AGL-CS-001 — Runway Edge Light Failure Due to Open Series Circuit
The AGL Knowledge Hub | Engineering Case Study Series. A systematic fault-finding walkthrough for a multi-fixture outage on a runway edge series circuit — from initial CCR check through to root cause and post-repair verification.
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 night operational inspection, several consecutive runway edge lights were found not illuminated.
The Constant Current Regulator (CCR) indicated that the lighting circuit was energized. The maintenance team initially suspected multiple defective light fittings.
Instead of replacing the fittings individually, the team followed a systematic fault-finding procedure beginning at the CCR and progressing toward the field fixtures.
The investigation ultimately identified an open circuit in the series lighting circuit.
2Learning Objectives
After completing this case study, the learner should be able to:
- Understand the basic AGL series-circuit arrangement.
- Interpret a pattern of multiple failed lights.
- Differentiate between a fixture failure and a circuit fault.
- Develop a systematic AGL troubleshooting sequence.
- Understand the role of the CCR during fault diagnosis.
- Use electrical measurements to narrow down the fault location.
- Identify the importance of post-repair verification.
3System Background
A typical AGL series circuit consists of a constant-current source feeding a series-connected field circuit through airport lighting components such as isolating transformers and light fittings.
A simplified arrangement is:
The exact configuration depends on the airport design and applicable installation requirements.
The important engineering characteristic for this case is that an interruption in the series circuit can affect the operation of multiple downstream fittings.
4Problem Statement
During an operational inspection:
- Runway edge lights in one section were not illuminated.
- Several consecutive lights were affected.
- Lights before the affected section were operating.
- The CCR was showing the circuit as energized.
- No obvious physical damage was initially visible.
Is the problem caused by multiple failed light fittings, or is there a common circuit fault?
5Initial Observations
The pattern of failure is the first important diagnostic clue.
If several consecutive fittings are affected, the maintenance engineer should investigate whether a common upstream component or circuit interruption is responsible.
6Initial Engineering Hypotheses
The team develops the following possible causes:
Multiple individual light fittings failed simultaneously.
An isolating transformer or connector failed.
An underground cable or cable joint has an open circuit.
CCR output is abnormal.
A control-system indication does not represent the actual field condition.
The investigation must eliminate these possibilities systematically.
7Troubleshooting Methodology
The recommended diagnostic sequence is:
Do not immediately start replacing field lights.
Check CCR
Verify:
- Output current
- Output voltage
- Selected brightness level
- Alarm indications
- Local/remote status
- Circuit selection
- Protection status
If the CCR itself is abnormal, field troubleshooting may produce misleading results.
Identify the Failure Pattern
Record exactly which lights are affected. For example:
| L1 | L2 | L3 | L4 | L5 | L6 | L7 | L8 | L9 |
| ON | ON | ON | OFF | OFF | OFF | OFF | OFF | ON |
This pattern is much more useful than simply recording: "Five runway lights not working." The engineer now has a defined fault section to investigate.
8Test Equipment
Depending on the airport's approved maintenance procedure, the investigation may require:
- True-RMS multimeter
- Insulation resistance tester
- Approved AGL circuit test equipment
- Cable fault locator, where applicable
- Clamp meter where appropriate
- Circuit drawings
- AGL circuit identification records
- Manufacturer maintenance documentation
- Appropriate PPE and electrical safety equipment
Testing must always follow the airport's approved isolation, switching and safety procedures.
9Investigation
The team first verifies the CCR output. The CCR appears to be operating normally. The investigation therefore moves toward the field circuit.
10Failure Pattern Analysis
The observed pattern is:
This immediately raises an important question: Why are L4 to L8 affected while L9 is operating?
In a real installation, the exact interpretation depends on the actual circuit topology and test results. The engineer should therefore not assume an open circuit solely from the visual pattern. The circuit drawing and field configuration must be verified first.
11Circuit Investigation
The team checks:
Light Fittings
Visual inspection reveals no obvious mechanical damage.
Connectors
One connector in the affected section shows evidence of moisture contamination.
Isolating Transformer
The transformer is visually intact, but electrical testing is required.
Cable
Continuity testing identifies an interruption associated with the connection between two circuit components.
The investigation is narrowed to the affected section.
12Root Cause
An open electrical connection in the series circuit interrupted current flow through the affected section. The failure was associated with a defective/moisture-affected connection.
The multiple light failures were therefore not five independent light-fitting failures. They were symptoms of a common circuit problem.
13Corrective Action
The maintenance team:
- Followed the approved circuit isolation procedure.
- Secured the affected circuit.
- Identified the defective connection.
- Removed the defective component/connection.
- Installed an approved replacement.
- Checked the associated connections.
- Performed the required electrical tests.
- Restored the circuit.
- Verified operation of the affected lights.
- Recorded the maintenance activity.
14Post-Repair Verification
After restoration, where required by the applicable airport maintenance programme, additional electrical and photometric verification should be completed.
15Root-Cause Analysis
Multiple runway edge lights not illuminated.
Multiple light fittings failed.
CCR checked → normal.
Failure pattern analysed.
Affected circuit section inspected.
Connector/circuit connection investigated.
Open circuit caused by defective electrical connection.
Repair/replace affected connection and perform required verification.
16What Could Have Gone Wrong?
A less systematic maintenance approach could have resulted in:
- Replacing several healthy light fittings.
- Unnecessary transformer replacement.
- Increased maintenance cost.
- Longer runway maintenance downtime.
- Failure to identify the actual circuit fault.
- Repeated failure after temporary restoration.
This is why fault diagnosis should precede component replacement.
17Engineering Lessons
Failure pattern matters.
One failed fitting and several consecutive failed fittings should not automatically be investigated in the same way.
Start with the system, not the component.
The CCR, circuit and field configuration should be understood before replacing individual fittings.
Don't confuse symptoms with root cause.
The extinguished lights were symptoms. The electrical interruption was the root cause.
Rain and moisture can be important clues.
Recent rainfall does not prove that moisture caused the fault, but it should encourage inspection of vulnerable connections, joints and components.
Always verify after repair.
Restoring one connection is not the end of the job. The complete affected circuit must be checked according to the approved procedure.
18Engineer's Decision Point
You arrive at an airport and find: 12 consecutive runway edge lights OFF. The CCR is operating normally. What would you investigate first?
19Knowledge Check
20Interview Questions
Quick Reference: Runway Lighting Series Circuit, Circuit Diagram, Resistance & Open Circuit
Runway lighting series circuit design puts all the lights on a loop in series, so they all carry the same current from the CCR. A runway lighting circuit diagram shows this as the CCR feeding a primary cable loop with an isolating transformer and light at each position.
An airport lighting open circuit breaks the loop, so the CCR cannot hold current and the lights on that circuit go out. Runway lighting circuit resistance, measured with the circuit isolated, helps show where continuity is lost, as described in this case.
21Frequently Asked Questions
Quick answers about open series circuit faults and multi-fixture runway edge light outages.
Q1What is AGL-CS-001?
AGL-CS-001 is an engineering training case study on a multi-fixture runway edge light outage caused by an open series circuit. It walks through the fault-finding sequence from the initial CCR check to root cause and post-repair verification. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Why do several runway edge lights go out at the same time on a series circuit?
In a series lighting circuit, an interruption can affect several downstream fittings at once. When consecutive lights are out, a common circuit fault such as an open connection, cable joint or isolating transformer problem should be investigated before assuming multiple independent fixture failures.
Q3How do you tell a fixture failure from a series circuit fault?
Start with the failure pattern. Record exactly which lights are ON and OFF, check the CCR, and compare the pattern against the circuit drawing. One failed fitting points toward the fixture; a run of consecutive failures points toward a common circuit path. Electrical tests confirm the diagnosis.
Q4What should you check first when runway edge lights are out?
Check the Constant Current Regulator first: output current and voltage, selected brightness level, alarm indications, local/remote status, circuit selection and protection status. If the CCR is abnormal, field troubleshooting can give misleading results.
Q5What is the recommended troubleshooting sequence in this case?
CCR, output, primary circuit, cable, isolating transformer, connector and then light fixture. Do not start by replacing field lights. Work from the system toward the component, using electrical measurements to narrow down the fault section.
Q6What was the root cause in AGL-CS-001?
An open electrical connection in the series circuit, associated with a defective or moisture-affected connection, interrupted current flow through the affected section. The extinguished lights were symptoms of that common circuit problem, not five independent fixture failures.
Q7Does recent rain prove that moisture caused the fault?
No. Recent rainfall does not prove moisture as the cause, but it is a useful clue. It should prompt inspection of vulnerable connections, joints and components, with the conclusion based on inspection and test results.
Q8Which test equipment may be needed to find an open series circuit?
Depending on the airport's approved maintenance procedure: a true-RMS multimeter, insulation resistance tester, approved AGL circuit test equipment, cable fault locator, clamp meter, circuit drawings, circuit identification records and manufacturer documentation, with appropriate PPE. All testing must follow the airport's approved isolation, switching and safety procedures.
Q9What should be done after repairing the circuit?
Verify operation of the affected lights, check the associated connections and perform the required electrical tests. Where the airport maintenance programme requires it, complete additional electrical and photometric verification, then record the maintenance activity. Restoring one connection is not the end of the job.
Q10What could go wrong if the fault is not diagnosed systematically?
Healthy light fittings or isolating transformers may be replaced unnecessarily, raising cost and runway maintenance downtime. The real circuit fault may stay unidentified, and the failure may repeat after a temporary restoration.
Engineering & Expert Notes
Draw the Circuit Before You Pick Up a Meter
MethodSketching the loop with the failed lights marked usually shows the common path within minutes. Test points chosen from the drawing beat test points chosen by habit.
Rain Is a Clue, Not a Verdict
MoistureRecent rainfall raises suspicion on joints and connectors, but confirm with measurement. Dry-weather faults at the same spot may still be moisture-related once water has entered.
Isolate and Prove Dead Before Opening the Loop
SafetyAn open series circuit can still present hazardous voltage at the break. Follow the lock-out and test procedure each time, regardless of how obvious the fault seems.
Re-Verify the Whole Circuit After the Repair
VerificationFixing one open point proves nothing about the rest of the loop. Check continuity, insulation and current at each brightness step before handing back.
22Case Study Conclusion
Do not treat every failed AGL light as an individual component failure. First understand the failure pattern and investigate the complete electrical system.
A systematic approach can reduce unnecessary component replacement, shorten troubleshooting time and improve the reliability of AGL maintenance.
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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.
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