Why Airfield Lights Fail Common Causes, Symptoms & Fixes
Work through the likely causes behind runway light outages, flickering, water ingress, damaged inset lights, transformer faults, CCR problems and control-system alarms, and learn how to investigate, verify and close an AGL fault with evidence.
Why AGL Failures Need a Structured Approach
Airfield Ground Lighting (AGL) failures can originate in the light fixture, secondary connection, isolating transformer, primary series cable, Constant Current Regulator (CCR), control interface or Airfield Lighting Control and Monitoring System (ALCMS).
A failed runway light does not always mean that the fixture itself is defective. The actual cause may be an electrical connection problem, damaged cable, transformer fault, circuit protection operation or control-system issue.
Effective fault diagnosis starts with recording the symptoms, identifying the affected asset and circuit, reviewing available monitoring information and following the approved fault-investigation procedure. Replacing a fixture without identifying the underlying cause may lead to repeated failures, unnecessary expenditure and incomplete corrective action.
This guide explains common AGL failure symptoms, their possible causes, evidence to review and general corrective-action considerations.
Important safety note: AGL systems support aircraft movement and airport operations. Troubleshooting, testing, isolation, repair and return-to-service decisions must be performed by authorized personnel under the airport’s approved safety, access, isolation, maintenance and operational procedures. The possible causes listed below are diagnostic starting points, not confirmation of a fault or instructions for intrusive electrical work.
Topics covered in this guide
Where a fault can start, and what to look at first.
Why Are Runway Lights Not Working?
When runway lights stop operating, the first task is to establish the extent of the failure. Determine whether the symptom affects one fitting, several fittings on a circuit or a wider lighting system.
The number and location of affected lights can help narrow the investigation, but the failure pattern alone does not establish the root cause.
Common causes of runway light failure
| Possible cause | Typical area to investigate | Evidence to review |
|---|---|---|
| Fixture failure | Individual light fitting | Inspection observations, asset history and relevant equipment records |
| Secondary connection fault | Fixture connection and associated wiring | Connector condition and approved inspection records |
| Isolating transformer fault | Transformer and its connections | Transformer identification, ratings and available test records |
| Primary cable fault | Series-circuit cable and joints | Circuit records, prior fault reports and approved cable-test results |
| CCR protection operation | CCR and circuit power equipment | Alarm history, operating indications and maintenance records |
| Supply or internal CCR problem | CCR power and internal equipment | Available event records and authorized inspection findings |
| Control command problem | Local or remote control interface | Command status, feedback, event logs and operating records |
| Multiple equipment defects | Affected lighting circuit or installation area | Failure distribution, asset history and inspection evidence |
Individual light failure versus multiple light failure
When one light is not working
A local fixture fault, secondary connection issue, isolating transformer fault or another associated equipment problem may be involved. The cause must be confirmed using the approved investigation method.
When several lights on the same circuit are affected
The investigation may need to include the CCR, circuit condition, primary cable, connections, protection indications and relevant control functions.
When several unrelated systems are affected
Review any common equipment, control interface, supply arrangement or event that could explain the symptoms, while checking each affected system according to its approved procedure.
Recommended investigation record
Record the following before concluding the fault investigation:
Three ways a series loop can fail, and what each one looks like.
AGL Visual Encyclopedia: Series-circuit fault path Lamp-out behaviour Earth fault
What Causes Runway Light Flickering?
Runway light flickering or intermittent operation may indicate a problem with the fixture, electrical connections, associated equipment, circuit or control system.
Because similar symptoms can result from different faults, an observation of flickering should be treated as a symptom rather than a definitive diagnosis.
Possible causes of flickering
| Possible cause | What to investigate |
|---|---|
| Intermittent connection | Available connector and connection inspection records |
| Fixture component problem | Fixture condition, product history and relevant manufacturer guidance |
| Isolating transformer issue | Transformer identification, condition and approved test results |
| Cable or joint deterioration | Circuit history, accessible installation condition and prescribed test records |
| CCR operating issue | Recorded output indications, alarms and applicable event history |
| Control or monitoring issue | Relationship between observed lighting behaviour and recorded commands or feedback |
| Equipment incompatibility | Compatibility of the installed components with the approved circuit design |
| Environmental damage | Evidence of moisture, contamination, impact or deterioration |
How to document flickering
Record whether the flickering is observed at one fitting, multiple fittings on the same circuit or multiple systems. Include the time, operating conditions, available equipment indications and any recurring pattern.
Review the information using the airport’s approved troubleshooting process. Do not assume that an apparent change in light output proves a CCR fault or that a control-system alarm proves a field-equipment failure.
Corrective action should follow the confirmed cause and the relevant equipment instructions.
A flicker is logged as an event, then traced to the likely cause.
AGL Visual Encyclopedia: Intermittent-fault troubleshooting Loose connection LED driver, thyristor CCR flicker
Airport Light Water Ingress
Water ingress can contribute to corrosion, connection problems, insulation deterioration and damage to electrical or optical components.
The significance of water ingress depends on the fixture design, the affected component, the degree of exposure and the manufacturer’s requirements.
Common contributors to water ingress
Inspection and corrective-action considerations
Record the equipment identification and observed condition. Where required, document visible moisture, corrosion, damaged seals, cable-entry concerns and relevant maintenance history.
The corrective-action process should establish the extent of the damage and whether associated electrical components require inspection, testing, replacement or other manufacturer-approved attention.
Three paths water can take into an in-pavement light.
AGL Visual Encyclopedia: Moisture ingress Water-ingress condition Water-ingress prevention Sealing system
Inset Light Damage and Failure
Inset airfield lights are installed within pavement and may experience mechanical loading, impact, contamination and environmental exposure. Their installation condition and compatibility with the approved mounting arrangement are important to reliable operation.
Common causes of inset light damage
| Possible cause | Inspection considerations |
|---|---|
| Pavement-related loading or impact | Look for evidence of damage to the fixture or surrounding pavement |
| Incorrect installation | Review installation records and the approved mounting arrangement |
| Damaged lens or optical surface | Record cracks, contamination or other visible abnormalities |
| Loose or damaged mounting arrangement | Review the prescribed inspection findings and installation details |
| Water ingress | Check relevant indicators and available sealing or maintenance records |
| Contamination | Record deposits or material that may affect the optical surface |
| Connection or component defect | Review relevant connection and equipment records |
| Inappropriate handling | Check maintenance and replacement history where available |
Why the surrounding pavement matters
A fitting and its surrounding pavement may need to be assessed together. Visible pavement damage, deterioration around the mounting area or evidence of physical impact may be relevant to the investigation.
Replacement decisions should take account of the specified fitting, approved installation details, required optical performance and any necessary pavement work. After corrective work, perform the inspections and acceptance checks required by the approved procedure.
Related Guides
AGL installation & commissioningInset light in section: window, body, flange, base, transformer and cable.
AGL Visual Encyclopedia: Inset runway light Rubber deposit build-up Structural deterioration
What Causes AGL Isolating Transformer Failure?
An airfield lighting isolating transformer forms part of the lighting circuit and must be suitable for its specified electrical and installation conditions.
Transformer-related failures may be associated with insulation deterioration, moisture ingress, physical damage, connection defects or operating conditions outside the equipment’s intended ratings.
Possible transformer failure causes
| Possible cause | Evidence to review |
|---|---|
| Insulation deterioration | Applicable electrical test results and maintenance history |
| Moisture ingress | Observed condition, connector details and relevant installation records |
| Physical damage | Inspection findings and equipment history |
| Connector or connection defect | Connector compatibility, assembly records and authorized inspection results |
| Incorrect equipment selection | Transformer rating, circuit schedule and associated fixture requirements |
| Equipment deterioration | Age, service history, previous faults and manufacturer guidance |
| Installation problem | Approved drawings, installation records and relevant inspection findings |
| External cable or circuit fault | Circuit records and results from the approved investigation process |
Transformer fault investigation checklist
Identify & verify
Review & record
Five checks on an isolating transformer, from primary to housing.
AGL Visual Encyclopedia: Transformer failure modes Transformer troubleshooting Isolation transformer principle
CCR Problems and Series Circuit Failures
The Constant Current Regulator (CCR) supplies the required current to the associated AGL series circuit. Its indications, alarms and event records can be useful evidence when investigating a lighting failure.
However, a CCR alarm or shutdown indication does not, by itself, identify the defective component.
Common CCR failure areas
| Possible issue | Evidence to review |
|---|---|
| Protection operation | Alarm history, recorded event information and applicable protection records |
| Internal component fault | Authorized maintenance findings and manufacturer guidance |
| Cooling or ventilation problem | Equipment indications and prescribed inspection records |
| Power supply issue | Available equipment indications and authorized inspection results |
| Incorrect configuration or settings | Approved configuration records and relevant event history |
| Control interface problem | Command, feedback and communication records |
| Circuit-related fault | Affected circuit information and approved investigation results |
| Recurring equipment problem | Maintenance history, fault frequency and prior corrective actions |
Series circuit failure investigation
Series-circuit failures may involve the primary cable, cable joints, connectors, isolating transformers, light fittings or associated power and control equipment.
Useful evidence may include
The investigation should distinguish between a confirmed component fault and a symptom that could have several causes.
Six CCR symptoms, each with a first check and a likely area.
AGL Visual Encyclopedia: CCR troubleshooting CCR to series-circuit interface Series-circuit troubleshooting
ALCMS and Control-System Failures
The Airfield Lighting Control and Monitoring System (ALCMS) may provide operator control, status indications, alarms and other approved functions. A failure within the control system can affect the command or indication path, while a field-equipment failure can generate an alarm or status abnormality within the system.
The investigation should distinguish between the reported indication and the actual condition of the lighting equipment.
Possible ALCMS failure causes
| Possible cause | Evidence to review |
|---|---|
| Communication loss | Recorded communications alarms and interface status |
| Incorrect configuration | Approved configuration records and documented changes |
| Interface failure | Relevant interface status and authorized test records |
| Faulty status feedback | Comparison of recorded indications with approved verification results |
| Operator command issue | Relevant event records and authorized operating information |
| Power or hardware problem | Equipment alarms and maintenance records |
| Field circuit fault | CCR indications, circuit history and approved field inspection results |
| Integration issue | Interface specifications, change records and integration test evidence |
ALCMS fault reporting checklist
Capture the indication
Verify & close
Related Guides
ALCMS guideWork through the control layers before blaming the field.
AGL Visual Encyclopedia: ALCMS troubleshooting flow ALCMS failure flow ALCMS event logging
Airfield Lighting Cable and Connector Failure Causes
AGL cable and connector faults may arise from physical damage, deteriorated insulation, poor joints, moisture exposure, installation defects or external excavation.
Primary series cables and secondary connections have different functions and should be investigated using the applicable circuit documentation and test methods.
Common cable and connector problems
| Possible issue | Evidence to review |
|---|---|
| Physical cable damage | Inspection findings, installation route information and maintenance history |
| Insulation deterioration | Applicable test results and condition history |
| Joint or termination problem | Jointing and termination records, plus authorized inspection findings |
| Connector damage | Connector identification, condition and assembly records |
| Water exposure | Environmental observations and relevant installation details |
| Excavation or civil works damage | Site records, reported incidents and cable-route information |
| Incorrect component selection | Approved material schedule and compatibility requirements |
| Undocumented modification | As-built drawings, work orders and circuit records |
Cable fault investigation record
A cable investigation record should include:
Where a primary cable or connector can fail.
AGL Visual Encyclopedia: Cable failure Cable troubleshooting Connector troubleshooting Cable fault location
Why LED Airfield Lights Fail
LED airfield lights can experience failures involving the light source, electronic components, power or control interfaces, optical components, connections or environmental protection.
The likely cause depends on the particular product design, installation and operating conditions.
Possible LED light failure causes
| Possible cause | What to review |
|---|---|
| Electronic component failure | Manufacturer’s diagnostic guidance and equipment history |
| Thermal stress | Installation conditions and relevant product requirements |
| Environmental exposure | Evidence of moisture, contamination or physical damage |
| Connection fault | Accessible inspection records and approved test findings |
| Power-system compatibility | Compatibility information for the installed circuit and equipment |
| Control-interface issue | Operating commands, indications and related event logs |
| Optical damage | Lens and other optical inspection findings |
| Product deterioration | Service history and available manufacturer assessment |
LED compatibility considerations
An LED fitting should not be assumed compatible with an existing AGL circuit simply because its physical dimensions are suitable.
Before a replacement or retrofit is accepted, verify the required electrical characteristics, approved system compatibility, control interfaces and any specific acceptance tests required by the project.
Related Guides
LED airfield ground lightingWhy a compatible-looking LED fitting can still flicker on a CCR.
AGL Visual Encyclopedia: LED driver, thyristor CCR flicker Retrofit and CCR resizing Mixed halogen and LED circuits
Common AGL Fault Symptoms and Their Possible Causes
The following table is a first-level reference for organizing fault investigations. Each symptom may have more than one cause, and the actual cause must be established from evidence.
| Observed symptom | Possible causes | Recommended investigation focus |
|---|---|---|
| One runway light not operating | Fixture, secondary connection, transformer or associated wiring fault | Asset history, local inspection records and approved test results |
| Several lights affected on a circuit | CCR, primary cable, circuit connection or other shared equipment issue | Circuit pattern, CCR indications and circuit records |
| Intermittent light output | Connection, equipment, circuit or control-related issue | Timing, affected locations, event records and approved inspection results |
| Water or moisture evidence | Damaged seals, compromised cable entry or environmental exposure | Equipment condition, assembly details and manufacturer requirements |
| Inset fitting damaged | Impact, loading, mounting or pavement-related issue | Fixture and surrounding pavement inspection |
| CCR alarm or trip indication | Protection operation, regulator fault or circuit-related issue | Event history and the approved investigation procedure |
| ALCMS indicates a fault | Field fault, interface problem, communications issue or inaccurate feedback | Alarm details, event history and authorized verification |
| Repeated transformer-related reports | Transformer defect, connections, installation conditions or another circuit problem | Transformer and circuit history, specifications and test evidence |
| Cable fault suspected | Physical damage, joint issue, insulation deterioration or external interference | Route records, incident history and approved cable tests |
| LED fitting failure | Electronic, environmental, optical, connection or compatibility issue | Product information, circuit compatibility and manufacturer guidance |
One light out versus many: the pattern points to where to start.
AGL Visual Encyclopedia: Multiple-fixture failure Fixture troubleshooting Series-circuit troubleshooting
A Systematic AGL Fault Investigation Process
A documented fault investigation helps teams identify the cause, avoid unnecessary component replacement and reduce recurring defects.
Record the reported symptom. Capture the observation, time, location, affected asset and any available operator or maintenance report. Avoid recording an assumed cause as if it were already confirmed.
Establish the failure scope. Identify whether the problem affects one fixture, multiple fittings, one circuit or several systems. Confirm equipment and circuit identities against approved records.
Review available indications. Review applicable CCR alarms, control-system status, circuit records and event history. Determine whether the available indications are consistent with the reported symptom.
Review previous maintenance. Check work orders, previous failures, equipment replacement history and any recent installation or configuration changes. Repeated faults may indicate an unresolved underlying cause, but the pattern alone is not sufficient to prove one.
Define the authorized investigation. Select the inspection or test method using the approved maintenance procedure, equipment instructions, applicable specifications and authorization requirements. Identify any access restrictions, isolation requirements, permits or operational coordination needed before work begins.
Record findings. Document observed conditions, tests performed, actual results, relevant instrument details, acceptance criteria and the procedure used.
Establish the cause. Separate the confirmed cause from other possible explanations. If the available evidence is inconclusive, record the investigation status and identify the next authorized action.
Carry out corrective action. Repair, adjust or replace equipment only under the approved procedure and within the responsible personnel’s authorization.
Verify the result. Perform the required post-maintenance tests, inspections or functional checks and record the results against the applicable acceptance criteria.
Close the record. Document the root cause where established, corrective work, verification evidence, outstanding items and authorized closure.
Isolate the fault in a fixed order.
AGL Visual Encyclopedia: Fault-isolation flowchart Corrective maintenance sequence Safe maintenance sequence
AGL Fault Report Template
Use a consistent record format for runway, taxiway, approach lighting, CCR, transformer, cable and control-system faults.
Fault investigation record
| Field | Details |
|---|---|
| Fault reference | |
| Date and time reported | |
| Airport / location | |
| System / equipment type | |
| Asset ID / circuit number | |
| Reported symptom | |
| Number / location of affected fittings, if applicable | |
| CCR or ALCMS indication | |
| Relevant operating context | |
| Previous fault or maintenance reference | |
| Investigation procedure reference | |
| Tests and inspections performed | |
| Measured results and units, where applicable | |
| Applicable acceptance criterion | |
| Confirmed cause or investigation status | |
| Corrective action taken | |
| Required post-maintenance verification | |
| Verification result | |
| Outstanding items | |
| Responsible person / reviewer | |
| Closure date and approval |
Example fault record
The following is an illustrative record structure, not a real incident or a confirmed technical diagnosis.
| Field | Example entry |
|---|---|
| Reported symptom | One lighting fitting reported as not operating |
| Initial assessment | Cause not yet confirmed |
| Evidence reviewed | Asset identification and available maintenance history |
| Investigation | To be completed under the approved procedure |
| Corrective action | To be determined from investigation findings |
| Verification | Required checks to be recorded against applicable criteria |
| Closure | Pending authorized review |
Related Guides
AGL inspection & audit checklistsFrom first report to closure, and what is recorded at each stage.
Preventing Recurring AGL Failures
Failure prevention depends on correct equipment selection, quality installation, suitable inspection and maintenance practices, reliable records and effective defect closure.
Useful reliability practices
Why root-cause analysis matters
Replacing a failed component may restore function, but it may not address the condition that caused the failure.
For example, if a replacement component fails repeatedly, the investigation may need to consider installation conditions, external damage, component compatibility, electrical conditions or maintenance history. These are hypotheses to investigate—not conclusions to make without evidence.
Engineering objective: Identify the actual cause, implement the appropriate corrective action and retain enough evidence to verify the result.
Prevention is a loop, not a one-off repair.
AGL Visual Encyclopedia: Predictive maintenance Preventive maintenance sequence Condition grading
Explore the AGL Engineering Knowledge Hub
Fundamentals
Standards & Compliance
Runway, Taxiway & Approach Lighting
Surface Movement & Safety
PAPI
Electrical Engineering & Power
Photometry & Testing
Maintenance, Inspection & Safety
Resources, Documentation & Careers
Frequently Asked Questions
Q1What should be checked first when runway lights fail?
Record the affected location and circuit, establish whether one or multiple lights are affected, and review available CCR and control-system indications. Follow the airport’s approved fault-investigation procedure to identify the cause.
Q2Why do runway lights keep failing?
Repeated failures may be associated with unresolved connection issues, equipment deterioration, environmental exposure, cable or installation problems, compatibility concerns or other system conditions. Review the failure history and evidence before determining the root cause.
Q3Why do airfield lights flicker?
Flickering may result from intermittent connections, equipment faults, circuit-related problems or control and power equipment issues. Establish the extent and timing of the symptom and review available records before selecting an authorized test method.
Q4What causes water ingress in airfield lights?
Possible contributors include damaged seals, compromised cable entries, incorrect assembly, physical damage or environmental conditions outside the equipment’s intended requirements. The corrective action should address the cause and assess any related electrical or optical damage.
Q5What causes isolating transformer failure?
Possible causes include insulation deterioration, moisture ingress, physical damage, connection defects, installation issues or operation outside the intended rating. Verify the actual cause using the applicable approved inspection and test procedures.
Q6Why does the CCR show a fault when some lights are not working?
A CCR indication can arise from protection operation, regulator-related issues or a circuit problem, among other possibilities. Review the recorded alarm and the affected lighting circuit. The indication alone does not prove which component has failed.
Q7Can an AGL fault be repaired while the circuit is energized?
All work must follow the airport-approved electrical safety, isolation, permit and maintenance procedures. Do not undertake intrusive electrical work simply because a lighting circuit appears to be operating normally.
Q8How can AGL failures be reduced?
Maintain accurate asset and circuit records, perform authorized preventive maintenance, use compatible specified equipment, investigate recurring symptoms and document corrective-action verification. Follow the approved programme for the particular airport and equipment.
Q9Does replacing a failed light solve the problem?
It may resolve a confirmed fixture defect, but it may not address an underlying connection, transformer, cable, installation or system problem. The failure cause and required post-maintenance verification should be documented.
Q10Why runway lights are not working: what are the common causes?
When runway lights are not working, common causes include a tripped or faulted CCR, an open circuit, a failed transformer or connector, water ingress, power loss and control problems. Work through the fault methodically from the supply and CCR to the circuit and the individual light.
Q11What causes AGL transformer failure?
AGL transformer failure can be caused by water ingress, insulation breakdown, overheating, mechanical damage, poor connections and surge damage. An isolating transformer fault can lead to an open circuit or an earth fault.
Q12How to document an AGL fault?
To document an AGL fault, record the date, location, circuit, symptoms, readings, suspected cause, action taken, parts used and who verified the repair. Add photographs and refer to the fault report template on this page.
Q13What are airfield lighting failure causes, airfield ground lighting failure causes and airport lighting failure causes?
The main airfield lighting failure causes are electrical (open circuits, insulation faults), environmental (water, heat, corrosion), mechanical (impact, vibration, jet blast), control-related and maintenance-related. Airfield ground lighting failure causes and airport lighting failure causes are the same in principle.
Q14What are runway lighting failure causes and runway edge lights failure causes?
Runway lighting failure causes include circuit faults, damaged fixtures, water ingress and CCR faults. Runway edge lights failure causes typically include lamp or LED failure, broken lenses, connector problems and transformer faults.
Q15What are runway centreline lights failure causes and threshold lights failure causes?
Runway centreline lights failure causes are often mechanical damage from aircraft wheels and water ingress in inset fixtures. Threshold lights failure causes include similar damage, plus aiming and cabling issues.
Q16What are taxiway lighting failure causes and approach lighting failure causes?
Taxiway lighting failure causes include damage from vehicles, water ingress, and cable and connector faults. Approach lighting failure causes include lightning, structural damage, cable faults and failures in remote elevated fixtures.
Q17What are AGL series circuit failure causes?
AGL series circuit failure causes include open circuits from a failed transformer or connector, insulation or earth faults, cable damage and incorrect loading. Series circuits are sensitive to any break in the loop.
Q18What are constant current regulator failure causes?
Constant current regulator failure causes include component failure, overheating, overload, supply problems, surges and control faults. Alarms and CCR trip indications help narrow down the cause.
Q19What are airfield lighting isolation transformer failure causes?
Airfield lighting isolation transformer failure causes include moisture, insulation damage, overheating, connector failure and mechanical stress.
Q20What are airfield lighting cable failure causes?
Airfield lighting cable failure causes include mechanical damage during excavation, insulation breakdown, water ingress at joints, rodent or ground movement damage and age.
Q21What are airfield lighting control system failure causes and ALCMS failure causes?
Airfield lighting control system failure causes include communication faults, power supply failures, software or configuration errors and hardware faults. ALCMS failure causes are similar and may also include network and interface problems.
Related AGL Resources
Explore these related technical pages.
Asset Lifecycle, Reliability Engineering and Failure Management
Reliability concepts, failure records and root-cause investigation.
Open guideFinal Note
Airfield lighting failures can have several possible causes, and similar symptoms may arise in different parts of the system. Reliable fault diagnosis depends on identifying the affected equipment, reviewing available records, using approved inspection and test methods, documenting evidence and verifying corrective action.
Use this guide as an educational starting point. The applicable airport procedures, approved engineering documents, manufacturer instructions and authorized maintenance practices take precedence for all operational work.
Explore More AGL Engineering Resources
Engineering & Expert Notes
Find the Pattern Before Replacing Parts
DiagnosisCheck whether failures cluster by circuit, location, age or weather. A pattern points to a shared cause such as a cable fault, a drainage problem or a batch issue, not a run of unrelated bad luck.
Water Is the Most Repeated Root Cause
Water IngressPoor seals, blocked drainage and damaged connectors let moisture into fixtures, transformers and bases. Fix the entry path as well as the failed component, or the fault will return.
Intermittent Faults Usually Mean a Connection Problem
ConnectionsFlickering or a light that works on some days is more often a marginal joint, corroded contact or movement in the base than a failed lamp or LED.
A Replaced Transformer Can Hide a Cable Fault
Series CircuitIf a secondary fault is repeated after replacement, test the cable and connector before accepting the transformer as the cause. Measure rather than swap by habit.
Keep a Fault Log That Others Can Learn From
RecordsRecord symptom, location, cause found and fix applied. After a few months the log shows which parts and areas deserve design changes or more frequent inspection.
Confirm Safe Isolation Before Touching the Circuit
SafetySeries circuits can carry hazardous voltage even when lights look off. Follow the lock-out and test procedure for every fault visit, however minor the job appears.
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