AGL-CS-076 — PAPI Indication Problem Investigation
The AGL Knowledge Hub | Engineering Case Study Series. A PAPI investigation where incorrect glide‑path indications were caused by beam‑angle misalignment — from pilot report analysis through beam‑angle measurement, mechanical alignment verification, foundation assessment, and calibration 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
Flight crews reported that the PAPI indication was inconsistent during final approach, with aircraft receiving three white and one red indications while established on the published glide path.
The PAPI system appeared operational from the ground, all four units were illuminated, and no electrical alarms were present.
Because the reported indication suggested a glide‑path guidance error, a detailed investigation was initiated covering beam‑angle calibration, unit intensity, optical performance, mechanical alignment, foundation stability, and chromaticity.
The investigation identified beam‑angle misalignment of one PAPI unit caused by mechanical settlement and incorrect recalibration during maintenance, resulting in an incorrect red/white transition sequence.
This case demonstrates that a PAPI system may appear electrically healthy while providing incorrect visual glide‑path information to pilots.
2Learning Objectives
After completing this case study, the learner should be able to:
- Investigate abnormal PAPI indications.
- Analyze pilot‑reported red/white sequences.
- Verify beam‑angle calibration.
- Evaluate unit‑to‑unit alignment.
- Assess intensity balance.
- Identify mechanical settlement effects.
- Restore ICAO‑compliant glide‑path guidance.
3System Background
A PAPI system consists of:
- Four optical units.
- Precision beam‑angle adjustment mechanisms.
- Red/white optical transition system.
- Mechanical mounting assemblies.
- Foundations.
- Power and control systems.
Correct operation requires:
- Correct beam angle.
- Correct unit spacing.
- Correct elevation.
- Correct intensity.
- Correct chromaticity.
- Correct transition sharpness.
Common indication problems include:
- Beam‑angle drift.
- Foundation settlement.
- Incorrect recalibration.
- Optical contamination.
- Unequal intensity.
- Mechanical movement.
- Unit replacement errors.
PAPI performance depends on precise geometric alignment.
4Problem Statement
The reported condition was:
- All four PAPI units ON.
- Pilots reporting 3W/1R on nominal glide path.
- Night visibility acceptable.
- Electrical system healthy.
- Visual guidance inconsistent.
Which PAPI unit is producing the incorrect indication, and why has the glide‑path transition changed?
5Initial Observations
| PAPI operation | Normal |
| Electrical status | Healthy |
| Pilot reports | Abnormal |
| Red/white sequence | Incorrect |
| Apparent brightness | Normal |
The problem was operational rather than electrical.
6Initial Engineering Hypotheses
The investigation considered:
Power‑supply instability.
Lamp/LED failure.
Beam‑angle misalignment.
Optical contamination.
Foundation settlement.
7PAPI Indication Principle
PAPI accuracy depends on beam‑angle precision.
8Test Equipment
The investigation used:
- PAPI beam‑angle measuring equipment.
- Optical alignment telescope.
- Digital inclinometer.
- Laser leveling system.
- Chromaticity meter.
- Photometric measurement equipment.
- ICAO Annex 14 PAPI reference data.
9Investigation
Step 1: Verify Electrical Operation
Results:
| Power supply | Normal |
| Unit operation | Normal |
| Control system | Normal |
| Intensity control | Normal |
No electrical fault was identified.
Step 2: Operational Observation
Aircraft positioned on the published glide path consistently observed:
3 White / 1 Red
Expected indication:
2 White / 2 Red
The abnormal indication was repeatable.
Step 3: Beam‑Angle Measurement
Measured beam angles:
| Unit | Measured | Required |
| 1 | 2.83° | 2.83° |
| 2 | 2.99° | 3.00° |
| 3 | 3.27° | 3.17° |
| 4 | 3.34° | 3.33° |
Unit 3 was outside tolerance.
Step 4: Mechanical Alignment Inspection
Inspection revealed:
- Mounting movement.
- Foundation settlement.
- Adjustment lock not fully secured.
- Vertical displacement.
Unit 3 had shifted after maintenance.
Step 5: Optical Inspection
Inspection showed:
- Clean lenses.
- Normal optical transmission.
- Correct chromaticity.
- Acceptable intensity.
The optical system was healthy.
Step 6: Intensity Balance Verification
Measured intensity:
| Unit | Intensity |
| 1 | Normal |
| 2 | Normal |
| 3 | Normal |
| 4 | Normal |
Intensity imbalance was not the cause.
Step 7: Controlled Recalibration
Unit 3 beam angle corrected to:
3.17°
Operational verification after recalibration produced:
2 White / 2 Red
The correct glide‑path indication was restored.
10Root Cause
The incorrect PAPI indication was caused by beam‑angle misalignment of Unit 3, resulting from:
- Mechanical settlement.
- Incomplete adjustment locking.
- Incorrect maintenance recalibration.
The electrical and optical systems remained healthy.
The primary failure mechanism was geometric beam‑angle error.
11Corrective Action
The maintenance team:
- Re‑leveled Unit 3.
- Corrected the beam angle.
- Secured adjustment mechanisms.
- Verified foundation stability.
- Performed beam‑angle measurements.
- Verified transition sequence.
- Conducted photometric verification.
- Confirmed chromaticity.
- Performed operational glide‑path checks.
- Updated calibration records.
12Post-Repair Verification
| Parameter | Before | After |
| Beam angle | 3.27° | 3.17° |
| Glide‑path indication | 3W/1R | 2W/2R |
| Transition accuracy | Incorrect | Correct |
| ICAO compliance | Fail | Pass |
Beam‑angle correction restored compliant PAPI operation.
13Root-Cause Analysis
Incorrect pilot indication
Healthy
Unit 3 incorrect
Settlement found
Indication restored
PAPI beam‑angle misalignment
Mechanical realignment and recalibration
14Why This Case Was Important
PAPI indication problems can cause:
- Incorrect glide‑path guidance.
- Unstable approaches.
- Increased pilot workload.
- ICAO non‑compliance.
- Reduced operational confidence.
- Potential approach safety risks.
PAPI troubleshooting requires both optical and geometric investigation.
15Engineering Lessons
A PAPI system can be electrically healthy while operationally incorrect.
A PAPI system can be electrically healthy while operationally incorrect.
Beam‑angle measurement is the primary diagnostic test.
Beam‑angle measurement is the primary diagnostic test.
Mechanical settlement can alter glide‑path indication.
Mechanical settlement can alter glide‑path indication.
Unit‑to‑unit calibration consistency is essential.
Unit‑to‑unit calibration consistency is essential.
Pilot reports are valuable diagnostic information.
Pilot reports are valuable diagnostic information.
16Engineer's Decision Point
You observe: All PAPI units ON. Pilots consistently report 3W/1R on the published glide path. Electrical system healthy. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about PAPI indication problems and beam-angle misalignment.
Q1What is AGL-CS-076?
AGL-CS-076 is an engineering training case study. It covers a PAPI where aircraft on the published glide path saw three white and one red instead of two white and two red. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2What should a PAPI show on the nominal glide path?
Two white and two red. In AGL-CS-076 crews consistently saw three white and one red on the published glide path, so the indication was wrong and repeatable.
Q3Can a PAPI look healthy but give wrong glide-path information?
Yes. All four units were on with no electrical alarms, and intensity, optics and chromaticity were normal, but one unit's beam angle was wrong.
Q4Which unit was at fault and by how much?
Unit 3. It measured 3.27° against a required 3.17°, outside tolerance. The other units were within tolerance.
Q5What caused the beam-angle error?
Mechanical settlement, an adjustment lock that was not fully secured, and incorrect recalibration during maintenance. Unit 3 had shifted after maintenance.
Q6Was intensity or optical condition involved?
No. Lenses were clean, transmission and chromaticity were normal, and intensity was balanced across the four units, so the primary failure was geometric beam-angle error.
Q7How was the correct indication restored?
Unit 3's beam angle was corrected to 3.17°, and operational verification then produced two white and two red.
Q8What corrective actions were taken?
The team re-leveled Unit 3, secured adjustment mechanisms, verified foundation stability, measured beam angles and checked the transition sequence. They also confirmed photometrics and chromaticity, ran operational glide-path checks and updated calibration records.
Q9What is the key lesson from this case?
PAPI accuracy depends on beam-angle precision. When the indication is wrong, measure each unit's beam angle and inspect its mounting rather than assuming an electrical fault.
Engineering & Expert Notes
Measure Each Unit's Angle Against Its Own Design Value
MethodCompare the readings for all units with the design table. One unit out of line shows up clearly this way.
Check the Adjustment Locking After Every Adjustment
MechanicalAn unlocked mechanism can drift. Confirm locks are tight and marked.
Check Foundation Level and Settlement
FoundationMovement of the base alters the angle. Compare with commissioning levels.
Record Angles Before and After Any Maintenance
RecordsThe before-and-after pair shows what was changed and supports later checks.
20Case Study Conclusion
PAPI indication problems are often geometric calibration failures caused by beam‑angle misalignment, mechanical settlement, and maintenance‑related adjustment errors, and a PAPI system can be electrically and optically healthy while providing incorrect glide‑path information to pilots.
Systematic beam‑angle measurement, mechanical alignment verification, foundation assessment, transition‑sequence analysis, photometric verification, and operational glide‑path validation can identify abnormal PAPI indications and restore compliant visual approach guidance.
In AGL engineering, the pilot does not judge a PAPI by whether four lights are illuminated; the pilot judges it by whether those four lights provide the correct glide‑path indication at exactly the right angle.
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