AGL-CS-079 — PAPI Unit Failure
The AGL Knowledge Hub | Engineering Case Study Series. A PAPI fault investigation where complete failure of one unit degraded visual guidance — from electrical verification through driver testing, LED module assessment, component isolation, and operational 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 routine night operations, air traffic control reported that one PAPI unit was not illuminated, and several pilots reported that the PAPI indication appeared incomplete and difficult to interpret during final approach.
The remaining three units were operating normally, beam angles were previously calibrated, and no major power-system alarms were present.
Because the failure affected one of the four PAPI units, a detailed investigation was initiated covering power supply, LED module condition, unit control circuitry, optical condition, cable continuity, and isolation of the failed unit.
The investigation identified failure of the LED driver module within Unit 2, resulting in complete loss of optical output from that unit.
This case demonstrates that a single PAPI unit failure can significantly degrade visual approach guidance and requires immediate technical investigation and operational assessment.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose complete failure of a PAPI unit.
- Differentiate electrical and optical failures.
- Verify unit power supply.
- Test LED driver circuits.
- Assess operational impact.
- Restore full PAPI functionality.
- Implement post-repair verification.
3System Background
A PAPI system consists of:
- Four optical units.
- Individual light sources.
- LED driver or lamp control modules.
- Optical assemblies.
- Beam-angle adjustment mechanisms.
- Power and control circuits.
- Cable and junction systems.
Complete unit failure may result from:
- LED driver failure.
- LED module failure.
- Power-supply failure.
- Cable damage.
- Junction-box fault.
- Connector failure.
- Water ingress.
- Internal electronic failure.
PAPI reliability depends on all four units operating correctly.
4Problem Statement
The reported condition was:
- Unit 2 OFF.
- Units 1, 3, and 4 ON.
- Electrical system generally healthy.
- Pilots reporting incomplete indication.
- Night visibility affected.
Has one PAPI unit suffered a complete electrical failure, and what is the operational impact on glide-path guidance?
5Initial Observations
| Unit 1 | ON |
| Unit 2 | OFF |
| Unit 3 | ON |
| Unit 4 | ON |
| Beam angles | Previously calibrated |
| Pilot reports | Incomplete indication |
The failure was isolated to a single unit.
6Initial Engineering Hypotheses
The investigation considered:
Power-supply failure.
Cable fault.
LED driver failure.
Optical blockage.
Control-circuit failure.
7PAPI Unit Failure Principle
Each PAPI unit contributes to the complete glide-path indication.
8Test Equipment
The investigation used:
- Digital multimeter.
- Insulation resistance tester.
- LED driver test equipment.
- Power-quality analyzer.
- Optical inspection tools.
- Beam-angle verification equipment.
- ICAO Annex 14 PAPI reference data.
9Investigation
Step 1: Verify Power Supply
Measurements:
| Parameter | Result |
| Incoming supply voltage | Normal |
| Control voltage | Normal |
| Distribution panel | Normal |
The main power system was healthy.
Step 2: Check Unit Input Voltage
Measured voltage at Unit 2:
230 V AC present
Power was reaching the failed unit.
Step 3: LED Driver Output Test
Measured driver output:
0 V DC
Expected output:
48 V DC
The LED driver was not operating.
Step 4: Cable Continuity Test
Results:
| Test | Result |
| Phase continuity | Good |
| Neutral continuity | Good |
| Earth continuity | Good |
| Connector condition | Good |
The cable system was healthy.
Step 5: Internal Unit Inspection
Inspection revealed:
- Burned driver components.
- Capacitor failure.
- Thermal discoloration.
- No significant optical damage.
The failure originated in the driver electronics.
Step 6: LED Module Verification
Using a temporary test supply:
LED module illuminated normally.
The LED optical module was healthy.
Step 7: Replacement Test
After replacing the LED driver:
Unit 2 illuminated normally.
Intensity matched the remaining units.
Operational observation restored the correct PAPI appearance.
10Root Cause
The complete PAPI unit failure was caused by failure of the LED driver module, resulting from:
- Internal electronic component failure.
- Thermal stress.
- Driver circuit breakdown.
The power supply, cable system, and optical module remained healthy.
The primary failure mechanism was loss of driver output.
11Corrective Action
The maintenance team:
- Isolated the failed unit.
- Replaced the LED driver module.
- Verified supply voltage.
- Tested LED module operation.
- Verified beam angle.
- Measured photometric intensity.
- Confirmed chromaticity.
- Conducted operational observation.
- Updated maintenance records.
- Recommended periodic driver-health inspections.
12Post-Repair Verification
| Parameter | Before | After |
| Unit 2 operation | OFF | ON |
| Driver output | 0 V | 48 V |
| Intensity | 0 cd | Normal |
| PAPI appearance | Incomplete | Complete |
| Operational status | Degraded | Restored |
Replacement of the driver restored full PAPI operation.
13Root-Cause Analysis
One PAPI unit failed
Normal
Present
Absent
Driver damaged
Operational verification — unit restored
LED driver failure
Replace driver and verify performance
14Why This Case Was Important
A single PAPI unit failure can cause:
- Incomplete visual guidance.
- Reduced transition clarity.
- Increased pilot workload.
- Night-operation degradation.
- ICAO maintenance concerns.
- Potential operational restrictions.
PAPI faults require rapid diagnosis and restoration.
15Engineering Lessons
One failed PAPI unit is operationally significant.
One failed PAPI unit is operationally significant.
Input voltage and driver output must be tested separately.
Input voltage and driver output must be tested separately.
LED modules and driver modules should be diagnosed independently.
LED modules and driver modules should be diagnosed independently.
Electrical restoration must be followed by photometric verification.
Electrical restoration must be followed by photometric verification.
Operational observation is an important final verification step.
Operational observation is an important final verification step.
16Engineer's Decision Point
You observe: One PAPI unit OFF. Supply voltage present. Remaining units operating normally. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about PAPI unit failure and LED driver diagnosis.
Q1What is AGL-CS-079?
AGL-CS-079 is an engineering training case study. It covers a PAPI where one of four units was not illuminated, giving pilots an incomplete indication at night. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Why is a single failed PAPI unit a problem?
Each unit contributes to the complete glide-path indication. With one out, the red/white pattern is incomplete, transition quality drops and pilots may misinterpret the guidance.
Q3How was the fault located in AGL-CS-079?
The team worked from supply to unit: incoming supply, control voltage and distribution were normal, and 230 V AC was present at Unit 2. The LED driver output measured 0 V DC against an expected 48 V DC.
Q4Were the cables or LED module at fault?
No. Phase, neutral and earth continuity and connectors were good. With a temporary test supply the LED module lit normally, so the optical module was healthy.
Q5What did internal inspection find?
Burned driver components, capacitor failure and thermal discoloration, with no significant optical damage. The failure originated in the driver electronics.
Q6What was the root cause?
Failure of Unit 2's LED driver module from internal electronic component failure, thermal stress and driver circuit breakdown, causing complete loss of optical output.
Q7How was the unit restored?
After replacing the LED driver, Unit 2 lit normally with intensity matching the other units, and the correct PAPI appearance was restored in operational observation.
Q8What corrective actions were taken?
The team isolated the failed unit, replaced the driver, verified supply voltage and tested the LED module. They also verified beam angle, measured intensity, confirmed chromaticity, conducted operational observation and updated records, and recommended periodic driver-health inspections.
Q9What is the key lesson from this case?
Trace the fault systematically: supply, unit input, driver output, cabling and module. A dark unit needs immediate technical and operational assessment.
Engineering & Expert Notes
Test Driver Input and Output Separately
MethodInput present with no output points to the driver. Measure both before swapping parts.
Look for Thermal Stress on Electronics
EnvironmentHeat shortens driver life. Check ventilation, mounting and the environment.
Verify Photometry and Alignment After Replacement
VerificationA working light may still be out of calibration. Confirm intensity and angle.
Hold Driver Spares for Critical Units
SparesHaving drivers on hand shortens the time a PAPI unit is out.
20Case Study Conclusion
PAPI unit failure is a critical visual-guidance fault that can occur even when the overall power system remains healthy, and systematic testing of supply voltage, driver output, LED module operation, cable continuity, and post-repair photometric performance is essential for restoring safe and ICAO-compliant PAPI operation.
Systematic electrical diagnosis, component isolation, driver testing, optical verification, photometric measurement, and operational observation can identify complete PAPI unit failures and restore reliable glide-path guidance.
In AGL engineering, a PAPI is only as reliable as its weakest unit, because one dark light can change the way a pilot sees the approach.
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