AGL-CS-048 — Touchdown Zone Light Failure
The AGL Knowledge Hub | Engineering Case Study Series. A precision-approach lighting investigation where one touchdown zone inset light failed while the rest of the circuit remained operational — from CCR verification through transformer secondary testing, fixture inspection, water-ingress assessment, and drainage 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 pre-operational CAT II/III inspection, one touchdown zone inset light was found completely extinguished, interrupting the paired touchdown zone lighting pattern.
The Constant Current Regulator (CCR) was operating normally, and the adjacent touchdown zone light pairs remained fully operational.
Because touchdown zone lights are installed as paired white inset fittings, the investigation focused on the individual transformer, secondary circuit, connector assembly, and inset touchdown zone fixture.
A systematic investigation identified water ingress into the fixture base-can assembly, causing corrosion of the secondary connector and failure of the transformer secondary connection.
This case demonstrates that touchdown zone light failures are commonly localized inset-fixture or secondary-circuit faults, with moisture ingress and sealing deterioration being major reliability factors in precision-approach lighting systems.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose touchdown zone light failures.
- Understand TDZ lighting system configuration.
- Investigate paired inset-light failures.
- Test transformer secondary circuits.
- Inspect connectors, base cans, and sealing systems.
- Assess water-ingress-related failures.
- Restore touchdown zone lighting safely.
3System Background
A touchdown zone light is supplied through:
Touchdown zone lights provide:
- White touchdown-zone reference.
- Landing-area definition.
- Flare guidance.
- Landing-distance perception.
- CAT II/III visual guidance.
Common failure causes include:
- LED module failure.
- Water ingress.
- Connector corrosion.
- Transformer secondary failure.
- Secondary cable damage.
- Base-can drainage failure.
- Fixture sealing deterioration.
4Problem Statement
The reported condition was:
- One TDZ light OFF.
- Adjacent TDZ light pairs operational.
- CCR healthy.
- No circuit alarms.
- CAT II/III inspection in progress.
Has the inset touchdown zone fixture failed, or has the secondary power supply to the fixture been interrupted?
5Initial Observations
| CCR status | Healthy |
| Selected current | 6.6 A |
| Actual current | 6.6 A |
| Output voltage | Normal |
| Adjacent TDZ lights | Operational |
| Failed TDZ fixture | Completely OFF |
The healthy CCR and adjacent TDZ light pairs indicated a localized field fault.
6Initial Engineering Hypotheses
The maintenance team considered:
LED module failure.
Inset fixture water ingress.
Transformer secondary failure.
Connector corrosion.
Base-can drainage failure.
7TDZ Diagnostic Principle
If adjacent TDZ light pairs remain operational, the fault is usually localized to the transformer secondary circuit or the inset fixture itself.
8Test Equipment
The investigation used:
- True-RMS multimeter.
- Secondary voltage test equipment.
- Continuity tester.
- Low-resistance ohmmeter.
- Insulation resistance tester.
- Thermal imaging camera.
- Moisture inspection equipment.
- Approved AGL circuit test equipment.
- Airport-approved PPE.
9Investigation
Step 1: Verify CCR Operation
Results:
| Current regulation | Normal |
| Voltage regulation | Normal |
| CCR alarms | None |
The CCR was healthy.
Step 2: Verify Transformer Primary
The associated transformer showed:
- Primary current present.
- Normal primary continuity.
- Normal operating temperature.
Primary power was available.
Step 3: Measure Transformer Secondary Voltage
Results:
| Primary current | Present |
| Secondary voltage | Absent |
The secondary circuit was interrupted.
Step 4: Secondary Continuity Testing
Testing showed:
- Open continuity.
- Elevated connector resistance.
- Moisture-related resistance instability.
The fault was localized near the fixture connection.
Step 5: Fixture Removal and Inspection
The inset TDZ fixture was removed.
Inspection revealed:
- Water accumulation.
- Corroded connector assembly.
- Oxidized terminals.
- Deteriorated sealing gasket.
- Contaminated electrical compartment.
The fixture had experienced progressive water-ingress deterioration.
Step 6: Base-Can Drainage Inspection
The base can showed:
- Blocked drainage holes.
- Standing water.
- Sediment accumulation.
- Poor water evacuation.
The drainage failure had increased long-term moisture exposure.
10Root Cause
The touchdown zone light failed because water ingress entered the inset fixture and base-can assembly through a deteriorated sealing gasket and blocked drainage system, causing corrosion of the secondary connector and interruption of the transformer secondary connection.
The primary circuit and transformer remained healthy.
The fault was localized to the inset fixture secondary connection.
11Corrective Action
The maintenance team:
- Isolated the circuit.
- Removed the affected inset fixture.
- Replaced the damaged connector assembly.
- Installed a new approved sealing gasket.
- Cleaned and dried the base can.
- Restored drainage.
- Verified secondary continuity.
- Tested photometric performance.
- Reinstalled the fixture.
- Updated maintenance records.
12Post-Repair Verification
The touchdown zone light returned to full operation.
13Root-Cause Analysis
TDZ light OFF
Healthy
Healthy
No output
Open
Water ingress found
Water-ingress-induced secondary connector failure
Replace connector and restore sealing and drainage
14Why This Case Was Important
Touchdown zone lights are critical for:
- CAT II operations.
- CAT III operations.
- Precision landing guidance.
- Touchdown-zone identification.
- Landing-distance perception.
Inset-fixture reliability directly affects precision-approach operational capability.
15Engineering Lessons
TDZ light failures are usually localized inset-fixture or secondary-circuit faults.
TDZ light failures are usually localized inset-fixture or secondary-circuit faults.
Water ingress is a major failure mechanism in TDZ systems.
Water ingress is a major failure mechanism in TDZ systems.
Base-can drainage is part of electrical reliability.
Base-can drainage is part of electrical reliability.
Secondary voltage testing is essential before fixture replacement.
Secondary voltage testing is essential before fixture replacement.
Connector corrosion often follows prolonged moisture exposure.
Connector corrosion often follows prolonged moisture exposure.
16Engineer's Decision Point
You observe: One touchdown zone light OFF. Adjacent TDZ light pairs ON. CCR healthy. No circuit alarms. What should you investigate first?
17Knowledge Check
18Interview Questions
Quick Reference: ICAO Touchdown Zone Lights & Intensity
ICAO touchdown zone lights are provided in the touchdown zone of precision approach runways, particularly for the lower-visibility categories. They are white, arranged in pairs of bars on each side of the centreline. Touchdown zone light intensity is checked as part of photometric testing, and a failed light in this zone should be treated as an operational defect for low-visibility operations.
19Frequently Asked Questions
Quick answers about touchdown zone light failure, based on this case study.
Q1What is AGL-CS-048?
AGL-CS-048 is an engineering training case study on a single touchdown zone (TDZ) inset light that was OFF during a pre-operational CAT II/III inspection. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Why is a single TDZ failure notable?
TDZ lights are installed as paired white inset fittings, so one extinguished light interrupts the paired lighting pattern. Adjacent TDZ pairs and the CCR were normal, which pointed to a localized field fault.
Q3What was the root cause?
Water entered the inset fixture and base-can assembly through a deteriorated sealing gasket and blocked drainage, corroding the secondary connector and interrupting the transformer secondary connection. The primary circuit and transformer stayed healthy.
Q4What should you investigate first?
Verify transformer secondary voltage and inspect the inset fixture, connector assembly, sealing system and base-can drainage. Replacing the CCR or all TDZ lights, or increasing the current setting, is not the right first step.
Q5Which test equipment was used?
The team used a true-RMS multimeter, secondary voltage test equipment, continuity tester, low-resistance ohmmeter, insulation resistance tester, thermal imaging camera and moisture inspection equipment.
Q6How was the light repaired?
The team isolated the circuit, removed the affected inset fixture and replaced the damaged connector assembly. They installed a new approved sealing gasket, cleaned and dried the base can, restored drainage, verified secondary continuity, tested photometric performance and reinstalled the fixture.
Q7How was the repair verified?
Transformer secondary and TDZ fixture operation were restored, the paired-light pattern was complete and drainage was restored.
Q8Why check drainage as well as the connector?
Blocked drainage and a deteriorated gasket together allowed the water in. Restoring drainage was part of the correction, not only replacing the corroded connector.
Engineering & Expert Notes
Check the Base-Can Drain Whenever You Open a Fitting
DrainageBlocked drains are a common cause of repeated moisture problems.
Replace the Gasket With the Correct Part
SealingUse the approved gasket and check the sealing surface for damage.
Test the Secondary First
MethodConfirm the fault location before changing the fixture.
Torque Fixings to Specification
InstallationUnder- or over-tightening affects sealing. Follow the manufacturer's values.
20Case Study Conclusion
A touchdown zone light failure is usually a localized inset-fixture or secondary-circuit fault, with water ingress, connector corrosion, and base-can drainage failure being among the most significant reliability factors affecting precision-approach touchdown zone lighting systems.
Systematic CCR verification, transformer secondary testing, fixture inspection, connector assessment, sealing evaluation, drainage inspection, and photometric verification can rapidly identify the failed component and restore touchdown zone lighting integrity.
In AGL engineering, a failed touchdown zone light often begins with moisture beneath the pavement long before it becomes visible on the runway surface.
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