Division 09 — Stop Bar / Guard Light Troubleshooting

01 · Stop Bar Faults

11 of 11 topics published

SPB-256 Stop Bar Not Illuminated

Symptom — A stop bar (or the full set of fixtures spanning the taxiway at the CAT II/III holding position) is commanded ON from ALCMS/ATC but fails to illuminate.

Equipment / System

Stop bar fixtures — in-pavement or elevated red L-852E lights spanning the taxiway at the CAT II/III holding position, fed via a dedicated stop bar series circuit and isolation transformers, commanded ON/OFF from the ALCMS/ATC control panel.

Possible Causes

HighCCR feeding the stop bar circuit not energised or at zero output

HighPrimary series cable fault on the loop feeding the bar, taking down the full fixture set

MedControl command from ALCMS not reaching the CCR/contactor (route to SPB-261 or SPB-263 if isolated there)

MedInterlock logic incorrectly holding the bar OFF due to a centreline circuit sensing fault

LowMultiple fixture-level transformer failures coinciding across the bar

LowALCMS displaying a stale or mismatched status not reflecting the true field state

Required Tools

ALCMS diagnostic terminalClamp-on series circuit ammeterTrue-RMS multimeter1000V insulation resistance tester (megger)Stop bar circuit as-built drawingBase-can lifting tool and PPE

Safety Precautions

Coordinate airside access with ATC given the stop bar's location at an active holding position.
LOTO the circuit before work.
Confirm with ATC that the stop bar will be treated as out of service, with a compensating procedure in place, before beginning work.

Diagnostic Procedure

ObservationCompare the ALCMS commanded state against the actual field state observed at the fixtures.
MeasurementCheck the CCR output for the stop bar circuit to confirm whether power is reaching the loop.
ObservationCheck the interlock status of the associated taxiway centreline circuit, since the interlock can legitimately hold the stop bar OFF.
MeasurementWith the circuit isolated, megger a sample of fixture transformers to rule out a coincidental multi-fixture failure.
DecisionCCR output absent → source-side fault, route to Division 01. CCR output correct but bar still dark → cable fault or interlock fault.
Fault LocationIsolate to the CCR/source, the primary cable, the interlock logic, or a coincidental multi-fixture failure.
Root CauseConfirm the specific cause before restoring the circuit to automatic control.

Test Points

CCR output at the panel

Interlock relay/logic state

Sample fixture transformer output

ALCMS commanded state vs. field state

Root Cause Verification

Confirm the stop bar illuminates correctly at commanded ON, and confirm the interlock correctly inhibits the downstream centreline lighting while the bar is lit, before returning to automatic control.

Corrective Action

Restore CCR output if the fault is source-side.
Repair the identified primary cable fault.
Correct the interlock fault causing the false OFF state.
Replace failed fixture transformers if a coincidental multi-fixture failure is confirmed.

Preventive Action

Include periodic testing of stop bar and interlock function in the PM cycle.
Verify ALCMS status accuracy against a field check on a defined schedule.
Trend CCR output for the stop bar circuit to catch a developing source-side fault early.

Related References

SOP: SOP-SPB-01 (Stop Bar Fault Isolation Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: ICAO Annex 14 Vol I (stop bar requirements), FAA AC 150/5340-30, FAA AC 150/5345-46 (L-852E)

Diagram: Stop Bar Circuit & Interlock Layout Drawing

Case Study: Division 01 — CCR Troubleshooting; Division 24 — Complete Runway Lighting Failure

Rev 0 — Initial issue

SPB-257 Stop Bar Partially Illuminated

Symptom — One or more individual fixtures within an otherwise correctly energised stop bar fail to illuminate, while the remainder of the bar lights normally.

Equipment / System

Individual stop bar fixture positions within the bar, and the series circuit segment feeding them.

Possible Causes

HighIndividual fixture transformer or lamp/LED failure at one or more positions

HighConnector fault isolated to a specific fixture

MedLocalised cable segment fault between two adjacent fixtures within the bar

MedBase can water ingress affecting a specific fixture

LowPhysical damage at a specific fixture position from a vehicle strike

LowUneven circuit loading from an incorrectly rated transformer installed at a previous replacement

Required Tools

Clamp-on series circuit ammeterTrue-RMS multimeter1000V insulation resistance tester (megger)Series circuit fault locator / TDRStop bar fixture layout drawingBase-can lifting tool and PPE

Safety Precautions

LOTO the circuit before work.
Coordinate access at the active holding position with ATC.

Diagnostic Procedure

ObservationIdentify exactly which fixture(s) within the bar are dark or affected via direct inspection.
MeasurementConfirm the overall circuit current is at the correct value for the commanded step, which rules out a full circuit-level fault.
MeasurementWith the circuit isolated, megger the transformer and test the connector at each affected fixture position.
ObservationInspect the base can at each affected position for water, debris, or physical damage.
DecisionFault isolated to one or two specific fixtures → fixture-level repair. Fault pattern suggests a cable segment issue between fixtures → investigate the intervening cable.
Fault LocationIsolate to the specific fixture(s) affected or to the cable segment between them.
Root CauseConfirm the specific failed component at each affected position before replacement.

Test Points

Affected fixture transformer

Connector continuity at affected positions

Cable segment between affected fixtures

Overall circuit current

Root Cause Verification

Re-energise and confirm every fixture in the bar illuminates uniformly at the commanded intensity step.

Corrective Action

Replace the failed transformer, lamp/LED, or connector at each affected fixture.
Repair the identified local cable fault.
Dry and reseal an affected base can.

Preventive Action

Include fixture-level inspection of the full stop bar in the periodic maintenance cycle.
Track individual fixture failure history to identify a chronically weak position.

Related References

SOP: SOP-SPB-01 (Stop Bar Fault Isolation Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5345-46

Diagram: Stop Bar Fixture Layout Drawing

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue

SPB-258 Stop Bar Flickering

Symptom — One or more stop bar fixtures flicker, pulse, or vary in brightness instead of holding a steady red output at the commanded state.

Equipment / System

Stop bar fixture, isolation transformer, connector, and the interlock relay/logic governing the circuit.

Possible Causes

HighLoose or intermittent L-823 connector, worsened by vehicle traffic vibration at the holding position

HighA partially failed or overheating isolation transformer with an intermittent internal short

MedLamp/LED driver instability or a failing LED module nearing end of life

MedIntermittent cable fault (chafed insulation making and breaking contact)

LowInterlock relay chatter causing rapid ON/OFF cycling — a cause specific to the interlocked nature of the stop bar circuit

LowCCR output instability rather than a fixture-level fault

Required Tools

True-RMS multimeter with min/max holdOscilloscope or power quality logger for waveform captureThermal imaging cameraL-823 connector test adaptorInterlock relay test equipment

Safety Precautions

Treat intermittent faults as live until proven otherwise.
LOTO the circuit before opening any base can or control cabinet.

Diagnostic Procedure

ObservationConfirm whether the flicker is on a single fixture or several, and whether it correlates with a particular commanded step or interlock transition.
MeasurementLog CCR output current over a period spanning the reported flicker to rule out a source-side instability.
MeasurementCheck the interlock relay for chatter or contact bounce as a distinct possible cause given the circuit's interlocked design.
MeasurementWith the circuit isolated, inspect and re-torque the L-823 connector, and megger the isolation transformer under light mechanical flexing.
DecisionFlicker follows CCR current logging → CCR/source-side issue, route to Division 01. Flicker correlates with interlock transitions → investigate interlock relay. Flicker isolated to the fixture → connector or transformer fault.
Fault LocationIsolate to the connector, transformer, lamp/LED module, cable, or interlock relay.
Root CauseConfirm by replacing the suspect component and re-testing under conditions that previously produced the flicker.

Test Points

L-823 connector contact resistance

Isolation transformer under mechanical flex

Interlock relay operation

CCR output current trend log

Root Cause Verification

Monitor the repaired fixture over several operating cycles, including interlock transitions, before closing out.

Corrective Action

Clean, re-grease, and properly seat the L-823 connector.
Replace a transformer confirmed to have an intermittent internal fault.
Replace a failing lamp/LED module.
Service or replace the interlock relay if chatter is confirmed.

Preventive Action

Torque-check connectors as a standard PM step given holding-position traffic vibration.
Include interlock relay condition in the periodic maintenance schedule.
Include thermal imaging of connectors and transformers in the PM cycle.

Related References

SOP: SOP-SPB-01 (Stop Bar Fault Isolation Procedure), SOP-SPB-03 (Interlock Function Test Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5345-46

Diagram: Stop Bar Circuit & Interlock Layout Drawing

Case Study: Division 01 — CCR Troubleshooting

Rev 0 — Initial issue

SPB-259 Stop Bar Wrong Color

Symptom — A stop bar fixture displays a colour other than red, creating a safety-critical guidance discrepancy at a CAT II/III holding position.

Equipment / System

Stop bar fixture lens/filter assembly and lamp/LED colour source.

Possible Causes

HighIncorrect lens/filter installed at a previous replacement (red specified per ICAO Annex 14 for stop bars)

HighLED module fitted with the wrong colour source or an incorrect part number

MedLens cracked or missing, letting the underlying white lamp/LED source show through uncoloured

MedFixture confused with an adjacent green or yellow fixture during a bulk parts replacement given the mixed colour inventory at a holding position

LowUV degradation shifting the red lens toward an orange or faded hue

LowSurface contamination causing an apparent colour shift without an actual component fault

Required Tools

Colour coordinate/chromaticity meterFixture parts catalogue and specification sheetStop bar as-built drawingReplacement lens/filter stock

Safety Precautions

LOTO the circuit before fixture access.
Treat a confirmed wrong-colour stop bar fixture as an immediate safety-critical discrepancy; notify ATC/airfield operations and consider taking the stop bar out of service until corrected, given its role in runway incursion prevention.

Diagnostic Procedure

ObservationConfirm the reported colour against the specification — red for stop bar fixtures per ICAO Annex 14.
MeasurementInspect the fixture's lens/filter and lamp/LED part number against the maintenance specification.
ObservationCheck lens integrity (cracking, missing lens) and confirm the lamp/LED source colour independent of the lens.
DecisionWrong lens/filter part installed → replace with the correct red part. Correct part but fixture confused with an adjacent colour zone during installation → verify against the as-built drawing.
Fault LocationIsolate the discrepancy to the lens/filter, the lamp/LED source, or an installation mix-up.
Root CauseVerify chromaticity meets the ICAO Annex 14 Vol I colour boundary for red after correction.

Test Points

Chromaticity coordinates vs. ICAO Annex 14 Vol I red colour boundary

Installed lens/filter part number

Lamp/LED source colour independent of lens

Fixture position vs. stop bar as-built drawing

Root Cause Verification

Re-check chromaticity after correction and confirm visually against the other fixtures in the same stop bar at night.

Corrective Action

Replace with the correct red lens/filter or fixture assembly.
Notify airfield operations if the discrepancy affected the stop bar's operational status.

Preventive Action

Verify part numbers against the stop bar specification before any bulk lens/filter replacement.
Segregate stop bar (red) parts stock from taxiway (green) and lead-on/off (yellow) parts stock to prevent mix-ups.
Include colour verification in the periodic inspection.

Related References

SOP: SOP-SPB-04 (Colour Verification Procedure)

Standards: ICAO Annex 14 Vol I (colour specifications), FAA AC 150/5345-46

Diagram: Stop Bar Fixture Layout Drawing

Case Study: Division 26 — Photometric & Intensity Compliance

Rev 0 — Initial issue

SPB-260 Stop Bar Intensity Failure

Symptom — A photometric survey or reported observation finds the stop bar, or a section of it, below the minimum required intensity for its operating category.

Equipment / System

Stop bar fixture photometric output as measured against ICAO Annex 14 Appendix 2 / FAA AC 150/5340-30 specifications.

Possible Causes

HighCumulative lens contamination across multiple fixtures reducing transmitted intensity below the compliance threshold

HighAggregate fixture ageing (lamp/LED lumen depreciation) across the bar approaching or past rated service life

MedCCR output intensity step below the value required for the stop bar's operating category

MedA batch of fixtures with a systematic transformer/lamp mismatch installed during a previous bulk replacement

LowSeries circuit voltage drop from cable resistance or route length affecting the far end of the bar

LowSurvey/measurement methodology or equipment calibration issue producing an apparent non-compliance

Required Tools

Calibrated mobile photometric survey unit or handheld luminance/intensity meterColour coordinate measurement equipmentICAO/FAA photometric compliance reference tablesFixture and transformer inventory records

Safety Precautions

LOTO the circuit before any fixture-level intervention following the survey.
Conduct photometric surveys per the applicable procedure and calibration schedule to ensure results are valid before acting on them.

Diagnostic Procedure

ObservationConfirm the survey methodology and equipment calibration are valid before treating the result as a genuine non-compliance.
ObservationIdentify whether non-compliance is isolated to specific fixtures, a specific installation batch, or the bar generally, by cross-referencing survey results against inventory and installation dates.
MeasurementPhysically inspect a representative sample of non-compliant fixtures for lens contamination, lamp/LED condition, and transformer rating.
MeasurementConfirm the CCR output intensity step matches the requirement for the stop bar's operating category.
DecisionNon-compliance traces to contamination → clean and re-survey before replacing components. Non-compliance traces to CCR intensity step → correct the CCR setting.
Fault LocationIsolate the dominant contributing cause across the affected fixtures.
Root CauseConfirm compliance is restored with a follow-up photometric re-survey after corrective action.

Test Points

Measured intensity vs. ICAO/FAA minimum per fixture

CCR output intensity step

Lens condition across the affected sample

Fixture/transformer age and specification records

Root Cause Verification

Conduct a follow-up photometric survey of the corrected fixtures, and cross-check a sample of unaffected fixtures to confirm the survey and correction are consistent.

Corrective Action

Clean lenses across the affected section as a first step where contamination is implicated.
Replace lamp/LED modules that have reached end-of-life lumen depreciation.
Correct the CCR intensity step setting.
Correct a transformer/lamp mismatch found across an installation batch.

Preventive Action

Schedule photometric compliance surveys on a defined recurring interval.
Track fixture installation dates and expected lumen-depreciation curves to plan proactive replacement.
Verify transformer/lamp specification consistency across every bulk procurement or replacement batch.

Related References

SOP: SOP-SPB-05 (Photometric Compliance Survey Procedure)

Standards: ICAO Annex 14 Vol I Appendix 2, FAA AC 150/5340-30

Diagram: Stop Bar Photometric Specification Sheet

Case Study: Division 26 — Photometric & Intensity Compliance

Rev 0 — Initial issue

SPB-261 Stop Bar Control Failure

Symptom — The stop bar's local control system fails to correctly execute an ON/OFF command, or reports a status to ALCMS that does not match the actual field state.

Equipment / System

Stop bar local control system — the CCR-integrated or standalone controller/PLC card, selector relays, and status feedback circuitry that executes ON/OFF commands and reports stop bar status to ALCMS.

Possible Causes

HighFailed controller/PLC card or relay driving the stop bar contactor

HighStatus feedback circuit reporting an incorrect field state to ALCMS while the commanded state is actually correct

MedLocal selector switch stuck or faulty at the CCR cabinet

MedFirmware or logic fault in the controller

LowPower supply fault to the local controller card

LowWiring fault between the controller and the CCR/contactor

Required Tools

True-RMS multimeterALCMS diagnostic terminalController/PLC diagnostic softwareSpare controller cardLocal control cabinet wiring diagram

Safety Precautions

LOTO the control cabinet before card or relay replacement.
Verify with ATC before taking the stop bar control system out of service.

Diagnostic Procedure

ObservationCompare the ALCMS commanded state against the physical state of the stop bar fixtures.
MeasurementCheck the controller/PLC card status LEDs and diagnostics.
MeasurementTest the selector switch and relay operation manually at the CCR cabinet.
MeasurementCheck the status feedback wiring from the controller to ALCMS.
DecisionField state correct but ALCMS status wrong → feedback circuit fault. Field state does not follow the command → controller/relay/selector switch fault.
Fault LocationIsolate to the controller/PLC card, selector switch/relay, status feedback wiring, or controller power supply.
Root CauseConfirm the specific failed component before replacement.

Test Points

Controller/PLC card diagnostics

Selector switch/relay operation

Status feedback signal to ALCMS

Control cabinet power supply

Root Cause Verification

Command the stop bar ON/OFF several times from ALCMS and confirm both the actual field state and the reported status match on each cycle.

Corrective Action

Replace the faulted controller/PLC card.
Repair or replace the selector switch/relay.
Correct the status feedback wiring.
Restore the controller power supply.

Preventive Action

Include controller diagnostics in the periodic maintenance cycle.
Keep a spare controller card on hand given the stop bar's safety-critical function.
Verify status feedback accuracy during routine testing.

Related References

SOP: SOP-SPB-06 (Stop Bar Control System Test Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5340-30

Diagram: Stop Bar Control Cabinet Wiring Diagram

Case Study: Division 01 — CCR Troubleshooting

Rev 0 — Initial issue

SPB-262 Stop Bar Interlock Failure

Symptom — The safety interlock between the stop bar and the downstream taxiway centreline lighting fails to prevent both from being illuminated simultaneously, or fails to allow the centreline lighting to activate when the stop bar is correctly switched OFF.

Equipment / System

Stop bar / taxiway centreline interlock logic — the control circuitry (relay-based or PLC-based) that ensures the stop bar and the taxiway centreline lighting beyond it are never illuminated simultaneously, per ICAO Annex 14 Vol I requirements for CAT II/III holding positions.

Possible Causes

HighInterlock relay contact welded or mechanically stuck, allowing both circuits to be commanded together

HighA maintenance bypass or override left engaged after previous work

MedInterlock logic firmware or programming fault

MedSensing circuit reporting incorrect stop bar or centreline status to the interlock logic

LowWiring fault between the two circuits' status feedback and the interlock controller

LowPower supply fault to the interlock controller

Required Tools

True-RMS multimeterInterlock logic diagnostic tool/PLC softwareRelay test equipmentInterlock wiring diagramALCMS diagnostic terminal

Safety Precautions

LOTO the interlock cabinet before work.
Treat any interlock discrepancy as safety-critical and notify ATC/airfield operations immediately — do not leave the stop bar in automatic operation until the interlock is verified correct.

Diagnostic Procedure

ObservationCommand the stop bar and centreline circuits through a full test cycle and verify only one is ever illuminated at a time.
MeasurementCheck interlock relay contacts for welding or mechanical failure.
MeasurementCheck the sensing/status feedback from both circuits into the interlock logic.
ObservationConfirm no maintenance override or bypass is engaged.
DecisionRelay contact welded/stuck → replace relay. Bypass found engaged → remove and re-secure. Feedback wiring or logic fault → correct wiring or reprogram logic.
Fault LocationIsolate to the interlock relay, the bypass/override, the status feedback wiring, or the interlock controller logic.
Root CauseConfirm with a full interlock test cycle that the fault is resolved before returning to automatic control.

Test Points

Interlock relay contact condition

Status feedback from stop bar and centreline circuits

Bypass/override switch position

Interlock controller power supply

Root Cause Verification

Run a complete interlock test cycle — stop bar ON/centreline OFF, then stop bar OFF/centreline ON — and confirm correct mutually exclusive operation before returning to automatic control.

Corrective Action

Replace the faulted interlock relay.
Correct the interlock logic/programming.
Repair the status feedback wiring.
Remove and re-secure a left-engaged bypass.

Preventive Action

Include a full interlock test cycle in the periodic maintenance schedule.
Log and formally close out any maintenance override/bypass use.
Verify interlock logic after any control system firmware update.

Related References

SOP: SOP-SPB-03 (Interlock Function Test Procedure)

Standards: ICAO Annex 14 Vol I (stop bar interlock requirements), FAA AC 150/5340-30

Diagram: Stop Bar Circuit & Interlock Layout Drawing

Case Study: Division 24 — Complete Runway Lighting Failure

Rev 0 — Initial issue

SPB-263 Stop Bar Remote Command Failure

Symptom — A command issued from the ATC tower or ALCMS workstation to select the stop bar ON or OFF fails to reach or be executed by the field control system.

Equipment / System

Remote command link between the ATC tower/ALCMS workstation and the stop bar's local control system — the communication network (serial/Ethernet/SCADA), protocol converters, and the field RTU/controller receiving the command.

Possible Causes

HighCommunication link failure (cable/fibre break, network switch fault) between ALCMS and the field controller

HighRTU or protocol converter fault at the field end

MedALCMS server/workstation software fault or a hung command queue

MedIncorrect addressing or configuration following a system change

LowPower failure at an intermediate communication node

LowElectromagnetic interference affecting a wireless or poorly shielded link segment

Required Tools

Network diagnostic tools (cable tester, ping/traceroute from the ALCMS network)ALCMS system log reviewRTU/protocol converter diagnostic softwareNetwork architecture drawing

Safety Precautions

Coordinate with ATC before testing since the stop bar may be under active operational control.
Do not disable remote control without an agreed fallback (local manual control) in place.

Diagnostic Procedure

ObservationConfirm from the ALCMS system log whether the command was sent and acknowledged.
MeasurementCheck the communication link status between ALCMS and the field RTU/controller.
MeasurementTest the physical link (cable, fibre, network switch) for continuity or faults.
MeasurementVerify RTU/protocol converter configuration and diagnostics.
DecisionCommand not acknowledged and link shows a fault → communication link/hardware fault. Command acknowledged but not executed → route to SPB-261 (local control system).
Fault LocationIsolate to the communication link, the RTU/protocol converter, or the ALCMS server/software.
Root CauseConfirm the specific failed link segment or component before repair.

Test Points

ALCMS command log and acknowledgement status

Communication link continuity

RTU/protocol converter diagnostics

Field controller local status vs. ALCMS commanded state

Root Cause Verification

Issue a series of test commands from ALCMS and confirm each is correctly received, executed, and acknowledged with accurate status feedback.

Corrective Action

Repair or replace the faulted communication link segment.
Replace the RTU/protocol converter.
Correct addressing/configuration.
Restore power to an intermediate communication node.

Preventive Action

Include communication link testing in the periodic maintenance cycle.
Maintain an accurate network architecture drawing.
Ensure a documented local manual-control fallback procedure is available and tested.

Related References

SOP: SOP-SPB-07 (Remote Command Link Test Procedure)

Standards: ICAO Annex 14 Vol I, ALCMS/SCADA vendor documentation

Diagram: ALCMS Network Architecture Drawing

Case Study: Division 01 — CCR Troubleshooting

Rev 0 — Initial issue

SPB-264 Stop Bar Circuit Failure

Symptom — A circuit-level fault affects the full stop bar series loop, taking out the entire bar or a large section of it, distinct from an isolated fixture-level fault.

Equipment / System

Stop bar series circuit — the CCR-fed series loop supplying the full stop bar fixture set, including the primary cable route and all in-line isolation transformers.

Possible Causes

HighCCR failure or fault at the source

HighPrimary series cable fault (cut, chafed, or moisture-degraded insulation) taking down the whole bar

MedA single shorted isolation transformer collapsing available voltage to the rest of the loop

MedDamaged or crushed duct/cable from recent excavation or heavy vehicle loading near the holding position

LowStorm or lightning-related surge damage to circuit components

LowA section isolator or tie point left open after previous maintenance, if the bar is circuit-segmented

Required Tools

Series circuit fault locator / TDR1000V/2500V insulation resistance testerClamp-on ammeterFull circuit as-built layout drawingExcavation permit and utility locate request if digging is required

Safety Precautions

LOTO the CCR before any circuit work.
Obtain excavation clearance and utility locates before opening pavement if the fault requires digging.
Coordinate the stop bar outage with ATC given its role in runway incursion prevention — a compensating procedure is typically required.

Diagnostic Procedure

ObservationConfirm at the CCR/ALCMS whether the fault is a complete loss of output (source-side) or a partial dark section with the CCR still delivering rated current (downstream fault).
ObservationCross-reference the extent of the fault against the circuit layout drawing to identify candidate cable segments.
MeasurementWith the circuit isolated, megger the primary cable in segments from accessible pull boxes or transformer base cans.
MeasurementUse a TDR or fault locator to estimate fault distance if a direct segment test is inconclusive.
DecisionInsulation resistance fails on a specific segment → cable/joint fault in that segment. All segments clear but a specific transformer reads shorted → that transformer is loading down the loop.
Fault LocationPhysically locate and expose the faulted cable segment or confirm the single shorted transformer.
Root CauseConfirm the cause of the cable fault (mechanical damage, water tracking, age-related insulation breakdown) once exposed.

Test Points

CCR output current and voltage

Primary cable insulation resistance by segment

Suspect transformer primary-to-secondary and to-earth

Section isolator continuity, if applicable

Root Cause Verification

After repair, megger the full circuit end-to-end, re-energise, and confirm every fixture illuminates correctly and the interlock function still operates correctly.

Corrective Action

Repair or replace the faulted cable segment.
Replace the shorted isolation transformer.
Restore duct/cable protection where mechanical damage was the cause.

Preventive Action

Schedule periodic full-loop insulation resistance testing and trend results over time.
Maintain accurate as-built records of every joint, pull box, and repair splice.
Coordinate with civil works teams before any excavation near the holding position.

Related References

SOP: SOP-SPB-01 (Stop Bar Fault Isolation Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: IEC 60364 (insulation testing), ICAO Annex 14 Vol I, FAA AC 150/5340-30

Diagram: Stop Bar Circuit & Interlock Layout Drawing

Case Study: Division 01 — CCR Troubleshooting; Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue

SPB-265 Stop Bar Connector Failure

Symptom — An L-823 connector at a stop bar fixture position is found faulted, contributing to an outage, intermittent fault, or a failed insulation reading at that position.

Equipment / System

L-823 connectors and base cans at individual stop bar fixture positions.

Possible Causes

HighMoisture ingress into the connector body degrading the seal

HighCorroded or pitted connector pins from age or de-icing chemical exposure

MedPlug not fully seated or locked after a previous maintenance visit

MedMechanical damage from vehicle or tug wheel contact given the connector's proximity to the holding position traffic path

LowAn incorrectly rated connector installed at a prior repair

LowContact plating worn from repeated connect/disconnect cycling

Required Tools

L-823 connector test adaptor1000V insulation resistance tester (megger)Contact resistance meterDielectric greaseReplacement connector stock

Safety Precautions

LOTO before disconnecting or inspecting a connector.
Treat a wet connector as energised until confirmed isolated.

Diagnostic Procedure

ObservationIsolate the circuit and inspect the connector body, pins, and boot for moisture, corrosion, or damage.
MeasurementMeasure contact resistance across the connector.
ObservationConfirm the connector part number/rating matches the specification.
ObservationCheck that the plug is fully seated and locked.
DecisionContact resistance elevated with visible corrosion → clean/replace connector. Correct connector but not fully seated → reseat and re-torque/lock.
Fault LocationIsolate the fault to the connector body, pins, boot, or seating.
Root CauseConfirm restored contact resistance and insulation resistance after correction.

Test Points

Connector contact resistance

Insulation resistance across the connector

Pin condition

Seating/locking engagement

Root Cause Verification

Re-energise and confirm the fixture illuminates correctly at all commanded states, and re-check the connector after a subsequent maintenance interval.

Corrective Action

Clean, re-grease, and reseat the connector.
Replace a corroded or damaged connector.
Correct an incorrectly rated connector with the specified part.

Preventive Action

Apply dielectric grease at every reconnection.
Include connector inspection in the periodic maintenance cycle given the holding-position traffic exposure.
Standardise connector part numbers across the stop bar circuit.

Related References

SOP: SOP-SPB-08 (Connector and Transformer Inspection Procedure)

Standards: FAA AC 150/5345-46, ICAO Annex 14 Vol I

Diagram: Stop Bar Base-Can Detail Drawing

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue

SPB-266 Stop Bar Transformer Failure

Symptom — An individual isolation transformer feeding a stop bar fixture is found faulted during investigation of an outage, dim output, or a failed insulation reading.

Equipment / System

Individual isolation transformers feeding stop bar fixtures off the series circuit.

Possible Causes

HighOpen primary or secondary winding from age-related insulation breakdown

HighA partial internal short developing into an intermittent or hard fault

MedAn incorrect wattage/tap installed at a previous replacement, mismatched to the fixture's lamp/LED module

MedMoisture ingress into the transformer housing degrading insulation

LowThermal stress from repeated high-intensity step operation

LowMechanical damage to the transformer housing or leads

Required Tools

1000V insulation resistance tester (megger)Isolation transformer test setTrue-RMS multimeterReplacement transformer stock matched to the fixture specification

Safety Precautions

LOTO before transformer removal or testing.
Allow the transformer to cool if recently energised at a high intensity step.

Diagnostic Procedure

ObservationIsolate the circuit and disconnect the transformer.
MeasurementMegger the primary winding, secondary winding, and primary-to-secondary insulation.
ObservationConfirm the transformer's rated wattage/tap matches the fixture specification.
ObservationInspect the housing and leads for physical damage or moisture.
DecisionWinding reads open or shorted → replace transformer. Transformer good but wattage/tap mismatched → correct with the specified part.
Fault LocationIsolate the fault to the primary winding, secondary winding, or a wattage/tap mismatch.
Root CauseConfirm the specific failed condition by bench-testing the removed transformer.

Test Points

Primary winding insulation resistance

Secondary winding insulation resistance

Primary-to-secondary insulation

Rated wattage/tap vs. fixture specification

Root Cause Verification

Re-energise and confirm the fixture illuminates correctly at the commanded intensity step, and re-megger the replacement transformer as a baseline reading.

Corrective Action

Replace the failed transformer with the correctly rated part.
Correct a mismatched wattage/tap installed previously.

Preventive Action

Verify transformer rating against the fixture specification at every replacement.
Include transformer megger testing in the periodic maintenance cycle.
Track transformer age for proactive replacement given the stop bar's safety-critical role.

Related References

SOP: SOP-SPB-08 (Connector and Transformer Inspection Procedure)

Standards: FAA AC 150/5345-26 (isolating transformers), ICAO Annex 14 Vol I

Diagram: Stop Bar Base-Can Detail Drawing

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue

02 · Guard Light Faults

4 of 4 topics published

GRD-267 Guard Light Failure

Symptom — A Runway Guard Light (RGL) fixture, or a pair/row of fixtures, fails to illuminate or flash when the installation is energised.

Equipment / System

Runway Guard Lights (RGL) — elevated Configuration A paired yellow flashing fixtures or in-pavement Configuration B row of yellow flashing fixtures, installed at the taxiway holding position to alert of proximity to the runway, fed via a dedicated series or parallel circuit and flash controller.

Possible Causes

HighFlash controller/synchroniser failure (route to GRD-270 if isolated to the controller)

HighLamp/LED module failure in one or both fixtures

MedIsolation transformer failure (in-pavement Configuration B) or ballast/power supply failure (elevated Configuration A)

MedConnector or base can fault

LowCircuit-level fault (source or cable) affecting the full RGL installation

LowFixture physically damaged by vehicle strike — elevated fixtures are particularly exposed

Required Tools

Clamp-on ammeterTrue-RMS multimeter1000V insulation resistance tester (megger)Flash controller diagnostic toolRGL as-built circuit drawingBase-can or elevated-fixture access tools and PPE

Safety Precautions

Coordinate airside access given the holding-position location.
LOTO the circuit before work.
Use appropriate fall protection/access equipment for elevated fixture work.

Diagnostic Procedure

ObservationConfirm from ALCMS/field inspection whether the fault is isolated to one fixture, both fixtures of a pair, or the full installation.
MeasurementCheck the flash controller status and output for the affected fixture(s).
MeasurementWith the circuit isolated, test the affected fixture's lamp/LED and transformer/ballast.
ObservationInspect the connector/base can for damage or moisture.
DecisionController output present but fixture dark → fixture-level fault (lamp, transformer/ballast, connector). Controller output absent for the channel → route to GRD-270.
Fault LocationIsolate to the fixture (lamp/LED, transformer/ballast, connector) or to the flash controller.
Root CauseConfirm the specific failed component before replacement.

Test Points

Flash controller output

Fixture lamp/LED condition

Transformer/ballast output

Connector/base can condition

Root Cause Verification

Re-energise and confirm both fixtures (or the full row) flash correctly in the specified alternating or sequential pattern before closing out.

Corrective Action

Replace the failed lamp/LED module.
Replace a failed transformer/ballast.
Repair or replace the connector.
Replace a physically damaged fixture.

Preventive Action

Include RGL fixture and flash pattern verification in the periodic night inspection.
Prioritise elevated-fixture condition checks given vehicle-strike exposure.
Track flash controller service history.

Related References

SOP: SOP-GRD-01 (Runway Guard Light Fault Isolation Procedure)

Standards: ICAO Annex 14 Vol I (runway guard lights), FAA AC 150/5340-30, FAA AC 150/5345-46 (L-804)

Diagram: RGL Circuit and Flash Pattern Drawing

Case Study: Division 24 — Complete Runway Lighting Failure

Rev 0 — Initial issue

GRD-268 Guard Light Wrong Color

Symptom — A Runway Guard Light fixture displays a colour other than yellow/amber, creating a discrepancy in the runway-proximity warning it is intended to provide.

Equipment / System

RGL fixture lens/filter assembly and lamp/LED colour source.

Possible Causes

HighIncorrect lens/filter installed at a previous replacement (yellow/amber specified per ICAO Annex 14 for RGLs)

HighLED module fitted with the wrong colour source or an incorrect part number

MedLens cracked or missing, letting the underlying white lamp/LED source show through uncoloured

MedFixture confused with a nearby stop bar (red) or centreline (green) fixture during a bulk parts replacement given the mixed colour inventory at a holding position

LowUV degradation shifting the yellow lens hue

LowSurface contamination causing an apparent colour shift

Required Tools

Colour coordinate/chromaticity meterFixture parts catalogue and specification sheetRGL as-built drawingReplacement lens/filter stock

Safety Precautions

LOTO the circuit before fixture access.
Treat a confirmed wrong-colour RGL fixture as a safety-critical discrepancy given its role as a runway-proximity warning; notify airfield operations.

Diagnostic Procedure

ObservationConfirm the reported colour against the specification — yellow/amber for RGL fixtures.
MeasurementInspect the fixture's lens/filter and lamp/LED part number against the maintenance specification.
ObservationCheck lens integrity and confirm the lamp/LED source colour independent of the lens.
DecisionWrong lens/filter part installed → replace with the correct yellow/amber part. Fixture confused with an adjacent colour zone → verify against the as-built drawing.
Fault LocationIsolate the discrepancy to the lens/filter, the lamp/LED source, or an installation mix-up.
Root CauseVerify chromaticity meets the ICAO Annex 14 Vol I colour boundary for yellow after correction.

Test Points

Chromaticity coordinates vs. ICAO Annex 14 Vol I yellow colour boundary

Installed lens/filter part number

Lamp/LED source colour independent of lens

Fixture position vs. RGL as-built drawing

Root Cause Verification

Re-check chromaticity after correction and confirm visually against the paired/adjacent RGL fixture at night.

Corrective Action

Replace with the correct yellow/amber lens/filter or fixture assembly.

Preventive Action

Verify part numbers against the RGL specification before any bulk lens/filter replacement.
Segregate RGL (yellow) parts stock from stop bar (red) and centreline (green) stock to prevent mix-ups.
Include colour verification in the periodic inspection.

Related References

SOP: SOP-GRD-02 (Colour Verification Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5345-46

Diagram: RGL Circuit and Flash Pattern Drawing

Case Study: Division 26 — Photometric & Intensity Compliance

Rev 0 — Initial issue

GRD-269 Guard Light Intermittent

Symptom — A Runway Guard Light fixture or its flash pattern comes and goes intermittently rather than holding a consistent, correctly timed flash.

Equipment / System

RGL fixture, connector, transformer/ballast, and the flash controller output feeding that fixture.

Possible Causes

HighLoose or intermittent connector connection, worsened by vehicle vibration at the holding position

HighA partially failed transformer/ballast with an intermittent internal fault

MedA flash controller output channel developing an intermittent fault

MedAn intermittent cable fault (chafed insulation making and breaking contact)

LowLamp/LED driver instability nearing end of life

LowMoisture inside the base can (in-pavement Configuration B) causing an intermittent leakage path

Required Tools

True-RMS multimeter with min/max holdOscilloscope or power quality logger for waveform captureThermal imaging cameraL-823 connector test adaptorFlash controller diagnostic tool

Safety Precautions

Treat intermittent faults as live until proven otherwise.
LOTO the circuit before opening any enclosure.

Diagnostic Procedure

ObservationConfirm whether the intermittent behaviour is isolated to one fixture or affects the flash controller output generally.
MeasurementLog the flash controller output over a period spanning the reported fault.
MeasurementWith the circuit isolated, inspect the connector and megger the transformer/ballast under mechanical flex.
MeasurementCheck the flash controller channel diagnostics for the affected fixture.
DecisionFault reproduced by flexing the connector/transformer → component-level intermittent fault. Fault correlates with the controller channel → route to GRD-270.
Fault LocationIsolate to the connector, transformer/ballast, cable, or flash controller channel.
Root CauseConfirm by monitoring the repaired fixture over several flash cycles and operating conditions.

Test Points

Connector contact resistance

Transformer/ballast under mechanical flex

Flash controller channel output

Cable segment continuity

Root Cause Verification

Monitor the repaired fixture over several flash cycles and operating conditions before closing out.

Corrective Action

Clean, re-grease, and reseat the connector.
Replace the transformer/ballast.
Repair the identified cable fault.
Replace the flash controller channel/module if confirmed faulty.

Preventive Action

Torque-check connectors given vehicle vibration exposure at the holding position.
Include flash controller diagnostics in the periodic PM cycle.
Include thermal imaging of connectors and transformers in the PM cycle.

Related References

SOP: SOP-GRD-01 (Runway Guard Light Fault Isolation Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5345-46

Diagram: RGL Circuit and Flash Pattern Drawing

Case Study: Division 01 — CCR Troubleshooting

Rev 0 — Initial issue

GRD-270 Guard Light Control Failure

Symptom — The Runway Guard Light flash controller/synchroniser fails to produce the correct flash pattern — showing no flash, a steady state, or an incorrect rate/sequence.

Equipment / System

RGL flash controller/synchroniser — the device generating and timing the alternating (Configuration A) or sequential (Configuration B) flash pattern for the runway guard lights, and any interface to ALCMS for status/command.

Possible Causes

HighFailed controller card or timing circuit producing no flash, a steady-on/steady-off state, or an incorrect flash pattern

HighPower supply fault to the controller

MedFirmware or logic fault following a configuration change

MedWiring fault between the controller and the fixture outputs

LowLoss of synchronisation between paired Configuration A fixtures

LowALCMS interface fault reporting incorrect controller status

Required Tools

Flash controller diagnostic softwareTrue-RMS multimeterOscilloscope for flash-timing verificationController wiring diagramSpare controller card/module

Safety Precautions

LOTO the control cabinet before card replacement.
Coordinate with ATC/airfield operations if the RGL will be out of service during correction, given its role as a runway-proximity warning.

Diagnostic Procedure

ObservationObserve the actual flash pattern in the field and compare against the specified pattern (alternating or sequential, correct rate).
MeasurementCheck the controller's diagnostic status and output channels.
MeasurementVerify the controller's power supply.
MeasurementCheck wiring between the controller and each fixture.
DecisionController diagnostics show a channel fault → replace controller card/module. Power supply fault confirmed → restore power. Wiring fault confirmed → repair wiring.
Fault LocationIsolate to the controller card/module, its power supply, the output wiring, or a synchronisation fault between paired fixtures.
Root CauseConfirm with an on-site flash pattern check that the corrected pattern matches the specification.

Test Points

Controller diagnostic status

Flash timing/rate at each output channel

Controller power supply

ALCMS status feedback, where applicable

Root Cause Verification

Confirm the corrected flash pattern matches the specification at the correct rate and synchronisation, observed directly at the fixture during a night check.

Corrective Action

Replace the failed controller card/module.
Restore the controller power supply.
Repair the wiring fault.
Reconfigure the controller after a logic/firmware fault.

Preventive Action

Include flash pattern verification in the periodic night inspection.
Keep a spare controller card on hand given the RGL's safety function.
Verify controller configuration after any firmware update.

Related References

SOP: SOP-GRD-03 (Flash Controller Test Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5340-30

Diagram: RGL Circuit and Flash Pattern Drawing

Case Study: Division 01 — CCR Troubleshooting

Rev 0 — Initial issue

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Frequently Asked Questions

Quick answers about Stop Bar (SPB) / Runway Guard Light (GRD) troubleshooting in the AGL Troubleshooting Atlas.

Q1What does Division 09 of the AGL Troubleshooting Atlas cover?

Division 09 is the Stop Bar (SPB) / Runway Guard Light (GRD) Troubleshooting reference. It publishes 15 diagnostic topics, SPB-256 to GRD-270, across 2 subsections: Stop Bar Faults (11); Guard Light Faults (4).

Q2What information does each topic include?

Every topic follows the same diagnostic record: the symptom, equipment/system, possible causes ranked by likelihood (high, medium or low), required tools, safety precautions, diagnostic procedure, test points, root cause verification, corrective action, preventive action and related references (SOPs, standards, diagrams and case studies).

Q3Which topics cover Stop Bar Faults?

SPB-256 to SPB-266 (11 topics): Stop Bar Not Illuminated; Stop Bar Partially Illuminated; Stop Bar Flickering; Stop Bar Wrong Color; Stop Bar Intensity Failure; Stop Bar Control Failure; Stop Bar Interlock Failure; Stop Bar Remote Command Failure; Stop Bar Circuit Failure; Stop Bar Connector Failure; Stop Bar Transformer Failure.

Q4Which topics cover Guard Light Faults?

GRD-267 to GRD-270 (4 topics): Guard Light Failure; Guard Light Wrong Color; Guard Light Intermittent; Guard Light Control Failure.

Q5How does a diagnostic procedure begin in Division 09?

Each procedure starts with observation before any intervention. For example, SPB-256 (Stop Bar Not Illuminated) begins: Compare the ALCMS commanded state against the actual field state observed at the fixtures.

Q6What safety precautions apply when troubleshooting Stop Bar (SPB) / Runway Guard Light (GRD)?

Each topic lists its own precautions. For example, SPB-256 (Stop Bar Not Illuminated) starts with: Coordinate airside access with ATC given the stop bar's location at an active holding position.

Q7Which SOPs are referenced in Division 09?

Related references in the topics include SOP-SPB-01 (Stop Bar Fault Isolation Procedure), SOP-CCR-04 (CCR LOTO Procedure), SOP-SPB-03 (Interlock Function Test Procedure), SOP-SPB-04 (Colour Verification Procedure), SOP-SPB-05 (Photometric Compliance Survey Procedure), SOP-SPB-06 (Stop Bar Control System Test Procedure), SOP-SPB-07 (Remote Command Link Test Procedure) and SOP-SPB-08 (Connector and Transformer Inspection Procedure), and 3 more.

Q8Which standards are referenced in the Stop Bar (SPB) / Runway Guard Light (GRD) topics?

Where a standard applies, it appears in the topic's Related References. Those cited in this division include ICAO Annex 14 Vol I, FAA AC 150/5340-30, FAA AC 150/5345-46, ICAO Annex 14 Vol I Appendix 2, IEC 60364 and FAA AC 150/5345-26.

Q9How can I find a specific fault in Division 09 quickly?

Use the search box at the top of the page to filter the 15 published topics by code, fault name or keyword; matching topics open automatically. Each diagnostic record can also be downloaded as a Word (.doc) file.