AGL-CS-077 — PAPI Unit Intensity Imbalance
The AGL Knowledge Hub | Engineering Case Study Series. A PAPI photometric investigation where unequal unit intensity degraded transition quality — from pilot reports through individual photometry, optical transmission assessment, chromaticity verification, intensity balancing, and operational 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
Flight crews reported that the PAPI appeared uneven during final approach, with one unit noticeably dimmer than the others.
The reported glide-path indication was technically correct, but pilots stated that the red/white transition was difficult to interpret, particularly during night approaches and wet-runway conditions.
A ground inspection showed that all four PAPI units were operating, beam-angle calibration was within tolerance, and no electrical alarms were present.
A detailed investigation was initiated covering unit intensity, photometric balance, chromaticity, optical transmission, power-supply performance, and optical-system condition.
The investigation identified significant photometric intensity imbalance caused by optical degradation and reduced output from one PAPI unit, resulting in degraded visual transition quality.
This case demonstrates that PAPI units may be correctly aligned geometrically while still providing poor operational guidance due to unequal photometric intensity.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose PAPI intensity imbalance.
- Compare photometric output between units.
- Evaluate intensity uniformity.
- Assess optical degradation.
- Verify chromaticity consistency.
- Restore balanced PAPI photometric performance.
- Improve maintenance quality control.
3System Background
A PAPI system consists of four optical units.
Correct operation requires:
- Correct beam angles.
- Correct unit spacing.
- Correct elevation.
- Correct chromaticity.
- Correct transition sharpness.
- Balanced intensity between units.
Intensity imbalance may result from:
- LED aging.
- Optical contamination.
- Reflector degradation.
- Power-supply variation.
- Driver deterioration.
- Optical-window contamination.
- Internal moisture.
- Incorrect replacement components.
Visual interpretation depends on relative brightness between units.
4Problem Statement
The reported condition was:
- All four units ON.
- Correct glide-path indication.
- One unit visibly dim.
- Transition difficult to interpret.
- Electrical system healthy.
- Calibration within tolerance.
Can unequal intensity between PAPI units degrade visual approach guidance even when beam angles are correct?
5Initial Observations
| Unit operation | Normal |
| Beam-angle calibration | Correct |
| Pilot reports | Uneven brightness |
| Transition quality | Poor |
| Electrical alarms | None |
The problem was photometric rather than geometric.
6Initial Engineering Hypotheses
The investigation considered:
Beam-angle error.
Power-supply instability.
Optical contamination.
Unit intensity imbalance.
Chromaticity shift.
7Intensity Balance Principle
PAPI effectiveness depends on photometric balance.
8Test Equipment
The investigation used:
- PAPI photometric measurement equipment.
- Calibrated photometer.
- Beam-angle measurement system.
- Chromaticity meter.
- Optical transmission meter.
- Power-quality analyzer.
- ICAO Annex 14 PAPI reference data.
9Investigation
Step 1: Verify Beam-Angle Calibration
Measured beam angles:
| Unit | Measured |
| 1 | Correct |
| 2 | Correct |
| 3 | Correct |
| 4 | Correct |
Beam-angle calibration was acceptable.
Step 2: Measure Unit Intensities
Measured peak intensity:
| Unit | Measured |
| 1 | 24,800 cd |
| 2 | 25,100 cd |
| 3 | 16,200 cd |
| 4 | 24,900 cd |
Unit 3 was approximately 35% lower than the other units.
Step 3: Chromaticity Verification
Measured chromaticity showed:
- Unit 3 slightly shifted toward yellow.
- Reduced color saturation.
- Optical aging effects.
Color performance had begun to degrade.
Step 4: Optical Inspection
Inspection of Unit 3 revealed:
- Internal lens haze.
- Reflector contamination.
- Optical-window deposits.
- Reduced optical transmission.
The optical system was degraded.
Step 5: Power-Supply Verification
Measurements showed:
| Parameter | Result |
| Supply voltage | Normal |
| Driver output | Normal |
| Current regulation | Normal |
Electrical supply was not the cause.
Step 6: Optical Transmission Measurement
Transmission:
| Unit | Transmission |
| 1 | 91% |
| 2 | 92% |
| 3 | 66% |
| 4 | 91% |
Unit 3 had substantially reduced optical efficiency.
Step 7: Controlled Replacement Test
After replacing the optical assembly of Unit 3:
Measured intensity:
24,700 cd
Intensity balance was restored.
10Root Cause
The PAPI intensity imbalance was caused by optical degradation of Unit 3, including:
- Internal optical contamination.
- Lens haze.
- Reflector degradation.
- Reduced optical transmission.
Beam-angle calibration remained correct.
The primary failure mechanism was unequal photometric output between units.
11Corrective Action
The maintenance team:
- Replaced the optical assembly of Unit 3.
- Restored optical transmission.
- Replaced degraded reflector components.
- Verified beam angles.
- Performed photometric balancing.
- Verified chromaticity.
- Conducted operational observation.
- Updated maintenance procedures.
- Established intensity-balance inspections.
- Recorded post-repair photometric data.
12Post-Repair Verification
| Parameter | Before | After |
| Unit 3 intensity | 16,200 cd | 24,700 cd |
| Intensity imbalance | 35% | Less than 2% |
| Transition quality | Poor | Uniform |
| Operational guidance | Degraded | Restored |
Photometric balancing restored correct PAPI performance.
13Root-Cause Analysis
Uneven PAPI appearance
Correct
Unit 3 low
Degraded
Reduced
Intensity remeasured — balanced
PAPI unit intensity imbalance
Restore optical performance
14Why This Case Was Important
Intensity imbalance can cause:
- Poor transition visibility.
- Reduced glide-path confidence.
- Uneven visual appearance.
- Increased pilot workload.
- ICAO compliance concerns.
- Degraded night operations.
PAPI systems require both geometric and photometric balance.
15Engineering Lessons
Correct beam angles do not guarantee correct visual guidance.
Correct beam angles do not guarantee correct visual guidance.
PAPI units must be photometrically balanced.
PAPI units must be photometrically balanced.
Internal optical degradation can affect one unit independently.
Internal optical degradation can affect one unit independently.
Chromaticity should be checked with intensity.
Chromaticity should be checked with intensity.
Periodic intensity comparison between units is essential.
Periodic intensity comparison between units is essential.
16Engineer's Decision Point
You observe: All four PAPI units ON. Beam angles correct. One unit visibly dim. Pilots report poor transition visibility. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about PAPI unit intensity imbalance.
Q1What is AGL-CS-077?
AGL-CS-077 is an engineering training case study. It covers a PAPI with correct beam angles and a correct glide-path indication, but with one unit noticeably dimmer than the others. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Can unequal intensity degrade PAPI guidance if the beam angles are correct?
Yes. Uneven intensity gives an uneven visual pattern and a red/white transition that is harder to interpret, especially at night and on wet runways.
Q3What did the intensity measurements show in AGL-CS-077?
Units 1, 2 and 4 measured 24,800, 25,100 and 24,900 cd, while Unit 3 measured 16,200 cd, roughly 35% lower.
Q4Was the electrical supply or calibration at fault?
No. Supply voltage, driver output and current regulation were normal, and all four beam angles were correct.
Q5What was wrong with Unit 3's optics?
Internal lens haze, reflector contamination and optical-window deposits. Optical transmission was 66% against 91–92% on the other units, and chromaticity had shifted slightly toward yellow.
Q6What was the root cause?
Optical degradation of Unit 3: internal contamination, lens haze, reflector degradation and reduced transmission. The primary mechanism was unequal photometric output between units.
Q7How was the cause confirmed?
Replacing Unit 3's optical assembly restored intensity to 24,700 cd, in line with the other units.
Q8What corrective actions were taken?
The team replaced Unit 3's optical assembly and degraded reflector components, verified beam angles, performed photometric balancing, verified chromaticity and conducted operational observation. They also established intensity-balance inspections and recorded post-repair data.
Q9What is the key lesson from this case?
PAPI effectiveness depends on photometric balance as well as geometry. Measure each unit's intensity, don't rely on beam angle alone.
Engineering & Expert Notes
Compare Intensity Between Units, Not Just Against a Limit
BalanceVisual guidance depends on how the units compare with each other. Check balance across the bar.
Open the Unit to Inspect Internal Optics
InspectionExternal cleaning will not remove internal film. Inspect inside during maintenance.
Check Colour Along With Intensity
ColourContamination can alter colour. Measure both when diagnosing.
Retest After Cleaning or Replacement
VerificationConfirm the balance is restored and record the result.
20Case Study Conclusion
PAPI unit intensity imbalance is a photometric-performance problem caused by unequal optical output between units, and a PAPI system can have correct beam angles while still providing degraded visual glide-path guidance due to reduced intensity and optical transmission of one unit.
Systematic individual unit photometry, intensity comparison, optical transmission measurement, chromaticity verification, and photometric balancing can identify unequal PAPI output and restore compliant visual approach guidance.
In AGL engineering, a PAPI is not only a system of correct angles; it is also a system of balanced light, where every unit must contribute equally to the pilot's perception of the glide path.
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