AGL-CS-046 — Runway Threshold Light Failure
The AGL Knowledge Hub | Engineering Case Study Series. A threshold-lighting investigation where one runway threshold light failed while the rest of the circuit remained operational — from CCR verification through transformer secondary testing, connector inspection, and secondary-circuit 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 night inspection, one runway threshold light was found completely extinguished, reducing the continuity of the green threshold lighting pattern.
The Constant Current Regulator (CCR) was operating normally, and all runway edge and adjacent threshold lights remained operational.
Because the failure affected a single threshold fitting, the investigation focused on the associated isolating transformer, secondary circuit, connector assembly, and threshold fixture.
A systematic investigation identified a failed transformer secondary connector caused by corrosion and moisture ingress within the threshold light pit, interrupting power to the individual threshold light.
This case demonstrates that runway threshold light failures are commonly localized secondary-side faults that can affect threshold lighting pattern integrity and operational visibility.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose single runway threshold light failures.
- Understand threshold lighting system configuration.
- Differentiate threshold lighting from runway edge lighting.
- Test transformer secondary circuits.
- Inspect connectors, secondary cables, and fixture terminals.
- Assess moisture-related connector deterioration.
- Restore threshold light operation safely.
3System Background
A runway threshold light is supplied through:
Threshold lights provide:
- Green light toward approaching aircraft.
- Threshold identification.
- Runway entry reference.
- Threshold-bar continuity.
Common failure causes include:
- LED module failure.
- Transformer secondary failure.
- Secondary cable damage.
- Connector corrosion.
- Water ingress.
- Fixture terminal deterioration.
- Mechanical damage.
4Problem Statement
The reported condition was:
- One threshold light OFF.
- Adjacent threshold lights ON.
- CCR healthy.
- No circuit alarms.
- Runway edge lights normal.
- Threshold pattern incomplete.
Has the threshold 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 threshold lights | Operational |
| Failed threshold light | Completely OFF |
The healthy CCR and adjacent threshold lights indicated a localized field fault.
6Initial Engineering Hypotheses
The maintenance team considered:
LED module failure.
Transformer secondary failure.
Secondary cable open circuit.
Connector corrosion.
Fixture terminal failure.
7Threshold Light Diagnostic Principle
If adjacent threshold lights remain operational, the fault is usually localized to the individual secondary circuit.
8Test Equipment
The investigation used:
- True-RMS multimeter.
- Secondary voltage test equipment.
- Continuity tester.
- Low-resistance ohmmeter.
- Insulation resistance tester.
- Thermal imaging camera.
- 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.
- Intermittent contact during connector movement.
The fault was localized near the transformer connection.
Step 5: Connector Inspection
Inspection revealed:
- Moisture contamination.
- Corroded contact surfaces.
- Oxidized terminals.
- Damaged sealing.
- Reduced contact pressure.
The connector had failed electrically.
Step 6: Fixture Verification
The threshold fixture showed:
- Normal internal continuity.
- No optical damage.
- No LED module failure.
The fixture itself was healthy.
10Root Cause
The runway threshold light failed because corrosion and moisture ingress in the transformer secondary connector interrupted power delivery to the individual threshold fixture.
The primary circuit, transformer primary, and threshold fixture remained healthy.
The fault was localized to the secondary connector assembly.
11Corrective Action
The maintenance team:
- Isolated the circuit.
- Removed the damaged connector.
- Cleaned the transformer secondary terminals.
- Installed a new approved connector.
- Restored environmental sealing.
- Verified secondary continuity.
- Measured secondary voltage.
- Tested threshold light operation.
- Conducted operational verification.
- Updated maintenance records.
12Post-Repair Verification
The runway threshold light returned to normal operation.
13Root-Cause Analysis
Threshold light OFF
Healthy
Healthy
No output
Open
Corrosion identified
Moisture-damaged secondary connector
Replace connector and restore sealing
14Why This Case Was Important
Threshold lights are operationally critical because they provide:
- Runway threshold identification.
- Green threshold reference.
- Approach guidance.
- Threshold-bar continuity.
A missing threshold light can reduce visual pattern integrity during night and low-visibility operations.
15Engineering Lessons
Single threshold light failures are usually localized secondary-circuit faults.
Single threshold light failures are usually localized secondary-circuit faults.
Threshold lighting pattern integrity is operationally important.
Threshold lighting pattern integrity is operationally important.
Connector corrosion is a common threshold-light failure mechanism.
Connector corrosion is a common threshold-light failure mechanism.
Secondary voltage testing is the most important localization step.
Secondary voltage testing is the most important localization step.
Moisture sealing is critical for threshold-pit reliability.
Moisture sealing is critical for threshold-pit reliability.
16Engineer's Decision Point
You observe: One threshold light OFF. Adjacent threshold lights ON. CCR healthy. No circuit alarms. What should you investigate first?
17Knowledge Check
18Interview Questions
Quick Reference: Runway Threshold Light Colour, Spacing & Intensity
Runway threshold lights mark the beginning of the usable landing area. Runway threshold light spacing is set by the standard for the runway type and category, so check Annex 14 or the project specification and do not copy it from a case study. Runway threshold light intensity is checked against the required photometric values at each brightness step, as part of periodic testing.
19Frequently Asked Questions
Quick answers about runway threshold light failure, based on this case study.
Q1What is AGL-CS-046?
AGL-CS-046 is an engineering training case study on a single runway threshold light that was OFF, breaking the green threshold pattern. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Was the threshold fixture or its secondary supply at fault?
The case starts with that question. The CCR was healthy at 6.6 A, adjacent threshold lights and runway edge lights were normal and there were no alarms, which indicated a localized field fault.
Q3What was the root cause?
Corrosion and moisture ingress in the transformer secondary connector, within the threshold light pit, interrupted power delivery to the individual fixture. The primary circuit, transformer primary and fixture stayed healthy.
Q4What should you investigate first?
Verify transformer secondary voltage and inspect the secondary connector and cable. Replacing the CCR or all threshold 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 and thermal imaging camera.
Q6How was the light repaired?
The team isolated the circuit, removed the damaged connector and cleaned the transformer secondary terminals. They installed a new approved connector, restored environmental sealing, verified secondary continuity, measured secondary voltage and tested threshold light operation.
Q7How was the repair verified?
Transformer secondary voltage and the threshold light were restored, the threshold pattern was complete and connector temperature was normal.
Q8Why does a single failed threshold light matter?
It leaves the threshold lighting pattern incomplete, so the case treats it as a fault to resolve even though the circuit and CCR are normal.
Q9What color are runway threshold lights?
Runway threshold lights are green. They show green to an aircraft approaching to land, and the lights at the end of the runway behind them show red in the opposite direction, so the threshold and runway end are clearly separated.
Engineering & Expert Notes
Threshold Patterns Need Prompt Restoration
PriorityGaps in the threshold pattern matter operationally. Treat the repair as a priority item.
Test Secondary Voltage at the Fixture
MethodThis identifies the connector as the cause quickly.
Re-Make the Connector With New Seals
RepairReplace the damaged parts and verify the seal.
Verify the Pattern After the Fix
VerificationCheck that all lights in the threshold group operate correctly.
20Case Study Conclusion
A single runway threshold light failure is usually a localized secondary-circuit fault involving the isolating transformer, secondary connector, cable, or fixture, while the CCR and primary circuit remain fully operational.
Systematic CCR verification, transformer secondary testing, connector inspection, secondary continuity testing, and moisture assessment can rapidly identify the failed component and restore threshold lighting integrity.
In AGL engineering, a missing threshold light is often a connector problem in the pit before it becomes a fixture problem on the runway.
Turn Case Studies Into Engineering Capability
A case study shows you how a problem is investigated. Precision AGL Academy helps you build the knowledge to understand why it happens, diagnose it systematically, and apply the right engineering approach.
Study real-world-style engineering scenarios, identify failure patterns, understand root causes, and develop a systematic approach to AGL troubleshooting.
Study the case. Understand the system. Build the skill.
Follow the Academy
Important Notice, Disclaimer & Limitation of Liability
Please read before relying on any content on this page or on any record downloaded from it.
- Purpose and intended users. This page is an independent educational and informational reference for competent, suitably trained and authorised airfield ground lighting (AGL) and aerodrome electrical personnel. It is not professional engineering, legal or regulatory advice, and using it does not create any professional or client relationship.
- Fictional training simulation. This case study is an engineering simulation created for training. It is not a report of an actual airport incident. Scenarios, sequences, dates, readings, measurements, outcomes and numerical values are illustrative and invented for teaching. No fault, failure, cause or liability is attributed to any person, company, airport, regulator, manufacturer or product, and any resemblance to a real event is coincidental. The content is not a forensic, legal, insurance, regulatory or accident-investigation finding and must not be cited as evidence, as a record of an event, or as a basis for disciplinary, contractual, insurance or legal action.
- Safety. Work on series circuits, constant current regulators (CCRs), isolating transformers, cables, connectors and control equipment can be fatal. Always apply the aerodrome’s permit-to-work and isolation/LOTO procedures, prove dead, use correctly rated instruments and PPE, and follow the manufacturer’s instructions. Nothing here authorises work on live equipment or the bypassing of any safety device or interlock. Where a case narrates an inspection, measurement or test, that narrative is not an instruction or an endorsement of the method.
- Values, test results and acceptance. Test voltages, insulation-resistance and current values, tolerances, thresholds, photometric and angular figures and pass/fail or “acceptable” wording in a case are specific to its invented scenario. They are not acceptance criteria, limits or targets. Use the project specification, the applicable standard and the manufacturer’s data for real work. Completing or following a case produces no test certificate, calibration record, acceptance or evidence of compliance.
- Aviation operations. Nothing here authorises return to service, a change of operating category (CAT I/II/III), a NOTAM, a flight inspection outcome or any aerodrome certification decision. These rest with the aerodrome operator, the air navigation service provider and the competent regulator.
- Standards and regulations. References to ICAO, FAA, IEC, IEEE, ISO and other standards are for orientation only. They are summarised, may be incomplete or out of date, and standards are revised from time to time. Consult the current official editions. Following this content does not guarantee compliance with any law, standard or regulatory approval. Official documents, regulations and manufacturers’ instructions always prevail.
- Learning aids. Knowledge checks, decision points, interview questions, FAQs and suggested answers are teaching aids with illustrative answers. They are not an assessment, certification, accreditation or competency or authorisation to work on any aerodrome, and they do not predict or guarantee the outcome of any job interview or application.
- Generic content; no warranty. Diagnostic sequences, cause rankings, tolerances and values are generic and indicative; real installations differ in design, equipment make and model, age and condition. Content is provided “as is” and “as available”, without warranty of any kind, express or implied, including accuracy, completeness, currency or fitness for a particular purpose.
- Limitation of liability. To the fullest extent permitted by applicable law, Precision AGL Academy and its owners, authors and contributors are not liable for any injury, death, loss, damage, downtime, penalty or regulatory consequence, whether direct or indirect, arising from the use of, reliance on, or inability to use this content. Nothing in this notice excludes or limits any liability that cannot lawfully be excluded or limited.
- Independence and trademarks. This is an independent publication, not affiliated with or endorsed by ICAO, the FAA, the IEC, DGCA, any airport, regulator or equipment manufacturer. Third-party names, standard designations and trademarks belong to their respective owners and are used for identification only.
- Copyright and use. © 2026 Precision AGL Academy. All rights reserved. You may read this page and use it for personal reference and internal training, keeping this notice attached. Any other copying, republication, redistribution, scraping or commercial use requires our prior written permission.
- Corrections. Content is reviewed periodically, with no obligation to update. To report an error, safety concern or rights issue, write to precisionaglsolutions@gmail.com.