filter degradation.
- Optical aging.
- Mechanical misalignment.
- Foundation settlement.
- LED/lamp lumen depreciation.
- Incorrect calibration.
- Environmental contamination.
4Problem Statement
The reported condition was:
- All PAPI units illuminated.
- Unstable red/white transition.
- Reduced brightness.
- Poor glide‑path consistency.
- CCR/power system healthy.
- Photometric inspection failed.
Are the PAPI units electrically healthy but photometrically and angularly non‑compliant with ICAO Annex 14 requirements?
5Initial Observations
| Electrical operation | Normal |
| PAPI units | ON |
| Red/white transition | Inconsistent |
| Brightness | Reduced |
| Glide‑path indication | Unstable |
| Beam appearance | Non‑uniform |
The PAPI units were electrically operational, but their optical and angular performance was degraded.
6Initial Engineering Hypotheses
The maintenance team considered:
Power‑supply fluctuation.
Light‑source degradation.
Lens contamination.
Beam‑angle misalignment.
PAPI calibration drift.
7PAPI Photometric Failure Principle
PAPI performance depends on both photometry and precision angular alignment.
8Test Equipment
The investigation used:
- PAPI photometric measurement equipment.
- Beam‑angle measurement instrument.
- Digital inclinometer.
- Optical alignment telescope.
- Chromaticity meter.
- Optical inspection tools.
- Lux meter.
- ICAO Annex 14 PAPI reference data.
- Airport‑approved PPE.
9Investigation
Step 1: Verify Electrical Operation
Results:
| Power supply | Normal |
| Current supply | Normal |
| Unit operation | Normal |
| Control system | Normal |
No electrical fault was identified.
Step 2: Photometric Measurement
Measured intensity:
14,000 cd
Required intensity:
≥ 24,000 cd (representative training value)
The unit failed the photometric requirement.
Step 3: Beam‑Angle Verification
Measured beam angle:
3.18°
Required beam angle:
3.00°
Angular error:
+0.18°
The beam angle exceeded acceptable tolerance.
Step 4: Transition Sharpness Measurement
Testing showed:
- Broad transition zone.
- Reduced red/white discrimination.
- Inconsistent transition between adjacent units.
The optical transition was degraded.
Step 5: Chromaticity Measurement
Measured chromaticity showed:
- Reduced red saturation.
- White color shift.
- Filter aging effects.
Color discrimination had deteriorated.
Step 6: Optical Inspection
Inspection revealed:
- Dust contamination.
- Insect deposits.
- Surface scratching.
- Lens hazing.
- Reduced optical transparency.
Optical transmission had deteriorated significantly.
Step 7: Mechanical Alignment Inspection
Inspection revealed:
- Minor foundation settlement.
- Unit tilt.
- Beam‑axis displacement.
- Inconsistent unit elevation.
Mechanical alignment had changed over time.
10Root Cause
The PAPI photometry failure was caused by combined optical degradation and beam‑angle misalignment, including:
- Lens contamination.
- Optical aging.
- Filter deterioration.
- Mechanical settlement.
- Calibration drift.
The electrical system remained healthy, but the PAPI units no longer produced ICAO‑compliant intensity, transition sharpness, chromaticity, and glide‑path indication.
11Corrective Action
The maintenance team:
- Cleaned the optical assemblies.
- Replaced degraded optical components.
- Replaced aged filters.
- Recalibrated beam angles.
- Re‑leveled the PAPI units.
- Verified transition sharpness.
- Performed photometric testing.
- Confirmed chromaticity compliance.
- Conducted operational flight‑path verification.
- Updated PAPI calibration records.
12Post‑Repair Verification
The PAPI system met ICAO photometric and angular requirements after recalibration.
13Root‑Cause Analysis
Inconsistent PAPI indication
Healthy
Below requirement
Out of tolerance
Contaminated and aged
Settlement identified
Optical degradation and beam‑angle misalignment
Restore optics and recalibrate beam angles
14Why This Case Was Important
PAPI photometric failures can cause:
- Incorrect glide‑path indication.
- Unstable visual approach guidance.
- Poor approach consistency.
- ICAO non‑compliance.
- Reduced pilot confidence.
- Increased operational risk.
PAPI calibration is a precision optical and geometric engineering activity.
15Engineering Lessons
An illuminated PAPI is not necessarily providing a correct glide‑path indication.
An illuminated PAPI is not necessarily providing a correct glide‑path indication.
Beam‑angle accuracy is as important as light intensity.
Beam‑angle accuracy is as important as light intensity.
Transition sharpness directly affects pilot interpretation.
Transition sharpness directly affects pilot interpretation.
Mechanical settlement can alter PAPI performance.
Mechanical settlement can alter PAPI performance.
PAPI systems require periodic photometric and angular calibration.
PAPI systems require periodic photometric and angular calibration.
16Engineer's Decision Point
You observe: All PAPI units ON. Pilots report unstable glide‑path indications. Electrical system healthy. Reduced brightness visible. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about PAPI photometry failure, beam angle and transition sharpness.
Q1What is AGL-CS-066?
AGL-CS-066 is an engineering training case study. It covers a PAPI system that was electrically healthy but gave an unreliable glide-path indication because of optical degradation and beam-angle misalignment. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Can a PAPI be electrically healthy but still give wrong glide-path information?
Yes. All four units were illuminated with no power abnormalities, but degraded optics and beam-angle error produced unstable red/white transitions and an unreliable indication.
Q3What did the measurements show in AGL-CS-066?
Intensity was 14,000 cd against a representative training requirement of at least 24,000 cd. The beam angle was 3.18° against 3.00°, an error of +0.18°, and the red/white transition was broad and inconsistent between units.
Q4Why does PAPI depend on both photometry and angular alignment?
A PAPI signals glide path through sharp red/white transitions at precise angles. Low intensity, filter aging or a beam-angle error blurs or shifts those transitions, even when the units are lit.
Q5What caused the failure in AGL-CS-066?
Combined optical degradation and beam-angle misalignment: lens contamination, optical aging, filter deterioration, mechanical settlement and calibration drift. The electrical system stayed healthy.
Q6How did foundation settlement contribute?
Minor foundation settlement caused unit tilt, beam-axis displacement and inconsistent unit elevation, so mechanical alignment had changed over time.
Q7What was found during optical inspection?
Dust contamination, insect deposits, surface scratching and lens hazing, plus reduced red saturation and a white colour shift from filter aging.
Q8How was the PAPI restored?
The team cleaned the optical assemblies, replaced degraded components and aged filters, recalibrated beam angles and re-leveled the units. They then verified transition sharpness, photometry and chromaticity, conducted operational flight-path verification and updated calibration records.
Q9What should you investigate first when PAPI indications are unstable but the electrical system is healthy?
Measure photometric intensity, verify beam-angle calibration, inspect the optical assemblies and check mechanical alignment. Replacing the power supply or all PAPI units would not address the cause.
Engineering & Expert Notes
Measure Angle and Intensity Together
MethodBoth must be correct for guidance to be right. Record each value.
Check the Sharpness of the Colour Transition
TransitionA blurred transition makes the indication harder to interpret.
Inspect Filters and Lenses for Ageing
OpticsDiscoloured or damaged filters affect colour and intensity.
Re-Check Alignment After Settlement or Impact
MountingFoundation movement can shift the beam. Verify after any disturbance.
20Case Study Conclusion
PAPI photometry failure is a combined optical and geometric alignment problem caused by lens contamination, optical aging, filter degradation, mechanical settlement, and beam‑angle calibration errors, and a PAPI system can be electrically operational while providing incorrect glide‑path information to approaching aircraft.
Systematic photometric measurement, beam‑angle verification, transition‑sharpness assessment, chromaticity testing, optical inspection, and mechanical recalibration can identify PAPI performance degradation and restore ICAO‑compliant visual approach guidance.
In AGL engineering, a PAPI system is not judged by whether all four lights are illuminated, but by whether every pilot receives the correct glide‑path indication at the correct angle, with the correct color transition, every time.
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