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Safety-critical technical reference — read before use. Independent educational guidance for qualified AGL personnel. It is not an approved procedure or regulatory instruction and does not replace your aerodrome’s permit-to-work, isolation/LOTO and maintenance procedures or the manufacturer’s instructions. Series-circuit AGL equipment can carry lethal voltages. Read the full disclaimer ↓
Division 10 of 30

Approach Lighting System Troubleshooting

Approach Lighting System (ALS) fault diagnosis — from a complete system outage through to partial/multiple fixture failures, flicker and intensity-step faults, colour and photometric compliance, circuit and cable faults, connector and transformer failure, alignment and structural damage (poles, crossbars, platforms), optical failure, control and remote-monitoring faults, earthing/lightning protection, and water ingress across the elevated mast and crossbar structures beyond the runway threshold. Each topic follows the full engineering diagnostic record: symptom → observation → measurement → decision → fault location → root cause → verification → corrective action → preventive action.

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01 · Approach Lighting System Faults (20)

01 · Approach Lighting System Faults

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ALS-271 Approach Light System Not Working

Symptom — The entire Approach Lighting System (ALS) fails to illuminate when commanded, reported via pilot report, ALCMS status, or routine night inspection — distinct from a partial or single-fixture outage.

Equipment / System

Approach Lighting System (ALSF-2/MALSR/simplified configuration as installed) — elevated crossbar and centreline barrette fixtures on frangible masts/poles beyond the runway threshold, sequenced flasher units, dedicated ALS feeder circuit(s) and control cabinet, fed from the CCR/ALS regulator and ALCMS.

Possible Causes
HighMain ALS feeder breaker tripped or the ALS regulator/CCR itself at zero output
HighALS control cabinet or master controller fault preventing the energise command from reaching the field circuit
MediumUpstream power supply failure (mains and standby/generator) affecting the ALS distribution board
MediumA main feeder cable fault between the control building and the ALS structure, particularly on systems with long feeder runs over unpaved or wetland approach terrain
LowALCMS command not being issued correctly to the ALS circuit, distinct from a true field-side power failure — verify command versus status feedback
Required Tools
Clamp-on ammeterTrue-RMS multimeter (CAT III)1000V/2500V insulation resistance testerALS as-built circuit and control schematicPortable generator or backup supply test point, if applicable
Safety Precautions
  • Coordinate all approach-area access through ATC/airfield operations — ALS structures are frequently in the approach surface and may require additional clearance beyond a standard runway/taxiway closure.
  • Treat all ALS circuits as energised until confirmed isolated and locked out at the regulator/CCR.
  • Where ALS structures are over water, wetland, or difficult terrain, follow the site's specific access and fall-protection procedures before approaching any fixture.
Diagnostic Procedure
  1. ObservationCheck the ALCMS/CCR panel to confirm whether the ALS regulator is commanded on and whether it is showing an output fault, a feedback fault, or genuinely zero output.
  2. MeasurementConfirm incoming supply to the ALS regulator/CCR and the main feeder breaker status before assuming a field-side fault.
  3. ObservationIf the regulator/CCR is confirmed healthy and outputting current, inspect the main ALS feeder cable route and control cabinet for a fault preventing power reaching the field.
  4. DecisionRegulator/CCR itself faulted or not commanded → route to Division 01 (CCR Troubleshooting). Regulator/CCR healthy and outputting but field remains dark → main feeder cable or field distribution fault, continue below.
  5. Fault LocationIsolate the fault to the regulator/CCR, the main feeder cable/control cabinet, or a full-system field distribution fault.
  6. Root CauseConfirm the specific point of failure before restoring the system, given the safety-critical nature of a complete ALS outage during low-visibility approach operations.
Test Points
ALS regulator/CCR output current and status
Main feeder breaker and supply status
Main feeder cable insulation resistance
ALCMS command vs. feedback status
Root Cause Verification

After repair, re-energise the system and confirm every crossbar, centreline barrette, and sequenced flasher operates correctly at all commanded intensity steps during a full night inspection.

Corrective Action
  • Reset or repair the tripped breaker or faulted regulator/CCR.
  • Repair the identified main feeder cable fault.
  • Restore the ALCMS command path if the fault was a control/communication issue rather than a power fault.
  • Escalate immediately to airfield operations given the operational significance of a complete ALS outage.
Preventive Action
  • Maintain a tested backup/standby power path for the ALS given its criticality to low-visibility approach operations.
  • Include full-system ALS energisation as part of the routine night flight-check.
  • Periodically megger the main feeder cable to catch a developing insulation fault before it causes a complete outage.
Related References

SOP: SOP-ALS-01 (ALS Fault Isolation Procedure), SOP-CCR-04 (CCR LOTO Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5340-30, FAA AC 150/5345-46 (ALSF-2/MALSR series)

Diagram: ALS System Circuit and Structure Layout Drawing

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

Rev 0 — Initial issue
ALS-272 Partial Approach Light Failure

Symptom — A defined section of the ALS — for example one or more crossbars, a segment of the centreline barrette run, or the sequenced flasher string — is dark or malfunctioning while the rest of the system operates normally.

Equipment / System

The affected ALS sub-circuit or structural section (crossbar group, centreline segment, or flasher branch) and its dedicated feeder from the control cabinet.

Possible Causes
HighA sub-circuit breaker or fuse specific to the affected section tripped or blown
HighA cable fault on the feeder branch serving only the affected section
MediumA junction box or distribution point serving that section showing a connection fault
MediumIsolation transformer failure at a specific structure if the affected section is fed as an individual series-circuit loop
LowA control cabinet relay or contactor specific to that section's control channel failed
Required Tools
Clamp-on ammeterTrue-RMS multimeter1000V/2500V insulation resistance testerALS sub-circuit schematic showing section breakdownSeries circuit fault locator / TDR if the affected section is series-wired
Safety Precautions
  • LOTO the affected sub-circuit at the control cabinet before working on it, even though the rest of the system remains live.
  • Clearly identify and isolate only the affected branch to avoid disrupting the operating sections of the ALS.
Diagnostic Procedure
  1. ObservationConfirm the exact extent of the affected section from a night inspection and cross-reference against the ALS structural/circuit layout drawing to identify which sub-circuit or branch is involved.
  2. ObservationCheck the control cabinet for a tripped breaker, blown fuse, or fault indication specific to that section's control channel.
  3. MeasurementWith the sub-circuit isolated, megger the feeder cable to that section and inspect the relevant junction box/distribution point.
  4. DecisionBreaker/fuse trips again on reset → downstream fault (cable, junction box, or fixture), continue field investigation. Breaker/fuse holds after reset → likely a transient event or nuisance trip; monitor closely.
  5. Fault LocationIsolate to the specific sub-circuit protective device, feeder cable segment, junction box, or fixture group affected.
  6. Root CauseConfirm the specific component before returning the section to service, given the importance of complete approach light coverage for low-visibility operations.
Test Points
Sub-circuit breaker/fuse status
Feeder cable insulation resistance to the affected section
Junction box/distribution point condition
Control cabinet relay/contactor status for that channel
Root Cause Verification

Re-energise the section and confirm it matches the operating pattern (steady, sequenced, or intensity-stepped as applicable) of the rest of the system before closing out.

Corrective Action
  • Reset or replace the tripped/blown protective device.
  • Repair the identified feeder cable fault.
  • Repair or replace the faulted junction box connection.
  • Replace a failed control cabinet relay/contactor for that channel.
Preventive Action
  • Include sub-circuit breaker and protective device checks in the periodic maintenance programme.
  • Maintain an accurate, current sub-circuit breakdown drawing to speed section-level fault isolation.
  • Trend nuisance trips on any given sub-circuit as an early indicator of a developing fault.
Related References

SOP: SOP-ALS-01 (ALS Fault Isolation Procedure), SOP-ALS-02 (Sub-Circuit Isolation Procedure)

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

Diagram: ALS Sub-Circuit Distribution Drawing

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

Rev 0 — Initial issue
ALS-273 Multiple Approach Lights Failure

Symptom — Two or more individual ALS fixtures — crossbar lights, centreline barrette elements, or flasher heads — are dark or malfunctioning together within an otherwise energised section, distinct from a full sub-circuit outage.

Equipment / System

The group of individual ALS fixtures affected and the series/parallel wiring segment feeding them within their sub-circuit.

Possible Causes
HighA shared series cable fault or bad connection upstream of the affected fixture group
HighA shared isolation transformer or distribution point failure feeding multiple fixtures
MediumWater ingress into a shared junction box or splice affecting several fixtures downstream
MediumStructural/mechanical damage to a mast or crossbar affecting the wiring to multiple fixtures on that structure
LowCoincidental multiple independent fixture failures — verify this is genuinely a shared-cause fault before assuming a common circuit issue
Required Tools
Series circuit fault locator / TDR1000V/2500V insulation resistance testerClamp-on ammeterALS circuit layout and wiring drawingExcavation permit and utility locate request if cable work requires digging
Safety Precautions
  • LOTO the affected sub-circuit before any cable or connection work.
  • Where the affected fixtures are on an elevated mast/crossbar structure, follow fall-protection and elevated-work procedures for the site.
Diagnostic Procedure
  1. ObservationConfirm the exact fixtures affected and their physical/electrical relationship — same crossbar, same mast, same series loop segment — from the ALS layout drawing.
  2. MeasurementWith the sub-circuit isolated, megger the cable segment feeding the affected fixture group, working from the last known-good point toward the affected group.
  3. ObservationInspect the shared junction box, splice, or distribution point serving the affected fixtures for water ingress or connection failure.
  4. DecisionInsulation resistance fails on the shared segment → cable/joint fault, proceed to physical fault location. Segment tests clear but fixtures remain out → investigate for a shared structural/mechanical cause (e.g., mast damage) instead.
  5. Fault LocationPhysically locate the faulted cable segment, joint, or structural damage point affecting the group.
  6. Root CauseConfirm cause once exposed — cable/joint failure, water ingress, or structural damage — before repair.
Test Points
Shared cable segment insulation resistance
Junction box/splice condition
Structural condition of the mast/crossbar serving the group
CCR/regulator output current before and during isolation
Root Cause Verification

After repair, megger the full affected segment, re-energise, and confirm every fixture in the previously affected group operates correctly.

Corrective Action
  • Repair or replace the faulted cable segment or splice.
  • Repair the shared junction box/distribution point.
  • Address structural damage to the mast/crossbar if identified as the root cause, coordinating with structural inspection as needed.
Preventive Action
  • Include periodic insulation resistance testing of ALS feeder and series segments in the PM programme.
  • Inspect junction boxes and splices on elevated structures for weatherproofing integrity as part of routine maintenance.
  • Coordinate structural inspection of ALS masts/crossbars on a defined interval given their exposure to wind and weather.
Related References

SOP: SOP-ALS-02 (Sub-Circuit Isolation Procedure), SOP-ALS-03 (Series Circuit Fault Location Procedure)

Standards: IEC 60364, ICAO Annex 14 Vol I, FAA AC 150/5345-46

Diagram: ALS Circuit and Structure Wiring Drawing

Case Study: Division 24 — Complete Runway Lighting Failure; Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue
ALS-274 Approach Light Flickering

Symptom — One or more ALS fixtures flicker, pulse, or vary in brightness instead of holding steady output (for steady-burning elements) or a consistent flash characteristic (for sequenced flashers) at the commanded intensity step.

Equipment / System

ALS fixture, its isolation transformer or flasher driver, connector, and the circuit supplying it.

Possible Causes
HighLoose or intermittent connection at the fixture connector, worsened by wind-induced vibration on elevated mast/crossbar structures
HighA partially failed or overheating isolation transformer or flasher driver with an intermittent internal fault
MediumLamp/LED driver instability or a failing flash tube/LED module nearing end of life
MediumIntermittent cable fault (chafed insulation making and breaking contact) rather than a solid open or short
LowCCR/regulator output instability itself rather than a fixture-level fault
LowMoisture inside a fixture or junction box causing an intermittent leakage path
Required Tools
True-RMS multimeter with min/max holdOscilloscope or power quality logger for waveform captureThermal imaging cameraConnector test adaptor as applicable to fixture type
Safety Precautions
  • Treat intermittent faults as live until proven otherwise.
  • Follow elevated-work and fall-protection procedures when inspecting fixtures on masts/crossbars.
  • LOTO the circuit before opening any connection point even if the light appears steady at the time of access.
Diagnostic Procedure
  1. ObservationConfirm whether the flicker/inconsistent flash is on a single fixture or several, and whether it correlates with wind conditions, a particular intensity step, or weather.
  2. MeasurementLog CCR/regulator output current over a period spanning the reported flicker to rule out a source-side instability.
  3. MeasurementWith the circuit isolated, inspect and re-torque the fixture connector, and test the isolation transformer or flasher driver under light mechanical flexing to try to reproduce an intermittent fault.
  4. ObservationUse thermal imaging on the connector and transformer/driver during a live period to spot a hot connection consistent with intermittent high resistance.
  5. DecisionFlicker follows CCR/regulator current logging → source-side issue, route to Division 01. Flicker isolated to the fixture and reproducible by flexing the connector → connector, transformer, or driver fault.
  6. Fault LocationIsolate to the connector, transformer/driver, lamp/LED/flash-tube module, or cable.
  7. Root CauseConfirm by replacing the suspect component and re-testing under conditions that previously produced the flicker, including a windy period if wind-induced vibration was implicated.
Test Points
Connector contact resistance
Isolation transformer/flasher driver under mechanical flex
CCR/regulator output current trend log
Lamp/LED/flash-tube module bench test
Root Cause Verification

Monitor the repaired fixture over several operating cycles, including at least one period of the wind or weather condition previously associated with the flicker, before closing out.

Corrective Action
  • Clean, re-grease, and properly seat the connector.
  • Replace a transformer/driver confirmed to have an intermittent internal fault.
  • Replace a failing lamp/LED/flash-tube module.
  • Repair the identified intermittent cable fault.
Preventive Action
  • Torque-check connectors as a standard step in periodic maintenance, given the wind-vibration exposure of elevated ALS structures.
  • Include thermal imaging of connectors and drivers in the periodic PM cycle.
  • Trend CCR/regulator output logs to distinguish source-side from fixture-side intermittent issues early.
Related References

SOP: SOP-ALS-01 (ALS Fault Isolation Procedure)

Standards: FAA AC 150/5345-46 (ALSF-2/MALSR), ICAO Annex 14 Vol I

Diagram: ALS Circuit Layout Drawing

Case Study: Division 01 — CCR Troubleshooting (intensity step control)

Rev 0 — Initial issue
ALS-275 Approach Light Dim

Symptom — An ALS fixture illuminates but appears noticeably dimmer than adjacent fixtures on the same crossbar or centreline run at the same commanded intensity step.

Equipment / System

ALS fixture optical/lamp assembly, isolation transformer or driver output, and lens condition.

Possible Causes
HighLens contaminated with dirt, salt spray, or weathering film, particularly on approach structures exposed to open terrain or coastal conditions
HighIsolation transformer/driver secondary output below rated value due to partial internal fault
MediumLamp/LED module degraded or nearing end of life without having fully failed
MediumIncorrect or mismatched transformer/driver rating installed at a previous replacement
LowSeries circuit current below the correct value for the commanded step, affecting the whole loop rather than one fixture
LowLens misaligned so peak beam intensity is directed away from the design approach sightline
Required Tools
Light output/luminance meterTrue-RMS multimeterClamp-on ammeterLens cleaning kitIsolation transformer/driver test set
Safety Precautions
  • LOTO the circuit before removing or cleaning a lens or accessing the fixture interior.
  • Follow elevated-work procedures on masts/crossbars, and allow the fixture to cool before handling if recently energised at a high intensity step.
Diagnostic Procedure
  1. ObservationCompare the reported fixture's apparent brightness against neighbouring fixtures on the same crossbar/run at the same commanded step, ideally with a luminance meter.
  2. MeasurementConfirm the circuit current at the CCR/regulator matches the correct value for the commanded intensity step, to rule out a circuit-wide cause.
  3. ObservationInspect the lens for contamination, weathering, or degradation, and clean if required.
  4. MeasurementWith the circuit isolated, check the isolation transformer/driver secondary output and confirm its rated wattage/tap matches the design.
  5. DecisionCircuit current correct and lens clean but output still low → transformer/driver or lamp/LED degradation. Lens found heavily contaminated → cleaning likely resolves it; re-check before replacing parts.
  6. Fault LocationIsolate to lens contamination, transformer/driver output, or lamp/LED degradation.
  7. Root CauseConfirm with a before/after luminance reading once the identified cause is addressed.
Test Points
Fixture luminance output vs. neighbouring fixtures
Circuit current at CCR/regulator
Isolation transformer/driver secondary voltage/wattage
Lens transmission condition
Root Cause Verification

Re-measure luminance output after corrective action and compare against neighbouring fixtures and the manufacturer's rated output for the commanded step.

Corrective Action
  • Clean the lens assembly.
  • Replace an isolation transformer/driver with an incorrect or degraded rating.
  • Replace a degraded lamp/LED module.
  • Correct any lens misalignment found during inspection.
Preventive Action
  • Include lens cleaning in the routine ALS maintenance cycle, given exposure to open-terrain weathering beyond the runway threshold.
  • Verify transformer/driver rating matches the specified lamp/LED module at every replacement.
  • Track luminance readings over time to catch gradual degradation before it becomes a reported fault.
Related References

SOP: SOP-ALS-04 (Lens Cleaning and Inspection Procedure)

Standards: ICAO Annex 14 Vol I (light intensity requirements), FAA AC 150/5340-30

Diagram: ALS Photometric Specification Sheet

Case Study: Division 26 — Photometric & Intensity Compliance

Rev 0 — Initial issue
ALS-276 Approach Light Wrong Color

Symptom — An ALS fixture shows a colour other than specified for its position — for example, a white steady-burning light showing amber/red-tinted output, or a fixture in the last 300 m section not showing the correct red side-row colour where applicable.

Equipment / System

ALS fixture lens/filter and lamp/LED module colour characteristics relative to its position in the approach lighting configuration (e.g., red terminating/side row barrettes in ALSF-2 systems).

Possible Causes
HighLens discolouration/fading from UV degradation or weathering over the fixture's service life
HighWrong lamp/LED module or colour filter fitted — a mismatch installed by mistake at a previous replacement
MediumContamination film producing an apparent colour shift
MediumFixture installed in the wrong position relative to the ALS colour scheme (e.g., a red side-row/terminating-bar fixture installed in a white steady-burning position)
LowLens cracked or partially melted, altering colour transmission
Required Tools
Colour temperature meter (if available)Torch/inspection lightReplacement lens/filter and correct-specification lamp/LED moduleALS colour-scheme layout drawing
Safety Precautions
  • LOTO the circuit before removing the lens or accessing the fixture internals.
  • Follow elevated-work procedures on masts/crossbars during inspection.
Diagnostic Procedure
  1. ObservationConfirm the colour discrepancy visually against neighbouring fixtures and against the ALS colour-scheme layout drawing for that fixture's specific position.
  2. ObservationInspect the lens/filter for discolouration, contamination, or damage, and clean before further diagnosis if deposits are present.
  3. MeasurementWith the circuit isolated, confirm the lamp/LED module and filter fitted match the specified part number and colour for this position in the ALS configuration.
  4. DecisionLens discoloured/contaminated but correct module fitted → clean or replace lens/filter. Lens clear but wrong module/filter or fixture position → replace with correct-specification part, or relocate if a position error is confirmed.
  5. Fault LocationIsolate to lens/filter condition, lamp/LED module mismatch, or fixture-position error.
  6. Root CauseConfirm the corrected fixture matches its designed colour for that position during a night inspection.
Test Points
Lens/filter condition and clarity
Lamp/LED module part number and colour specification
Fixture position vs. ALS colour-scheme drawing
Comparative colour appearance vs. neighbouring fixtures
Root Cause Verification

Visually confirm the correct colour is displayed at the correct position during a night inspection, alongside the fixtures immediately either side.

Corrective Action
  • Clean or replace the discoloured/contaminated lens/filter.
  • Replace a mismatched lamp/LED module with the correct specification.
  • Relocate the fixture if it is confirmed to be in the wrong position for the colour scheme.
Preventive Action
  • Include lens/filter colour checks in the periodic maintenance schedule.
  • Verify lamp/LED module and filter part numbers against specification before stocking or fitting.
  • Maintain the ALS colour-scheme layout drawing current, particularly if the approach configuration has been modified.
Related References

SOP: SOP-ALS-04 (Lens Cleaning and Inspection Procedure)

Standards: ICAO Annex 14 Vol I (approach lighting colour requirements), FAA AC 150/5340-30

Diagram: ALS Colour-Scheme Layout Drawing

Case Study: Division 26 — Photometric & Intensity Compliance

Rev 0 — Initial issue
ALS-277 Approach Light Intensity Step Failure

Symptom — The ALS fails to change output correctly when a different intensity step is commanded — either remaining at a fixed intensity, failing to reach the commanded step, or responding incorrectly compared to neighbouring circuits.

Equipment / System

ALS regulator/CCR step control output and the fixture/transformer response across the system's intensity range.

Possible Causes
HighRegulator/CCR step-control fault — a specific tap or output stage not functioning correctly
HighALCMS command not correctly reaching the regulator for a specific step
MediumA fixture or transformer group mismatched to a specific step's current level, causing correct steps elsewhere but incorrect response at this fixture
MediumA control relay or contactor within the regulator dedicated to a specific step failed
LowFeedback/monitoring fault reporting an incorrect step status while actual field output is correct — verify with an independent field measurement before assuming a genuine fault
Required Tools
Clamp-on ammeterTrue-RMS multimeterALCMS command/feedback log accessRegulator/CCR step-control schematic
Safety Precautions
  • LOTO the regulator/CCR before any internal inspection.
  • Coordinate step-change testing with ATC, as commanding intensity changes during testing affects the operational lighting state.
Diagnostic Procedure
  1. ObservationCommand each intensity step in turn and record actual field current/output at the regulator, comparing against the expected value for each step.
  2. MeasurementCross-check the ALCMS commanded step against the regulator's actual output using an independent ammeter reading, to distinguish a genuine field fault from a feedback/reporting fault.
  3. ObservationIf a specific step is confirmed faulty at the regulator, inspect the corresponding internal stage, relay, or contactor for that step.
  4. DecisionField output confirmed incorrect for a specific step → regulator internal fault, escalate to Division 01. Field output confirmed correct but ALCMS reports it incorrectly → feedback/monitoring fault, route to ALS-288.
  5. Fault LocationIsolate to the specific regulator stage/step, the ALCMS command path, or the feedback/monitoring path.
  6. Root CauseConfirm correct operation across every intensity step, not just the one originally reported, before closing out.
Test Points
Field current output for each commanded intensity step
ALCMS commanded step vs. actual field output
Regulator internal stage/relay condition for the affected step
Root Cause Verification

Cycle through every available intensity step and confirm correct field output and correct ALCMS feedback at each before returning the system to normal operation.

Corrective Action
  • Repair or replace the faulted regulator stage, relay, or contactor.
  • Correct the ALCMS command path if the fault is confirmed to be a control/communication issue.
  • Escalate to CCR-level troubleshooting per Division 01 if the fault is within the regulator itself.
Preventive Action
  • Include a full intensity-step cycle test in the periodic maintenance and night flight-check programme, not just a single-step check.
  • Log and trend step-change response times and accuracy to catch a developing regulator fault early.
Related References

SOP: SOP-ALS-01 (ALS Fault Isolation Procedure), SOP-CCR-08 (Intensity Step Verification)

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

Diagram: ALS Regulator Step-Control Schematic

Case Study: Division 01 — CCR Troubleshooting (Intensity Step Control)

Rev 0 — Initial issue
ALS-278 Approach Light Circuit Failure

Symptom — A defined ALS circuit — a series loop, sub-circuit, or feeder branch — shows a confirmed electrical fault (open circuit, short circuit, or protection trip) at the circuit level, distinct from an individual fixture or cable-segment fault already isolated to a single point.

Equipment / System

The affected ALS circuit as a whole, including its protective device, feeder, and the fixture group it serves.

Possible Causes
HighProtective device (breaker/fuse) tripped or failed for the affected circuit
HighA short circuit somewhere on the circuit loading down or tripping protection
MediumAn open circuit at any point in the loop preventing current flow to the fixture group
MediumA regulator output stage feeding this specific circuit faulted
LowA control/command fault preventing the circuit from being energised even though the circuit itself is electrically sound
Required Tools
Clamp-on ammeterTrue-RMS multimeter1000V/2500V insulation resistance testerSeries circuit fault locator / TDRALS circuit schematic
Safety Precautions
  • LOTO the affected circuit at the regulator/control cabinet before investigation.
  • Do not attempt to reset a tripped protective device more than once without investigating the cause — repeated reset attempts on a genuine fault risk further damage.
Diagnostic Procedure
  1. ObservationConfirm the protective device status and reported symptom (no output, tripped breaker, partial output) at the control cabinet.
  2. MeasurementWith the circuit isolated, megger the full circuit loop to check for a short circuit or insulation breakdown, and check for continuity to confirm there is no open circuit.
  3. ObservationIf insulation resistance and continuity are both acceptable, inspect the regulator output stage and control/command path for this circuit.
  4. DecisionMegger fails or continuity is broken → field-side cable/fixture fault, proceed to physical fault location per ALS-273/279. Circuit tests electrically sound → regulator or control-side fault, escalate to Division 01.
  5. Fault LocationIsolate to a field-side circuit fault or a regulator/control-side fault.
  6. Root CauseConfirm the specific cause before re-energising, particularly if a protective device tripped, to avoid repeated nuisance trips or further damage.
Test Points
Circuit insulation resistance (full loop)
Circuit continuity
Protective device trip history/status
Regulator output stage for this circuit
Root Cause Verification

After repair, re-energise the circuit and confirm the protective device holds under normal load, and that every fixture on the circuit operates correctly across all intensity steps.

Corrective Action
  • Reset the protective device once the underlying fault is repaired.
  • Repair the identified cable/fixture fault causing the short or open circuit.
  • Repair or replace the faulted regulator output stage if the issue is source-side.
Preventive Action
  • Include full-circuit megger and continuity testing in the periodic PM programme for each ALS circuit.
  • Investigate any protective device trip fully before reset, rather than assuming a nuisance trip.
  • Maintain accurate circuit schematics reflecting the current as-built configuration.
Related References

SOP: SOP-ALS-02 (Sub-Circuit Isolation Procedure), SOP-ALS-03 (Series Circuit Fault Location Procedure)

Standards: IEC 60364, ICAO Annex 14 Vol I

Diagram: ALS Circuit Schematic

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

Rev 0 — Initial issue
ALS-279 Approach Light Cable Failure

Symptom — A confirmed insulation or conductor fault in ALS feeder or series cable, requiring cable-level repair or replacement, distinct from a connector or fixture fault.

Equipment / System

ALS feeder and series cable routed through ducts, pull boxes, and structures across the approach lighting area, which frequently spans unpaved, wetland, or difficult-access terrain beyond the runway threshold.

Possible Causes
HighInsulation breakdown from age, moisture tracking, or cumulative thermal cycling under load
HighMechanical damage from ground movement, vehicle access tracks, or wildlife/rodent activity in the often less-controlled terrain beyond the threshold
MediumA degraded or improperly made field splice/joint failing under load or moisture exposure
MediumFlood or standing-water exposure in low-lying or wetland approach areas degrading buried cable over time
LowManufacturing defect or an original installation with an undersized bend radius causing insulation stress over time
Required Tools
1000V/2500V/5000V insulation resistance tester as appropriate to cable ratingSeries circuit fault locator / TDRALS cable route and joint location drawingExcavation permit and utility locate request if digging is requiredCable splicing kit rated for the cable classSite access equipment appropriate to the terrain (e.g., all-terrain vehicle, boat, or walkway access for wetland sections)
Safety Precautions
  • LOTO the regulator/CCR before any cable testing or repair work.
  • Obtain excavation clearance and utility locates before opening any pavement, verge, or ground in the approach area.
  • Assess terrain and access conditions before dispatching personnel to remote sections of the approach lighting area, particularly wetland or flood-prone sections.
Diagnostic Procedure
  1. ObservationConfirm from circuit-level symptoms and prior fault-location work that the fault is confirmed to be in the cable itself rather than a fixture, connector, or transformer.
  2. MeasurementMegger the cable in accessible segments from pull boxes or structure bases, and use a TDR/fault locator to pinpoint distance to fault along the identified segment.
  3. ObservationOnce physically exposed, inspect the cable and any joints for the specific failure mode — insulation breakdown, mechanical damage, or a failed splice.
  4. DecisionDamage isolated to a short section → repair by splice. Cable degraded over an extended length, of an age/condition warranting it, or in a flood-prone section with repeated failure history → replace the full segment rather than repeated spot repairs.
  5. Fault LocationConfirm the precise fault location by physical exposure before cutting or splicing.
  6. Root CauseDetermine and record the failure mode (age, mechanical damage, water exposure, joint failure) to inform whether adjacent cable sections need proactive attention.
Test Points
Cable insulation resistance by segment
TDR distance-to-fault reading
Splice/joint condition at exposed points
Duct/route condition, including flood or ground-movement exposure
Root Cause Verification

After repair, megger the full repaired segment and the adjoining sections, re-energise, and confirm all fixtures on the loop operate correctly across all intensity steps.

Corrective Action
  • Repair the cable by a correctly rated splice where damage is localised.
  • Replace the full cable segment where damage or age warrants it.
  • Restore duct protection where mechanical damage was the cause.
  • Consider elevated or protected cable routing for chronically flood-prone approach sections.
Preventive Action
  • Include periodic insulation resistance testing and trending of ALS feeder and series cable in the PM programme, given the criticality and often difficult access of the approach lighting area.
  • Maintain accurate as-built cable route and splice records covering the full approach lighting corridor.
  • Review drainage or flood exposure for any section with a repeated cable failure history.
Related References

SOP: SOP-ALS-03 (Series Circuit Fault Location Procedure), SOP-ALS-05 (Cable Splice and Repair Procedure)

Standards: IEC 60502 (power cable), ICAO Annex 14 Vol I, FAA AC 150/5340-30

Diagram: ALS Cable Route and Structure Drawing

Case Study: Division 24 — Complete Runway Lighting Failure

Rev 0 — Initial issue
ALS-280 Approach Light Transformer Failure

Symptom — The individual isolation transformer feeding an ALS fixture has failed — open-circuit, short-circuit, or significantly out of tolerance on secondary output — distinct from a fixture lamp/LED or connector fault.

Equipment / System

ALS series-circuit isolation transformer (rated per the specific fixture type — crossbar, centreline barrette, or flasher unit — within the ALSF-2/MALSR configuration).

Possible Causes
HighWinding failure from age-related insulation breakdown, particularly in transformers exposed to repeated moisture at exposed mast/crossbar locations
HighInternal short circuit from moisture tracking across the windings
MediumThermal damage from prolonged operation at a mismatched (too high) wattage rating for the fitted lamp/LED module
MediumPhysical/impact damage to the transformer body or leads during structure/junction box work, or from wind-induced vibration over time
LowManufacturing defect or a transformer that was already marginal at installation
LowCascading damage from a lightning strike, which is a materially elevated risk for tall ALS mast structures compared to inset runway/taxiway fixtures
Required Tools
1000V/2500V insulation resistance testerTrue-RMS multimeterClamp-on ammeterReplacement transformer, correct wattage/tap ratingElevated-access equipment as required for mast/crossbar-mounted transformers
Safety Precautions
  • LOTO the circuit before disconnecting or removing any transformer.
  • Handle a transformer suspected of internal arcing or lightning damage carefully — do not assume it is fully de-energised even after the circuit is isolated, until tested.
  • Follow fall-protection and elevated-work procedures for transformers mounted on masts or crossbars.
Diagnostic Procedure
  1. ObservationConfirm via CCR/regulator and a field inspection that the fault pattern (single dark fixture, no downstream effect) is consistent with a fixture-level transformer fault rather than a broader circuit fault (cross-reference ALS-273).
  2. MeasurementWith the circuit isolated, disconnect the transformer at both primary and secondary connectors and megger primary-to-earth, secondary-to-earth, and primary-to-secondary.
  3. ObservationInspect the transformer body for physical damage, discolouration, or evidence of internal arcing/overheating, and note if recent lightning activity in the area could be a contributing factor.
  4. MeasurementConfirm the transformer's wattage/tap rating matches the design specification for the fitted lamp/LED module.
  5. DecisionTransformer fails insulation test or shows physical damage → replace. Transformer tests good but wattage/tap mismatched → replace with correctly rated unit even though the current one has not fully failed.
  6. Fault LocationConfirm the transformer itself as the fault, distinct from the connector or fixture lamp/LED.
  7. Root CauseBench-test the removed transformer if practical to confirm the specific failure mode for the maintenance record, noting any suspected lightning-related cause for the incident log.
Test Points
Transformer primary winding insulation resistance
Transformer secondary winding insulation resistance
Primary-to-secondary insulation resistance
Transformer wattage/tap vs. design specification
Root Cause Verification

After replacement with the correctly rated transformer, re-energise and confirm the fixture operates correctly across all intensity steps, and re-check insulation resistance at the next scheduled maintenance visit.

Corrective Action
  • Replace the failed transformer with a correctly rated unit.
  • Correct a wattage/tap mismatch even where the existing transformer has not yet fully failed.
  • Verify surge/lightning protection at the structure if lightning damage is suspected as the cause.
Preventive Action
  • Include transformer insulation resistance testing in the periodic PM programme, trending results to catch degrading units before failure.
  • Verify surge protection devices at ALS structures are in good condition given their lightning exposure.
  • Maintain correctly rated transformer spares on hand.
Related References

SOP: SOP-ALS-01 (ALS Fault Isolation Procedure), SOP-ALS-06 (Transformer Test and Replacement Procedure)

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

Diagram: ALS Circuit Design Specification

Case Study: Division 01 — CCR Troubleshooting (series circuit fundamentals)

Rev 0 — Initial issue
ALS-281 Approach Light Connector Failure

Symptom — A fixture, transformer, or junction connector within the ALS is found failed — corroded, cracked, arced, or not maintaining a reliable connection.

Equipment / System

Fixture, transformer, and junction connectors within ALS structure bases, junction boxes, and mast-mounted enclosures.

Possible Causes
HighWater/condensation ingress into the connector body leading to corrosion and increased contact resistance, particularly at exposed elevated connections
HighConnector not fully mated or seated during a previous installation or maintenance visit
MediumConnector boot/seal perished or damaged, allowing moisture and contamination in over time
MediumArcing damage from a marginal connection carrying current under partial contact for an extended period, aggravated by wind-induced vibration on mast/crossbar structures
LowWrong connector type or rating fitted, mismatched to the cable/fixture combination
LowPhysical damage to the connector body from structure/maintenance work
Required Tools
Connector test adaptor as applicable1000V insulation resistance testerDielectric greaseReplacement connector, correct type and ratingTorque/crimp tool as specified by connector manufacturer
Safety Precautions
  • LOTO the circuit before disconnecting or inspecting any connector.
  • Treat a connector showing arc/heat damage as a sign of a fault that may have been present for some time — inspect adjacent components for related damage before re-energising.
  • Follow elevated-work procedures for connectors on masts/crossbars.
Diagnostic Procedure
  1. ObservationWith the circuit isolated, open the relevant enclosure and visually inspect the connector for corrosion, cracking, arc marks, discolouration, or an incompletely mated condition.
  2. MeasurementMegger across the connector and check continuity/contact resistance if a suitable test adaptor is available.
  3. ObservationInspect the connector boot/seal for damage and check whether water was present in the enclosure (cross-reference ALS-290).
  4. DecisionConnector shows corrosion/arc damage or fails a contact-resistance check → replace. Connector appears sound but was simply not fully mated → reseat correctly with fresh dielectric grease and re-test.
  5. Fault LocationIsolate to the specific connector, and to the connector itself versus the boot/seal.
  6. Root CauseConfirm the corrected connection holds under a follow-up inspection, particularly after the next significant weather event if water ingress was implicated.
Test Points
Connector contact resistance/continuity
Insulation resistance across the connector
Boot/seal condition
Enclosure water presence
Root Cause Verification

Re-test insulation resistance and contact integrity after reconnection, and schedule a follow-up check after the next significant weather event to confirm the seal is holding.

Corrective Action
  • Replace the failed connector with the correct type and rating.
  • Reseat and re-grease a connector found simply mismated.
  • Replace a perished boot/seal even if the connector body itself is serviceable.
Preventive Action
  • Apply dielectric grease at every connector mating as standard practice.
  • Include connector contact-resistance checks in the periodic maintenance programme, given the vibration exposure of elevated ALS structures.
  • Stock the correct connector types and boots as standard spares.
Related References

SOP: SOP-ALS-01 (ALS Fault Isolation Procedure), SOP-ALS-07 (Connector Maintenance Procedure)

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

Diagram: ALS Structure and Enclosure Wiring Detail Drawing

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue
ALS-282 Approach Light Alignment Error

Symptom — An ALS fixture (crossbar element, centreline barrette, or flasher head) is correctly powered and mechanically sound, but is not correctly aimed relative to the approach path and design sightline, reducing apparent intensity to an approaching aircraft.

Equipment / System

ALS fixture mounting/aiming on its mast, crossbar, or ground-level frangible base relative to the design approach azimuth and elevation.

Possible Causes
HighFixture rotated or tilted on its mount during installation or after a maintenance disturbance
HighStructural movement of the mast/crossbar (settlement, wind loading fatigue, ground movement in soft approach-area terrain) shifting the reference position
MediumFixture mounting hardware not correctly torqued, allowing drift under wind loading over time
MediumOriginal installation aiming not correctly set against the design approach azimuth/elevation reference
LowFrangible coupling partially sheared, tilting the fixture without fully failing
Required Tools
Inclinometer/level and azimuth reference toolFixture alignment jig (if specified by manufacturer)Approach lighting survey/aiming reference documentationTorque wrench for mounting fixingsElevated-access equipment as required
Safety Precautions
  • LOTO the circuit before adjusting or removing the fixture for realignment.
  • Follow fall-protection and elevated-work procedures for any fixture on a mast or crossbar structure.
  • Coordinate any realignment work through the same approach-area access and clearance process as other ALS maintenance.
Diagnostic Procedure
  1. ObservationConfirm the alignment issue via a night inspection from the design approach direction, comparing the affected fixture's apparent brightness/beam direction against neighbours and against the aiming reference documentation.
  2. MeasurementCheck fixture level/inclination and azimuth against the manufacturer's specified tolerance and the original design aiming data.
  3. ObservationInspect the mast/crossbar structure for settlement, movement, or fatigue that may have shifted the reference position for the fixture.
  4. DecisionFixture mount rotated/mis-torqued but structure position correct → realign and re-torque the fixture. Structure itself has moved or is compromised → escalate to structural inspection under ALS-283/284 before or alongside fixture realignment.
  5. Fault LocationIsolate to fixture mounting/aiming on an otherwise sound structure, or to a structural movement issue affecting the reference position.
  6. Root CauseConfirm the corrected alignment restores the design beam pattern from the intended approach sightline.
Test Points
Fixture level/inclination and azimuth vs. specification
Structure (mast/crossbar) condition and position
Comparative apparent brightness vs. neighbouring fixtures
Beam pattern check from the design approach sightline
Root Cause Verification

Re-inspect from the design approach sightline at night after realignment, and confirm the fixture remains correctly aligned after the next scheduled maintenance interval, particularly following any significant wind event.

Corrective Action
  • Realign and re-torque the fixture to the correct level and azimuth.
  • Escalate to structural assessment if mast/crossbar movement is identified as the underlying cause.
  • Update the aiming reference documentation if the original design data is found to be inaccurate.
Preventive Action
  • Torque-check fixture mountings as part of the periodic maintenance schedule, given the wind-loading exposure of elevated ALS structures.
  • Include periodic structural condition surveys of masts and crossbars.
  • Include a beam-alignment spot-check in the periodic night inspection, especially after severe weather.
Related References

SOP: SOP-ALS-08 (Fixture Alignment Verification Procedure)

Standards: ICAO Annex 14 Vol I Appendix 2 (approach light beam characteristics), FAA AC 150/5340-30

Diagram: ALS Aiming and Alignment Reference Drawing

Case Study: Division 26 — Photometric & Intensity Compliance

Rev 0 — Initial issue
ALS-283 Approach Light Pole Damage

Symptom — An ALS support pole (mast supporting a crossbar or individual fixture) is found physically damaged — bent, cracked, corroded, or partially collapsed — distinct from wider structural damage affecting the crossbar or platform itself (ALS-284).

Equipment / System

ALS support pole/mast, its frangible base coupling, and immediate foundation.

Possible Causes
HighWind loading fatigue or storm damage, given the height and exposure of ALS poles in open approach terrain
HighVehicle or maintenance equipment impact during ground-level access work
MediumCorrosion-weakened pole material failing under normal wind or structural loading, particularly in coastal or high-humidity environments
MediumFrangible coupling activated (as designed) by an aircraft strike or a vehicle exceeding the coupling's intended load path
LowManufacturing or installation defect in the pole or coupling
Required Tools
Visual inspection kitStructural assessment tools (as required, potentially involving structural engineering input for significant damage)Torque wrench and pole-specific toolingReplacement pole/frangible coupling assembly, correct specificationElevated-access and rigging equipment as required for pole replacement
Safety Precautions
  • LOTO the circuit before working on or near a damaged pole.
  • Treat a significantly damaged or leaning pole as a structural safety hazard — barricade the area and restrict access until assessed.
  • Follow fall-protection and rigging procedures for any pole replacement or major repair work.
Diagnostic Procedure
  1. ObservationDocument the extent and apparent cause of the pole damage, photographing for the maintenance and incident record, and barricade the area if there is any risk of further collapse.
  2. MeasurementWith the circuit isolated, check the condition of the wiring and connectors at the pole base and along its length, since pole damage often accompanies wiring disturbance.
  3. ObservationAssess whether the frangible coupling has functioned as designed (clean shear at the intended break point) or whether the damage is a structural failure of the pole itself.
  4. DecisionFrangible coupling sheared cleanly, pole itself undamaged above the break → straightforward coupling/fixture repair. Pole structurally damaged (bent, cracked, corroded) → full pole replacement required, likely with structural engineering input for significant cases.
  5. Fault LocationIsolate the damage to the frangible coupling, the pole structure itself, or the base/foundation.
  6. Root CauseWhere damage is wind/storm-related and not a designed frangible activation, report to airfield operations and consider whether pole specification or foundation design needs review for this location.
Test Points
Frangible coupling condition — clean break vs. structural failure
Pole structural integrity (bend, crack, corrosion extent)
Wiring and connector condition at the pole base and along its length
Foundation condition
Root Cause Verification

After pole repair or replacement, confirm correct electrical operation (megger and functional test) and correct fixture alignment per ALS-282 before returning the section to service.

Corrective Action
  • Replace the damaged pole and/or frangible coupling assembly.
  • Repair any wiring or connector damage found along the pole.
  • Realign the fixture per ALS-282 as part of the reinstallation.
  • Escalate to structural engineering for foundation or specification review if damage indicates an underlying design or condition issue.
Preventive Action
  • Include pole structural condition in the periodic visual inspection, with particular attention after severe weather events.
  • Investigate recurring pole damage at a given location for a wind-exposure or foundation-design cause.
  • Maintain pole and frangible coupling spares given the exposure of ALS structures to open-terrain weather.
Related References

SOP: SOP-ALS-09 (Pole Replacement Procedure), SOP-ALS-10 (Frangible Coupling Inspection)

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

Diagram: ALS Pole and Frangible Coupling Assembly Drawing

Case Study: Division 27 — FOD & Physical Damage Case Studies

Rev 0 — Initial issue
ALS-284 Approach Light Structural Damage

Symptom — A wider ALS structural element — a crossbar, elevated platform, walkway, or support framework carrying multiple fixtures — is found damaged, distinct from damage isolated to a single support pole (ALS-283).

Equipment / System

ALS crossbar, elevated platform/walkway, and support framework carrying multiple fixtures and their associated wiring.

Possible Causes
HighWind/storm loading exceeding the structure's design tolerance, particularly for larger crossbar assemblies
HighCorrosion-weakened structural members failing under normal loading over an extended service life
MediumFatigue cracking at welds or connection points from repeated wind-induced vibration
MediumImpact damage from maintenance equipment, vehicles, or (in low-lying approach areas) flood debris
LowOriginal design or construction defect becoming apparent over the structure's service life
Required Tools
Visual inspection kitStructural engineering assessment (required for significant structural damage)Elevated-access and rigging equipmentTorque wrench and structure-specific toolingReplacement structural components as specified by structural engineering assessment
Safety Precautions
  • LOTO the circuit serving all fixtures on the affected structure before any work.
  • Treat significant structural damage as a safety hazard requiring barricading and access restriction until a qualified assessment is complete.
  • Do not attempt fixture-level repairs on a structurally compromised platform/crossbar until the structure itself has been assessed and, if necessary, repaired or shored.
Diagnostic Procedure
  1. ObservationDocument the extent and apparent cause of the structural damage, photographing for the maintenance and incident record, and immediately restrict access to the affected structure.
  2. ObservationArrange a structural engineering assessment for any damage beyond minor, cosmetic issues, given the safety implications of a compromised elevated structure carrying multiple energised fixtures.
  3. MeasurementWith the circuit isolated, check wiring and connector condition across the affected structure once access is confirmed safe, since structural damage often disturbs multiple fixture connections simultaneously.
  4. DecisionDamage confirmed minor/cosmetic by assessment → proceed with standard fixture-level repair. Damage confirmed structurally significant → full structural repair or replacement required before fixtures are reinstated.
  5. Fault LocationIsolate the extent of structural damage and confirm which fixtures/wiring runs are affected.
  6. Root CauseDetermine the cause (wind/storm, corrosion, fatigue, impact) to inform whether design or maintenance interval changes are warranted for this or similar structures.
Test Points
Structural integrity assessment (qualified engineering input)
Wiring and connector condition across the affected structure
Foundation condition
Corrosion/fatigue extent at connection points
Root Cause Verification

After structural repair and re-certification (where a formal assessment was required), confirm correct electrical operation and alignment of every fixture on the structure before returning it to service.

Corrective Action
  • Complete structural repair or replacement per the engineering assessment.
  • Repair wiring and connector damage across the affected structure.
  • Realign and re-verify every fixture on the repaired structure.
  • Review structure specification or maintenance interval if the root cause indicates a systemic issue.
Preventive Action
  • Include a structural condition survey of ALS crossbars/platforms on a defined interval, with particular attention after severe weather.
  • Coordinate with structural engineering proactively for any ALS structure approaching or exceeding its design service life.
  • Maintain an incident and repair history for each major ALS structure to track condition trends over time.
Related References

SOP: SOP-ALS-11 (Structural Damage Assessment and Repair Procedure)

Standards: FAA AC 150/5345-46, ICAO Annex 14 Vol I, applicable structural design codes

Diagram: ALS Structure Engineering Drawing

Case Study: Division 27 — FOD & Physical Damage Case Studies

Rev 0 — Initial issue
ALS-285 Approach Light Optical Failure

Symptom — An ALS fixture receives correct electrical power and the lamp/LED/flash source operates, but the fixture fails to produce the correct beam pattern — light appears scattered, absent in the expected direction, or grossly asymmetric.

Equipment / System

ALS fixture optical assembly — reflector, lens system, and internal beam-forming components, including flash-tube reflector assemblies on sequenced flasher units.

Possible Causes
HighReflector detached, corroded, or coated with condensation/contamination, scattering rather than focusing the beam
HighLens cracked, fogged, or partially melted from a previous overheating event
MediumLamp/LED/flash-tube module not correctly seated in its optical focal position after a replacement
MediumInternal fixture component (baffle, mounting bracket) displaced by wind loading, vibration, or impact, altering the optical path
LowWrong lamp/LED/flash-tube module type fitted, with a different beam geometry than the fixture's optics were designed for
Required Tools
Beam pattern/goniophotometer test set (if available) or field beam-check procedureTorch/inspection lightFixture manufacturer optical assembly diagramReplacement optical componentsElevated-access equipment as required
Safety Precautions
  • LOTO the circuit before opening the fixture for optical inspection.
  • Allow the fixture to cool fully before handling the optical assembly, particularly if overheating is suspected.
  • Follow elevated-work procedures for fixtures mounted on masts/crossbars.
Diagnostic Procedure
  1. ObservationConfirm the light source itself is operating correctly (verify against ALS-273/275) before concluding this is an optical rather than electrical fault.
  2. ObservationWith the circuit isolated, open the fixture and visually inspect the reflector, lens, and (for flasher units) the flash-tube assembly for damage, contamination, or displacement.
  3. MeasurementConfirm the lamp/LED/flash-tube module is correctly seated at the designed focal position per the manufacturer's optical assembly diagram.
  4. DecisionPhysical damage or contamination found on optics → clean or replace the affected component. Optics appear intact but source seating is incorrect → reseat correctly and re-check beam pattern.
  5. Fault LocationIsolate to the reflector, lens, source seating, or an internal displaced component.
  6. Root CauseConfirm by a field beam-pattern check after correction, comparing against a known-good fixture of the same type.
Test Points
Reflector condition and alignment
Lens condition
Lamp/LED/flash-tube module seating position
Comparative beam check against a known-good fixture
Root Cause Verification

Verify beam pattern visually from the design approach sightline (or with a goniophotometer if available) after repair, and re-inspect after several operating cycles, including a windy period, to confirm the fix is stable.

Corrective Action
  • Clean or replace the reflector.
  • Replace a cracked or fogged lens.
  • Reseat the lamp/LED/flash-tube module to the correct focal position.
  • Replace any displaced internal optical component.
Preventive Action
  • Include an internal optical inspection in the periodic deep-clean/major maintenance cycle.
  • Handle optical components with clean gloves during any fixture work to avoid contamination.
  • Use only manufacturer-approved lamp/LED/flash-tube modules matched to the fixture's optical design.
Related References

SOP: SOP-ALS-12 (Optical Assembly Inspection Procedure)

Standards: FAA AC 150/5345-46 (photometric requirements), ICAO Annex 14 Vol I Appendix 2

Diagram: ALS Fixture Optical Assembly Diagram

Case Study: Division 26 — Photometric & Intensity Compliance

Rev 0 — Initial issue
ALS-286 Approach Light Photometric Failure

Symptom — An ALS fixture or section, while electrically and mechanically functional, fails to meet the required photometric output (intensity, beam spread, or flash characteristics for sequenced flashers) when measured against ICAO/FAA standards during a compliance survey.

Equipment / System

ALS fixture photometric performance as measured against ICAO Annex 14 Appendix 2 / FAA AC 150/5340-30 specifications for approach lighting.

Possible Causes
HighCumulative lens contamination or weathering reducing transmitted intensity below the compliance threshold across multiple fixtures
HighAggregate fixture ageing (lamp/LED lumen depreciation, or flash-tube output degradation) across a section approaching or past its rated service life
MediumA batch of fixtures with a systematic transformer/driver wattage or lamp/LED mismatch installed during a previous bulk replacement
MediumBeam or flash-timing alignment drift across multiple fixtures due to structural movement (cross-reference ALS-282/284)
LowSurvey/measurement methodology or equipment calibration issue producing an apparent non-compliance that does not reflect the true fixture output
Required Tools
Calibrated mobile photometric survey unit or handheld luminance/intensity meterFlash-rate/timing measurement equipment for sequenced flasher unitsICAO/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.
  • Follow elevated-work procedures for any fixture-level intervention on masts/crossbars.
Diagnostic Procedure
  1. ObservationConfirm the survey methodology and equipment calibration are valid before treating the result as a genuine fixture-level non-compliance.
  2. ObservationIdentify whether non-compliance is isolated to specific fixtures, a specific installation batch, or the section generally, by cross-referencing survey results against the fixture/transformer inventory and installation dates.
  3. MeasurementPhysically inspect a representative sample of the non-compliant fixtures for lens condition, lamp/LED/flash-tube condition, transformer/driver rating, and beam/timing alignment.
  4. DecisionNon-compliance traces to contamination/weathering → clean and re-survey before replacing components. Non-compliance traces to aged sources or a batch mismatch → plan targeted or bulk replacement. Isolated to alignment → route to ALS-282.
  5. Fault LocationIsolate the dominant contributing cause (contamination, ageing, mismatch, alignment) across the affected fixtures.
  6. Root CauseConfirm compliance is restored with a follow-up photometric re-survey after corrective action.
Test Points
Measured intensity vs. ICAO/FAA minimum for each surveyed fixture
Flash rate/timing for sequenced flasher units
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 neighbouring 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/flash-tube modules that have reached end-of-life output depreciation.
  • Correct a transformer/driver mismatch found across an installation batch.
  • Correct beam or flash-timing alignment on fixtures affected by structural movement.
Preventive Action
  • Schedule photometric compliance surveys on a defined recurring interval rather than only reactively.
  • Track fixture installation dates and expected output-depreciation curves to plan proactive replacement before compliance margins are lost.
  • Verify transformer/driver specification consistency across every bulk fixture procurement or replacement batch.
Related References

SOP: SOP-ALS-13 (Photometric Compliance Survey Procedure)

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

Diagram: ALS Photometric Specification Sheet

Case Study: Division 26 — Photometric & Intensity Compliance

Rev 0 — Initial issue
ALS-287 Approach Light Control Failure

Symptom — The ALS master controller or sequenced flasher control unit fails to correctly command the system — for example, incorrect flash sequencing, failure to synchronise with the runway lighting state, or failure to respond to ALCMS commands — while field power distribution itself is confirmed sound.

Equipment / System

ALS master controller / sequenced flasher control unit and its command interface to the CCR/ALCMS and field circuits.

Possible Causes
HighMaster controller internal fault (processor, power supply, or output card failure)
HighLoss of synchronisation input, causing the sequenced flasher pattern to run incorrectly or not at all
MediumFirmware or configuration fault following a software update or configuration change
MediumCommunication fault between the controller and the ALCMS, distinct from a field power fault
LowA single output channel on the controller failed while the rest of the unit functions correctly
Required Tools
Manufacturer diagnostic software/interface for the controllerTrue-RMS multimeterCommunication network tester (as applicable to the control protocol in use)Controller technical manual and wiring schematic
Safety Precautions
  • LOTO relevant field circuits before working inside the controller cabinet, even though the fault is control-side rather than a field power fault.
  • Coordinate any controller reset, reconfiguration, or firmware work with ATC given the operational impact of an ALS control fault.
Diagnostic Procedure
  1. ObservationConfirm field power distribution and circuit health are sound (cross-reference ALS-271/278) before concluding the fault is control-side rather than a field power issue.
  2. ObservationCheck the controller's status/diagnostic display or software interface for fault codes, communication status, and synchronisation status.
  3. MeasurementVerify the controller's command outputs with a multimeter at the relevant terminals, comparing against expected values for the current commanded state.
  4. DecisionController reports an internal fault code → follow manufacturer fault-code guidance, likely requiring card/module replacement. Controller reports healthy but field response is still incorrect → investigate the specific output channel or communication path further.
  5. Fault LocationIsolate to the controller's internal fault, a specific output channel, the synchronisation input, or the ALCMS communication path.
  6. Root CauseConfirm correct sequencing/command behaviour is restored and stable across a full observation period before closing out.
Test Points
Controller diagnostic/fault code status
Command output values at relevant terminals
Synchronisation input signal
ALCMS communication status
Root Cause Verification

Observe the ALS through at least one full commanded cycle (including any intensity-step and flasher-sequencing changes) to confirm correct, stable operation before closing out.

Corrective Action
  • Replace the faulted controller card/module per manufacturer guidance.
  • Restore correct firmware/configuration if a software issue is identified.
  • Repair the synchronisation input or communication path if that is the isolated fault.
Preventive Action
  • Maintain controller firmware/configuration backups to allow rapid restoration after a fault.
  • Include a controller diagnostic self-test in the periodic maintenance programme, where the manufacturer provides one.
  • Keep manufacturer support/escalation contacts readily available given the specialised nature of controller-level faults.
Related References

SOP: SOP-ALS-14 (Master Controller Fault Diagnosis Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5345-46 (sequenced flasher systems)

Diagram: ALS Master Controller Wiring and Communication Schematic

Case Study: Division 01 — CCR Troubleshooting; Division 28 — Control System Diagnostics

Rev 0 — Initial issue
ALS-288 Approach Light Remote Monitoring Failure

Symptom — The ALCMS or remote monitoring system shows incorrect, missing, or stale status for the ALS — for example, reporting a fault when the field system is actually operating correctly, or failing to report a genuine field fault — indicating a monitoring/reporting issue rather than a true field fault.

Equipment / System

ALS remote monitoring interface — status sensors, communication links, and the ALCMS display/logging system as it relates to ALS status reporting.

Possible Causes
HighA status sensor or monitoring point at the field circuit failed or reading incorrectly, independent of the actual lighting circuit's true condition
HighA communication link fault between the field monitoring point and the ALCMS (cable, network, or radio link as applicable)
MediumALCMS software/configuration fault causing incorrect display or logging of an otherwise correctly reported status
MediumA monitoring point left in an incorrect configuration after previous maintenance or a system upgrade
LowA genuine intermittent field fault that only manifests briefly, being correctly reported by monitoring but difficult to distinguish from a monitoring glitch without further investigation
Required Tools
Independent field measurement equipment (ammeter, multimeter) to verify actual field statusCommunication network testerALCMS diagnostic/configuration accessMonitoring point wiring schematic
Safety Precautions
  • LOTO relevant circuits before any physical monitoring-point inspection, treating the field circuit as live until independently confirmed otherwise.
  • Do not rely solely on ALCMS status when planning field work — always independently verify the actual field condition before assuming a circuit is de-energised or energised.
Diagnostic Procedure
  1. ObservationIndependently verify the true field status (using an ammeter or direct observation) against what the ALCMS is reporting, to confirm this is genuinely a monitoring discrepancy rather than a real field fault.
  2. ObservationCheck the specific status sensor/monitoring point at the field circuit for correct operation and correct wiring.
  3. MeasurementTest the communication link between the field monitoring point and the ALCMS for correct signal transmission.
  4. DecisionField status confirmed correct but ALCMS reports incorrectly → monitoring/communication fault, continue below. Field status confirmed to genuinely match a fault that was previously under-reported → treat as a field fault under the relevant ALS topic, not purely a monitoring issue.
  5. Fault LocationIsolate to the field sensor/monitoring point, the communication link, or the ALCMS display/logging configuration.
  6. Root CauseConfirm the ALCMS status now correctly and consistently matches the verified true field condition across a follow-up observation period.
Test Points
Independent field status verification vs. ALCMS display
Status sensor/monitoring point condition
Communication link signal integrity
ALCMS configuration/logging settings for this monitoring point
Root Cause Verification

Monitor ALCMS status against independently verified field condition over a follow-up period, including at least one intensity-step change and, if applicable, a flasher sequencing cycle, to confirm the fix is stable.

Corrective Action
  • Repair or replace the faulted status sensor/monitoring point.
  • Repair the communication link between the field point and the ALCMS.
  • Correct the ALCMS configuration/logging setting if that is the isolated fault.
Preventive Action
  • Include a periodic cross-check of ALCMS reported status against independent field verification as part of the PM programme, given the operational reliance on accurate remote status.
  • Maintain accurate monitoring-point wiring and configuration documentation.
  • Flag any monitoring point with a history of discrepancies for closer, more frequent verification.
Related References

SOP: SOP-ALS-15 (Remote Monitoring Verification Procedure)

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

Diagram: ALS Remote Monitoring and Communication Schematic

Case Study: Division 28 — Control System Diagnostics

Rev 0 — Initial issue
ALS-289 Approach Light Earthing Failure

Symptom — An ALS structure, fixture, or circuit is found with an earthing/bonding fault — high resistance to earth, a broken earth conductor, or a missing bond — identified either during a scheduled earth resistance test or following an electrical fault or lightning event.

Equipment / System

ALS earthing and bonding system, including structure earth electrodes, bonding conductors between structural elements, and the connection to the overall airfield earthing grid.

Possible Causes
HighCorrosion of the earth electrode or bonding conductor connections, particularly in exposed or coastal-influenced approach areas
HighA broken or disconnected bonding conductor following structural work, a previous fault, or a lightning strike
MediumSoil condition changes (drying, seasonal moisture variation) increasing earth electrode resistance beyond the acceptable threshold
MediumAn earth electrode damaged or displaced during ground works near the structure
LowOriginal installation deficiency — inadequate electrode depth or configuration for the local soil conditions
Required Tools
Earth resistance tester (fall-of-potential or clamp-on method as appropriate)Continuity tester for bonding conductorsVisual inspection kit for above-ground connectionsReplacement earthing hardware and conductor as required
Safety Precautions
  • LOTO the circuit before any earthing system work that involves disconnecting a bond, since removing a bond can create an unexpected touch-voltage hazard if the circuit is not fully isolated.
  • Treat any structure with a confirmed earthing fault as an elevated shock/lightning risk until repaired — restrict unnecessary contact with the structure in the interim.
  • Follow standard electrical safety and, where applicable, confined-space or elevated-access procedures for accessing earth test points.
Diagnostic Procedure
  1. ObservationConfirm the earthing fault from the scheduled test result or triggering event (electrical fault, lightning strike, ground works near the structure).
  2. MeasurementTest earth resistance at the structure and check continuity of each bonding conductor back to the main earthing grid connection point.
  3. ObservationInspect all above-ground earthing and bonding connections for corrosion, physical damage, or looseness.
  4. DecisionHigh resistance but conductors intact → likely a soil/electrode condition issue, may require additional electrodes or treatment. Broken or disconnected conductor found → straightforward conductor repair/replacement.
  5. Fault LocationIsolate to a specific broken/corroded conductor, a degraded electrode, or a genuine soil-resistivity issue requiring system enhancement.
  6. Root CauseConfirm earth resistance is restored to within the required threshold and record the result for the maintenance history.
Test Points
Earth resistance at each structure electrode
Bonding conductor continuity between structural elements
Above-ground connection condition
Comparison against the site's required earth resistance threshold
Root Cause Verification

Re-test earth resistance and full bonding continuity after repair, and repeat the test at the next scheduled interval to confirm the result remains stable, particularly through a seasonal soil-moisture cycle.

Corrective Action
  • Repair or replace corroded or broken bonding conductors.
  • Add supplementary earth electrodes or treatment if soil resistivity is the root cause of a persistent high-resistance reading.
  • Re-secure and clean any loose or corroded above-ground connections.
Preventive Action
  • Include earth resistance testing of every ALS structure in the periodic maintenance programme, given the elevated lightning exposure of tall approach lighting structures.
  • Test earthing systems after any significant lightning event in the vicinity, even without an obvious fault symptom.
  • Maintain accurate earthing system as-built records for each structure.
Related References

SOP: SOP-ALS-16 (Earthing and Bonding Test Procedure)

Standards: IEC 62305 (lightning protection), ICAO Annex 14 Vol I, applicable national electrical earthing standards

Diagram: ALS Structure Earthing and Bonding Drawing

Case Study: Division 29 — Earthing & Lightning Protection

Rev 0 — Initial issue
ALS-290 Approach Light Water Ingress

Symptom — Water is found inside a fixture housing, junction box, or structure base enclosure during inspection or maintenance, with or without an associated electrical fault already having occurred.

Equipment / System

ALS fixture housing, junction box, structure base enclosure, and their gasket/seal systems, across an area that frequently includes low-lying, unpaved, or wetland-adjacent terrain beyond the runway threshold.

Possible Causes
HighEnclosure gasket degraded, cracked, or incorrectly seated, allowing rainfall or splash to enter
HighStructure base or junction box not properly sealed against the surrounding ground, allowing groundwater or surface runoff to collect — a heightened risk in low-lying or wetland approach areas
MediumCable gland or entry point seal damaged or not fully tightened
MediumEnclosure mounting bolts loose, breaking the seal between housing sections
LowEnclosure drainage (weep hole) blocked, causing water to pool and rise above the acceptable level inside
LowSeasonal flooding or high water table temporarily submerging a low-lying structure base beyond its designed ingress protection rating
Required Tools
Insulation resistance tester (1000V/2500V)Wet/dry vacuum for enclosure pump-outReplacement gaskets, seals, and cable glandsTorque wrench for enclosure fixing bolts
Safety Precautions
  • LOTO the circuit before opening any enclosure with standing water.
  • Do not assume a wet fixture or enclosure is de-energised; test with a rated meter before hand contact.
  • Assess flood/access conditions before dispatching personnel to low-lying approach-area structures during or after significant rainfall.
  • Dispose of pumped-out water per site environmental procedure if contamination is suspected.
Diagnostic Procedure
  1. ObservationConfirm the extent of water ingress — fixture housing, junction box, or structure base — and note whether any electrical fault (dim, dark, flickering) is already present.
  2. MeasurementWith the circuit isolated, megger the fixture, transformer, and any affected cable to confirm whether insulation has already been compromised.
  3. ObservationInspect the relevant gasket, cable gland, and enclosure seal for the specific ingress path, and check whether the structure base is in a known flood-prone or low-lying section.
  4. DecisionInsulation resistance already low → treat as an active electrical fault, replace affected components before re-energising. Insulation resistance still acceptable → address the sealing issue and monitor, with particular attention if the location is flood-prone.
  5. Fault LocationIsolate the specific ingress path — gasket, cable gland, enclosure seal, or drainage blockage — or confirm seasonal flooding as the cause.
  6. Root CauseConfirm the enclosure is fully dry and re-test insulation resistance before closing out.
Test Points
Insulation resistance, fixture/transformer/cable to earth
Enclosure gasket and cable gland condition
Enclosure drainage (weep hole) function
Structure base flood/ground-water exposure history
Root Cause Verification

Re-test insulation resistance after drying and resealing, and re-inspect the enclosure after the next significant rainfall or flood-risk period to confirm the ingress path has been fully resolved.

Corrective Action
  • Replace the degraded gasket or cable gland.
  • Clear or restore enclosure drainage.
  • Torque the enclosure fixing bolts to specification.
  • Pump out and dry the enclosure fully before closing up.
  • Consider raising or better-protecting enclosures at structures with a confirmed history of seasonal flooding.
Preventive Action
  • Include gasket, seal, and drainage condition in the periodic maintenance checklist, replacing proactively rather than only on failure.
  • Identify and track structures in low-lying or flood-prone sections of the approach area for closer monitoring, particularly before the wet season.
  • Review enclosure ingress-protection ratings against actual site exposure conditions for chronically affected locations.
Related References

SOP: SOP-ALS-17 (Water Ingress Response Procedure)

Standards: FAA AC 150/5345-46 (environmental sealing requirements), ICAO Annex 14 Vol I

Diagram: ALS Structure and Enclosure Detail Drawing

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue
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Frequently Asked Questions

Quick answers about Approach Lighting System (ALS) troubleshooting in the AGL Troubleshooting Atlas.

Q1What does Division 10 of the AGL Troubleshooting Atlas cover?

Division 10 is the Approach Lighting System (ALS) Troubleshooting reference. It publishes 20 diagnostic topics, ALS-271 to ALS-290, for Approach Lighting System (ALS) faults on airfield ground lighting systems.

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 Approach Lighting System (ALS) faults are covered?

The 20 topics are ALS-271 to ALS-290: Approach Light System Not Working; Partial Approach Light Failure; Multiple Approach Lights Failure; Approach Light Flickering; Approach Light Dim; Approach Light Wrong Color; Approach Light Intensity Step Failure; Approach Light Circuit Failure; Approach Light Cable Failure; Approach Light Transformer Failure; Approach Light Connector Failure; Approach Light Alignment Error; Approach Light Pole Damage; Approach Light Structural Damage; Approach Light Optical Failure; Approach Light Photometric Failure; Approach Light Control Failure; Approach Light Remote Monitoring Failure; Approach Light Earthing Failure; Approach Light Water Ingress.

Q4How does a diagnostic procedure begin in Division 10?

Each procedure starts with observation before any intervention. For example, ALS-271 (Approach Light System Not Working) begins: Check the ALCMS/CCR panel to confirm whether the ALS regulator is commanded on and whether it is showing an output fault, a feedback fault, or genuinely zero output.

Q5What safety precautions apply when troubleshooting Approach Lighting System (ALS)?

Each topic lists its own precautions. For example, ALS-271 (Approach Light System Not Working) starts with: Coordinate all approach-area access through ATC/airfield operations — ALS structures are frequently in the approach surface and may require additional clearance beyond a standard runway/taxiway closure.

Q6Which SOPs are referenced in Division 10?

Related references in the topics include SOP-ALS-01 (ALS Fault Isolation Procedure), SOP-CCR-04 (CCR LOTO Procedure), SOP-ALS-02 (Sub-Circuit Isolation Procedure), SOP-ALS-03 (Series Circuit Fault Location Procedure), SOP-ALS-04 (Lens Cleaning and Inspection Procedure), SOP-CCR-08 (Intensity Step Verification), SOP-ALS-05 (Cable Splice and Repair Procedure) and SOP-ALS-06 (Transformer Test and Replacement Procedure), and 11 more.

Q7Which standards are referenced in the Approach Lighting System (ALS) 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, IEC 60364, IEC 60502, FAA AC 150/5345-26, ICAO Annex 14 Vol I Appendix 2 and IEC 62305.

Q8How can I find a specific fault in Division 10 quickly?

Use the search box at the top of the page to filter the 20 published topics by code, fault name or keyword; matching topics open automatically. Each diagnostic record can also be downloaded as a Word (.doc) file.

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