Division 20 — CAT I / CAT II / CAT III Troubleshooting

01 · CAT I / II / III & LVO Faults

15 of 15 topics published

LVO-491 CAT I Lighting System Failure

Symptom — The lighting configuration required to support CAT I approach and landing operations can no longer be confirmed serviceable, triggering a category downgrade or a NOTAM restricting CAT I operations.

Equipment / System

The full suite of CAT I-required lighting systems — approach lighting system, runway edge lights, runway threshold/end lights, and PAPI — evaluated against the CAT I serviceability criteria for the runway.

Possible Causes

HighA critical approach lighting segment (e.g. the inner 300m) with unserviceable units exceeding the allowed tolerance

HighRunway edge lighting serviceability percentage below the CAT I threshold

MedPAPI unit(s) unserviceable per Division 11 findings, removing the required vertical guidance

MedCCR failure affecting a CAT I-critical circuit without a functioning standby

LowA monitoring/reporting gap preventing confirmation of serviceability rather than an actual lighting fault

LowAn isolated single-unit failure incorrectly escalated to a full category failure due to a miscount

Required Tools

ALCMS/CCR serviceability reportCAT I lighting standard reference (ICAO Annex 14, FAA AC 150/5340-30)Circuit-by-circuit inspection checklistNOTAM drafting reference

Safety Precautions

Coordinate with ATC/airfield operations before any NOTAM action.
LOTO before any corrective work on affected circuits.
Treat any CAT I serviceability question as flight-safety critical requiring immediate assessment.

Diagnostic Procedure

ObservationPull the current ALCMS/CCR serviceability report for all CAT I-required systems on the affected runway.
ObservationIdentify which specific system(s) and circuit(s) are driving the shortfall.
MeasurementCount unserviceable units against the CAT I tolerance for each system (approach, edge, threshold, PAPI).
DecisionShortfall isolated to one system/circuit → route to the relevant division-specific procedure for repair. Shortfall is a monitoring/reporting error → verify actual field condition before amending the NOTAM.
Fault LocationConfirm the specific circuit(s) or unit(s) responsible for the shortfall.
Root CauseConfirm repair restores serviceability to within CAT I tolerance before any NOTAM is lifted.

Test Points

Serviceability percentage per system vs. CAT I threshold

Adjacent-unit-outage pattern check

PAPI unit status

CCR/standby circuit status for CAT I-critical circuits

Root Cause Verification

After repair, re-run the full serviceability check across all CAT I-required systems and confirm the runway meets the CAT I tolerance before requesting NOTAM cancellation.

Corrective Action

Repair or replace the specific failed unit(s)/circuit(s) identified.
Correct a monitoring/reporting discrepancy.
Restore CCR/standby function.

Preventive Action

Monitor serviceability trend proactively rather than waiting for a threshold breach.
Prioritise repair of CAT I-critical circuits ahead of routine work.
Maintain an up-to-date CAT I equipment criticality list.

Related References

SOP: SOP-LVO-01 (Category Serviceability Assessment Procedure), NOTAM Issuance Procedure

Standards: ICAO Annex 14 Vol I, FAA AC 150/5340-30, ICAO Annex 6

Diagram: CAT I lighting system schematic and serviceability criteria table

Case Study: Division 24 — Complete Runway Lighting Failure; Division 26 — Photometric & Intensity Compliance

Rev 0 — Initial issue

LVO-492 CAT II Lighting System Failure

Symptom — The lighting configuration required to support CAT II approach and landing operations can no longer be confirmed serviceable against the tighter CAT II tolerance, triggering a category downgrade to CAT I or below.

Equipment / System

The full suite of CAT II-required lighting systems — approach lighting with sequenced flashers, runway centreline lighting, touchdown zone lighting, runway edge lighting, and PAPI — evaluated against the CAT II serviceability criteria, which is stricter than CAT I and typically requires standby power redundancy.

Possible Causes

HighRunway centreline or touchdown zone lighting serviceability below the CAT II threshold (tighter adjacent-outage tolerance than CAT I)

HighStandby power or redundant circuit not confirmed available, a mandatory CAT II requirement

MedApproach lighting sequenced flasher system faulted, removing a CAT II-specific element

MedAGL monitoring system unable to confirm real-time serviceability status, a CAT II operational requirement

LowA single critical unit failure at a position where CAT II tolerance is zero (no adjacent outage permitted)

LowA documentation/reporting gap rather than an actual field fault

Required Tools

ALCMS/CCR serviceability reportCAT II lighting standard referenceStandby power status logSequenced flasher controller diagnostic tool

Safety Precautions

Coordinate with ATC/airfield operations before any category downgrade NOTAM.
LOTO before corrective work.
Treat CAT II serviceability assessment as flight-safety critical.

Diagnostic Procedure

ObservationPull the current serviceability report for all CAT II-required systems, including standby power status.
ObservationIdentify which system(s) are driving the shortfall against the CAT II (not CAT I) tolerance.
MeasurementVerify standby power/redundant circuit availability, a CAT II-specific requirement.
MeasurementCount unserviceable units against the stricter CAT II adjacent-outage tolerance for centreline and touchdown zone lighting.
DecisionShortfall isolated to a specific system → route to the relevant division-specific procedure. Standby power unavailable → route to LVO-503 (backup power failure).
Fault LocationConfirm the specific circuit(s), unit(s), or redundancy element responsible.
Root CauseConfirm repair restores serviceability to within CAT II tolerance, including confirmed standby availability, before NOTAM cancellation.

Test Points

Serviceability percentage per system vs. CAT II threshold

Standby power/redundant circuit availability

Sequenced flasher system status

Centreline/TDZ adjacent-outage pattern

Root Cause Verification

After repair, re-run the full serviceability check including standby power confirmation, and verify against the CAT II tolerance before requesting NOTAM cancellation.

Corrective Action

Repair or replace the specific failed unit(s)/circuit(s).
Restore standby power/redundant circuit availability.
Repair the sequenced flasher controller.

Preventive Action

Monitor CAT II-specific serviceability trends, including standby power readiness, on a tighter interval than CAT I.
Test standby power transfer regularly rather than assuming availability.
Maintain a CAT II equipment criticality list distinct from CAT I.

Related References

SOP: SOP-LVO-01 (Category Serviceability Assessment Procedure), SOP-LVO-02 (Standby Power Verification Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5340-30, ICAO Annex 6

Diagram: CAT II lighting system schematic and serviceability criteria table

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

Rev 0 — Initial issue

LVO-493 CAT III Lighting System Failure

Symptom — The lighting configuration required to support CAT III approach and landing operations can no longer be confirmed serviceable against the most stringent tolerance, triggering an immediate category downgrade given CAT III's near-zero fault tolerance.

Equipment / System

The full suite of CAT III-required lighting systems — approach lighting, runway centreline lighting (mandatory for CAT III), touchdown zone lighting, runway edge lighting, stop bars, and full dual/redundant power and monitoring systems.

Possible Causes

HighAny single unserviceable unit at a position where CAT III permits zero tolerance, unlike the small allowances under CAT I/II

HighLoss of confirmed dual redundancy (standby CCR, backup power, or monitoring system) required specifically for CAT III operation

MedStop bar system fault at an active LVP-controlled intersection, a CAT III-mandatory element

MedAGL monitoring system unable to provide the real-time, individual-unit status confirmation required for CAT III

LowRunway centreline lighting variable intensity/colour-change zone fault in the final approach segment

LowA reporting or configuration error incorrectly flagging a CAT III shortfall that does not exist in the field

Required Tools

ALCMS/CCR serviceability report with individual-unit statusCAT III lighting standard referenceDual power/redundancy status logStop bar control system diagnostic tool

Safety Precautions

Coordinate immediately with ATC/airfield operations given the operational impact of a CAT III downgrade.
LOTO before any corrective work.
Treat any CAT III serviceability question as immediate flight-safety critical requiring real-time assessment.

Diagnostic Procedure

ObservationPull the current individual-unit serviceability report for all CAT III-required systems, including confirmed dual redundancy status.
ObservationIdentify the specific unit(s), circuit(s), or redundancy element failing to meet the zero-tolerance CAT III criteria.
MeasurementVerify dual power supply and standby CCR are both confirmed available and automatically transferring correctly.
MeasurementVerify stop bar and centreline colour-change zone status at all LVP-controlled points.
DecisionShortfall isolated to a specific unit/circuit → route to the relevant division-specific procedure for immediate repair. Redundancy not confirmed → route to LVO-501 (redundancy system failure).
Fault LocationConfirm the exact unit, circuit, or redundancy element responsible.
Root CauseConfirm full restoration to CAT III zero-tolerance criteria, including verified dual redundancy, before any NOTAM cancellation.

Test Points

Individual-unit serviceability status for all CAT III-required systems

Dual power/standby CCR automatic transfer confirmation

Stop bar status at all LVP intersections

Centreline colour-change zone status

Root Cause Verification

After repair, re-run the full individual-unit serviceability check and confirm dual redundancy transfer under test before requesting NOTAM cancellation.

Corrective Action

Repair or replace the specific failed unit(s)/circuit(s) with zero delay given CAT III criticality.
Restore confirmed dual redundancy.
Repair the stop bar or centreline colour-change zone fault.

Preventive Action

Monitor CAT III systems continuously via real-time individual-unit status rather than periodic reporting.
Test dual redundancy transfer on a defined interval.
Maintain a CAT III equipment criticality list with the shortest response-time targets in the AGL system.

Related References

SOP: SOP-LVO-01 (Category Serviceability Assessment Procedure), SOP-LVO-03 (CAT III Redundancy Test Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5340-30, ICAO Annex 6

Diagram: CAT III lighting system schematic and zero-tolerance serviceability criteria table

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

Rev 0 — Initial issue

LVO-494 Low Visibility Lighting Failure

Symptom — A lighting fault is identified specifically during an active Low Visibility Procedures (LVP) period, where the operational configuration required for LVP — elevated intensity, stop bars active, restricted vehicle movement lighting — cannot be fully confirmed.

Equipment / System

The complete LVP-configured lighting system, including intensity step settings, stop bar activation status, and any LVP-specific circuit configuration distinct from normal operations.

Possible Causes

HighIntensity step command not correctly applied across all LVP-required circuits when LVP was activated

HighStop bar(s) not confirmed illuminated/functional at LVP activation

MedA circuit or system that operates normally under standard visibility but has a marginal fault only exposed under the LVP intensity/configuration setting

MedLVP activation procedure itself not fully executed, with a step missed in the control room

LowA monitoring system configuration mismatch between normal and LVP operational modes

LowA fault that exists regardless of LVP but is only being detected now due to increased attention during the LVP period

Required Tools

ALCMS LVP activation logCCR intensity step confirmationStop bar status panelLVP activation checklist

Safety Precautions

Coordinate immediately with ATC given the active LVP context.
LOTO before any corrective work.
Treat any LVP-period fault as immediate flight-safety critical.

Diagnostic Procedure

ObservationConfirm which specific element of the LVP configuration is not verified: intensity, stop bars, or restricted lighting.
ObservationReview the LVP activation log for the sequence of steps executed.
MeasurementVerify current intensity step settings against the LVP requirement for each affected circuit.
MeasurementVerify stop bar status at all controlled intersections.
DecisionLVP activation step missed or incomplete → re-execute the activation procedure and verify. Genuine equipment fault under LVP conditions → route to the relevant specific procedure (LVO-495 through LVO-500 as applicable).
Fault LocationConfirm the specific circuit or system not meeting the LVP configuration requirement.
Root CauseConfirm full LVP configuration verified across all required systems before continuing LVP operations.

Test Points

Intensity step setting per circuit vs. LVP requirement

Stop bar status at all controlled points

LVP activation log completeness

Monitoring system LVP mode configuration

Root Cause Verification

Re-verify the full LVP configuration across all systems and confirm with a follow-up check before the LVP period concludes or at the next LVP activation.

Corrective Action

Re-execute the LVP activation procedure fully.
Repair the specific equipment fault identified.
Correct the monitoring system's LVP mode configuration.

Preventive Action

Use a formal LVP activation checklist with positive confirmation of each step.
Conduct periodic LVP configuration drills outside of actual low-visibility conditions.
Review LVP activation logs regularly for procedural compliance.

Related References

SOP: SOP-LVO-04 (LVP Activation and Verification Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 120-118 (Low Visibility Operations), ICAO Doc 9365

Diagram: LVP activation checklist and circuit configuration reference

Case Study: Division 01 — CCR Troubleshooting

Rev 0 — Initial issue

LVO-495 Runway Centreline Failure During LVO

Symptom — A runway centreline lighting fault — unserviceable unit, incorrect intensity, or colour-change zone malfunction — is identified while Low Visibility Operations are active, where centreline guidance is critical given reduced visual reference.

Equipment / System

Runway centreline lighting circuit, including the variable intensity and red/white alternating colour-change zone in the final approach segment required for CAT II/III.

Possible Causes

HighIndividual centreline fixture failure at a position where LVP tolerance is zero or near-zero

HighCCR circuit fault affecting a section of the centreline circuit during the period of peak reliance

MedColour-change zone fixture showing the wrong colour (red/white) due to a lens or filter fault

MedIntensity step not correctly matching the LVP-required setting

LowA fixture recently serviced and not yet re-verified before LVP activation

LowA monitoring system reporting delay masking a fault that has just occurred

Required Tools

ALCMS real-time centreline statusCCR circuit logDivision 05 (Runway Centreline Light Troubleshooting) referenceColour-change zone inspection tools

Safety Precautions

Coordinate immediately with ATC given the active LVP context and centreline guidance's criticality.
LOTO before corrective work.
Prioritise response given the flight-safety implications.

Diagnostic Procedure

ObservationIdentify the specific centreline fixture(s) or zone reported unserviceable via the real-time monitoring system.
MeasurementVerify the fault against the CAT II/III centreline tolerance for the runway's current operating category.
MeasurementDetermine whether the fault is a single-fixture issue or a circuit-level fault using Division 05 diagnostic steps.
DecisionSingle fixture → immediate replacement per Division 05 procedure. Circuit-level fault → route to CCR/circuit troubleshooting with LVP-priority response.
Fault LocationConfirm the specific fixture(s) or circuit section responsible.
Root CauseConfirm centreline serviceability restored to the required tolerance for the active category before continuing LVP operations.

Test Points

Real-time centreline fixture status

Colour-change zone verification

Intensity step vs. LVP requirement

CCR circuit current for the centreline feed

Root Cause Verification

After repair, re-verify centreline serviceability against the active category's tolerance and confirm with the ATC/airfield operations coordination point.

Corrective Action

Replace the failed centreline fixture per Division 05 procedure.
Repair the CCR circuit fault.
Correct a colour-change zone lens/filter fault.

Preventive Action

Prioritise centreline circuit health monitoring during any period of forecast low visibility.
Verify fixture status proactively ahead of anticipated LVP activation.
Maintain expedited spare parts availability for centreline fixtures.

Related References

SOP: SOP-LVO-05 (LVO-Priority Fault Response Procedure), Division 05 procedures

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

Diagram: Runway centreline lighting schematic with colour-change zone detail

Case Study: Division 05 — Runway Centreline Light Troubleshooting

Rev 0 — Initial issue

LVO-496 TDZ Lighting Failure During LVO

Symptom — A touchdown zone (TDZ) lighting fault is identified while Low Visibility Operations are active, where TDZ lighting provides critical visual reference for the touchdown point under reduced visibility.

Equipment / System

Touchdown zone lighting circuit, consisting of paired barrettes symmetrically arranged along the runway within the touchdown zone.

Possible Causes

HighIndividual TDZ barrette/fixture failure at a position where LVP tolerance is zero or near-zero

HighCCR circuit fault affecting a section of the TDZ circuit

MedA barrette showing asymmetric or unbalanced brightness relative to its paired barrette on the opposite side of the runway

MedIntensity step not correctly matching the LVP-required setting

LowA fixture recently serviced and not yet re-verified before LVP activation

LowA monitoring system reporting delay masking a fault that has just occurred

Required Tools

ALCMS real-time TDZ statusCCR circuit logDivision 06 (Touchdown Zone Light Troubleshooting) reference

Safety Precautions

Coordinate immediately with ATC given the active LVP context.
LOTO before corrective work.
Prioritise response given the flight-safety implications.

Diagnostic Procedure

ObservationIdentify the specific TDZ fixture(s) or barrette pair reported unserviceable via the real-time monitoring system.
MeasurementVerify the fault against the CAT II/III TDZ tolerance for the runway's current operating category.
MeasurementDetermine whether the fault is a single-fixture issue or a circuit-level fault using Division 06 diagnostic steps.
DecisionSingle fixture → immediate replacement per Division 06 procedure. Circuit-level fault → route to CCR/circuit troubleshooting with LVP-priority response.
Fault LocationConfirm the specific fixture(s) or circuit section responsible.
Root CauseConfirm TDZ serviceability restored to the required tolerance for the active category before continuing LVP operations.

Test Points

Real-time TDZ fixture status

Barrette pair symmetry/brightness balance

Intensity step vs. LVP requirement

CCR circuit current for the TDZ feed

Root Cause Verification

After repair, re-verify TDZ serviceability against the active category's tolerance and confirm with the ATC/airfield operations coordination point.

Corrective Action

Replace the failed TDZ fixture per Division 06 procedure.
Repair the CCR circuit fault.

Preventive Action

Prioritise TDZ circuit health monitoring during any period of forecast low visibility.
Verify fixture status proactively ahead of anticipated LVP activation.
Maintain expedited spare parts availability for TDZ fixtures.

Related References

SOP: SOP-LVO-05 (LVO-Priority Fault Response Procedure), Division 06 procedures

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

Diagram: Touchdown zone lighting schematic

Case Study: Division 06 — Touchdown Zone Light Troubleshooting

Rev 0 — Initial issue

LVO-497 Stop Bar Failure During LVO

Symptom — A stop bar fails to illuminate, fails to extinguish on ATC command, or shows an inconsistent state at a runway-holding position while Low Visibility Procedures are active and stop bars are the primary means of preventing runway incursion under reduced visibility.

Equipment / System

Stop bar lighting circuit (typically red, elevated or in-pavement fixtures) at a runway-holding position, and its selective switching control from the tower/ALCMS.

Possible Causes

HighStop bar control relay or selective switching module failure preventing illumination or extinguishing on command

HighIndividual stop bar fixture failure at a position where any single unserviceable unit compromises the incursion-prevention function

MedCommunication fault between the tower control system and the stop bar's local controller

MedA stop bar left illuminated after being commanded off, or vice versa, indicating a control logic or relay sticking fault

LowCCR circuit fault affecting the stop bar circuit

LowA taxiway centreline lead-on light interlock fault causing an inconsistent state relative to the stop bar

Required Tools

ALCMS/tower stop bar control panelTrue-RMS multimeterControl relay/switching module diagnostic toolStop bar circuit diagram

Safety Precautions

Treat any stop bar malfunction as an immediate runway incursion risk during LVP.
Coordinate immediately with ATC.
LOTO before corrective work.
Consider immediate operational restriction (e.g. escort procedures) at the affected holding position until resolved.

Diagnostic Procedure

ObservationIdentify the specific stop bar and the nature of the malfunction — fails to illuminate, fails to extinguish, or inconsistent state — via the tower control panel.
MeasurementVerify the command signal reaching the stop bar's local controller from the tower/ALCMS.
MeasurementCheck the control relay/switching module and individual fixture status at the stop bar location.
DecisionCommand signal not reaching the controller → communication/control system fault. Command signal correct but stop bar not responding → local relay, switching module, or fixture fault.
Fault LocationConfirm the specific point of failure between the tower command and the physical stop bar state.
Root CauseConfirm the stop bar responds correctly to on/off commands before continuing LVP operations at that holding position.

Test Points

Tower/ALCMS command signal to the stop bar

Local control relay/switching module status

Individual stop bar fixture status

Taxiway centreline lead-on interlock consistency

Root Cause Verification

After repair, cycle the stop bar on and off from the tower control panel multiple times and confirm correct, consistent response before returning the holding position to normal LVP service.

Corrective Action

Replace the failed control relay or switching module.
Repair the communication link between the tower and the local controller.
Replace the failed fixture.
Correct the interlock logic fault.

Preventive Action

Test stop bar on/off response as part of the pre-LVP activation checklist.
Include stop bar control relay/switching module inspection in periodic PM.
Maintain a documented escort/restriction procedure for any holding position with a known stop bar issue.

Related References

SOP: SOP-LVO-06 (Stop Bar Verification Procedure), SOP-LVO-04 (LVP Activation and Verification Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5340-30, ICAO Doc 9365

Diagram: Stop bar control and switching circuit diagram

Case Study: Division 07 — Taxiway Light Troubleshooting

Rev 0 — Initial issue

LVO-498 Approach Lighting Failure During LVO

Symptom — An approach lighting system fault — unserviceable unit, sequenced flasher malfunction, or circuit failure — is identified while Low Visibility Operations are active, where the approach lighting system provides the primary visual reference during the final approach segment under reduced visibility.

Equipment / System

Approach lighting system, including the centreline light bars, crossbars, sequenced flashers (for CAT II/III configurations), and their supporting circuits.

Possible Causes

HighIndividual approach light unit failure at a position within the zero-or-near-zero-tolerance zone for the active category

HighSequenced flasher controller fault, removing the CAT II/III-specific flasher function

MedCCR circuit fault affecting a section of the approach lighting array

MedApproach lighting structure/mast physically disturbed, e.g. by wind or a maintenance vehicle, affecting alignment of a unit

LowA fixture recently serviced and not yet re-verified before LVP activation

LowA monitoring system reporting delay masking a fault that has just occurred

Required Tools

ALCMS real-time approach lighting statusSequenced flasher controller diagnostic toolCCR circuit logApproach lighting system layout drawing

Safety Precautions

Coordinate immediately with ATC given the active LVP context and approach lighting's criticality.
LOTO before corrective work.
Prioritise response given the flight-safety implications, particularly for units close to the runway threshold.

Diagnostic Procedure

ObservationIdentify the specific approach light unit(s) or the sequenced flasher system reported unserviceable via real-time monitoring.
MeasurementVerify the fault against the CAT II/III approach lighting tolerance for the active operating category.
MeasurementDetermine whether the fault is a single-unit issue, a circuit-level fault, or a sequenced flasher controller fault.
DecisionSingle unit → immediate replacement. Circuit-level fault → route to CCR/circuit troubleshooting with LVP-priority response. Sequenced flasher fault → investigate controller and flash-tube/capacitor bank.
Fault LocationConfirm the specific unit, circuit, or flasher element responsible.
Root CauseConfirm approach lighting serviceability restored to the required tolerance for the active category before continuing LVP operations.

Test Points

Real-time approach light unit status

Sequenced flasher controller status and flash rate

CCR circuit current for the approach lighting feed

Structural/alignment check for wind or impact disturbance

Root Cause Verification

After repair, re-verify approach lighting serviceability against the active category's tolerance, including confirmed correct sequenced flasher operation, before continuing LVP operations.

Corrective Action

Replace the failed approach light unit.
Repair the sequenced flasher controller or flash-tube/capacitor bank.
Repair the CCR circuit fault.
Correct structural alignment.

Preventive Action

Prioritise approach lighting circuit health monitoring during any period of forecast low visibility.
Verify sequenced flasher operation as part of the pre-LVP activation checklist.
Maintain expedited spare parts availability for approach lighting units and flasher components.

Related References

SOP: SOP-LVO-05 (LVO-Priority Fault Response Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5340-30 (ALSF-2), ICAO Doc 9365

Diagram: Approach lighting system layout drawing

Case Study: Division 24 — Complete Runway Lighting Failure

Rev 0 — Initial issue

LVO-499 PAPI Failure During LVO

Symptom — A PAPI fault — unit failure, wrong indication, or alignment issue — is identified while Low Visibility Operations are active, where vertical glide path guidance is critical given reduced visual reference on approach.

Equipment / System

PAPI wing bar, evaluated for its criticality during the active LVP period alongside its normal serviceability requirements per Division 11.

Possible Causes

HighIndividual PAPI unit failure reducing the wing bar below its required serviceable configuration

HighPAPI circuit/CCR fault affecting the wing bar during the period of peak reliance

MedA wrong indication or alignment issue not previously flagged because it only becomes operationally significant under LVP-level scrutiny

MedPAPI power supply or communication fault coinciding with LVP activation

LowA unit recently serviced and not yet re-verified before LVP activation

LowA monitoring system reporting delay masking a fault that has just occurred

Required Tools

ALCMS real-time PAPI statusDivision 11 (PAPI/VASI Troubleshooting) referenceCCR circuit log

Safety Precautions

Coordinate immediately with ATC given the active LVP context and PAPI's role in providing vertical guidance under reduced visibility.
LOTO before corrective work.
Prioritise response given the flight-safety implications.

Diagnostic Procedure

ObservationIdentify the specific PAPI fault type via real-time monitoring, referencing the applicable Division 11 topic (unit failure, wrong indication, alignment error, etc.).
MeasurementVerify the fault against the CAT I/II/III PAPI serviceability requirement for the active operating category, which may mandate full four-unit serviceability during LVP.
MeasurementApply the relevant Division 11 diagnostic procedure to isolate the specific cause.
DecisionFault isolated to a specific unit or component → proceed with the applicable Division 11 corrective action under LVP-priority response. Fault indicates a systemic circuit/power issue → route to CCR/power troubleshooting.
Fault LocationConfirm the specific unit, circuit, or system responsible.
Root CauseConfirm PAPI serviceability restored to the required configuration for the active category before continuing LVP operations.

Test Points

Real-time PAPI unit status for all four units

PAPI circuit/CCR status

Applicable Division 11 test points for the identified fault type

Root Cause Verification

After repair, re-verify full PAPI serviceability against the active category's requirement and confirm with the ATC/airfield operations coordination point.

Corrective Action

Apply the applicable Division 11 corrective action — unit replacement, angle correction, power supply repair, etc. — under LVP-priority response.

Preventive Action

Prioritise PAPI circuit health monitoring during any period of forecast low visibility.
Verify PAPI serviceability proactively ahead of anticipated LVP activation.
Maintain expedited spare parts availability for PAPI units.

Related References

SOP: SOP-LVO-05 (LVO-Priority Fault Response Procedure), Division 11 procedures

Standards: ICAO Annex 14 Vol I Attachment A, FAA AC 150/5345-28

Diagram: PAPI wing bar layout and LVP serviceability requirement reference

Case Study: Division 11 — PAPI/VASI Troubleshooting

Rev 0 — Initial issue

LVO-500 AGL Monitoring Failure During LVO

Symptom — The AGL monitoring/ALCMS system itself fails, degrades, or loses connectivity while Low Visibility Operations are active, removing the real-time serviceability visibility that LVP operations depend on, even though the underlying lighting fixtures may still be functioning correctly.

Equipment / System

ALCMS/AGL monitoring system server, field controllers, communication network, and the real-time status display used by the tower and maintenance during LVP.

Possible Causes

HighALCMS server or software fault causing a loss of status reporting or display

HighCommunication network fault (fibre, copper, or wireless link) between field controllers and the central monitoring system

MedA field controller/RTU (remote terminal unit) failure at a specific circuit or system, causing a localised loss of monitoring visibility rather than a full system outage

MedA software update or configuration change coinciding with the onset of the fault

LowA display/workstation-level fault at the tower or maintenance office rather than a true system-wide failure

LowA power fault at the ALCMS server/UPS distinct from the AGL circuits themselves

Required Tools

ALCMS server diagnostic accessNetwork diagnostic tools (ping, traceroute, link status)Field controller/RTU status checkUPS status check for the monitoring system

Safety Precautions

Coordinate immediately with ATC and airfield operations, since a monitoring failure during LVP removes real-time assurance of lighting serviceability regardless of actual field condition.
LOTO before hands-on work on monitoring infrastructure.
Consider a physical field verification of critical systems while monitoring is unavailable.

Diagnostic Procedure

ObservationConfirm the scope of the monitoring failure: full system, a specific communication segment, or a single field controller.
ObservationDetermine whether the underlying lighting systems are still operating normally despite the loss of monitoring visibility, via physical/visual verification if necessary.
MeasurementCheck ALCMS server status, network link status, and affected field controller/RTU status.
DecisionServer/software fault → restart/recover per vendor procedure or escalate to IT/vendor support. Communication network fault → trace and repair the affected link. Field controller/RTU fault → replace or repair the specific unit.
Fault LocationIsolate to the server, network segment, or specific field controller responsible.
Root CauseConfirm full monitoring visibility is restored across all LVP-critical systems before relying solely on the monitoring system again.

Test Points

ALCMS server status and logs

Network link status to each field controller

Field controller/RTU individual status

UPS status for the monitoring system

Root Cause Verification

After repair, confirm real-time status is correctly reporting for all LVP-critical systems by cross-checking against a physical field verification, then monitor system stability over the following operating period.

Corrective Action

Restart/recover the ALCMS server per vendor procedure.
Repair the communication network link.
Replace the failed field controller/RTU.
Restore power/UPS to the monitoring system.

Preventive Action

Maintain redundant communication paths for LVP-critical monitoring segments where feasible.
Include ALCMS server and network health checks in periodic PM.
Establish a documented fallback physical-verification procedure for use during a monitoring outage.

Related References

SOP: SOP-LVO-07 (Monitoring System Failure Fallback Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5340-30, ALCMS/monitoring system technical manual

Diagram: ALCMS network architecture diagram

Case Study: Division 01 — CCR Troubleshooting

Rev 0 — Initial issue

LVO-501 Redundancy System Failure

Symptom — A redundant system — standby CCR, backup power, dual monitoring path, or duplicate circuit — designed to automatically take over on primary failure does not activate or does not fully restore function when tested or called upon, compromising the resilience required for CAT II/III operations.

Equipment / System

The full redundancy chain for a CAT II/III-critical system, including automatic transfer switches, standby CCRs, backup generators/UPS, and dual monitoring/communication paths.

Possible Causes

HighAutomatic transfer switch/mechanism failure preventing switchover from primary to standby on a primary fault

HighThe standby component itself faulted and unable to perform when called upon, despite the primary having failed correctly triggering the transfer

MedA redundancy system that was manually placed out of service for maintenance and not returned to automatic mode

MedA configuration or wiring error causing the standby to be connected to the wrong circuit or to not fully replicate the primary's function

LowInsufficient testing interval allowing a latent standby fault to go undetected until an actual primary failure

LowA control logic fault causing the transfer to occur but not to complete fully (partial transfer)

Required Tools

Automatic transfer switch diagnostic toolStandby CCR/generator test procedureRedundancy system wiring diagramTest/maintenance log for the redundancy chain

Safety Precautions

LOTO before testing or repairing any component of the redundancy chain.
Coordinate testing to avoid an unintended service interruption to the primary system during the test.
Treat a confirmed redundancy failure as removing CAT II/III eligibility until resolved.

Diagnostic Procedure

ObservationIdentify the specific redundancy element that failed to activate or complete transfer, either from an actual event or a scheduled test.
MeasurementTest the automatic transfer switch/mechanism in isolation to confirm its function.
MeasurementTest the standby component itself (CCR, generator, monitoring path) independently of the transfer mechanism to isolate whether the fault is in the standby unit or the switching logic.
DecisionTransfer switch/mechanism fault → repair or replace the switching component. Standby unit itself faulted → repair or replace the standby component.
Fault LocationConfirm whether the fault lies in the switching mechanism, the standby unit, or the control logic coordinating them.
Root CauseConfirm a full, successful automatic transfer test before restoring CAT II/III eligibility.

Test Points

Automatic transfer switch function under simulated primary failure

Standby CCR/generator independent function test

Control logic sequencing during transfer

Redundancy system maintenance/test log history

Root Cause Verification

Conduct a full end-to-end redundancy test simulating primary failure and confirm automatic, complete transfer to standby before returning the system to CAT II/III service.

Corrective Action

Repair or replace the automatic transfer switch/mechanism.
Repair or replace the standby component itself.
Correct a wiring/configuration error.
Correct the control logic sequencing.

Preventive Action

Test the full redundancy chain on a defined, regular interval rather than relying on periodic component checks alone.
Ensure any manual override of automatic mode is tracked and reversed promptly after maintenance.
Maintain a maintenance log specific to redundancy system testing separate from routine circuit PM.

Related References

SOP: SOP-LVO-03 (CAT III Redundancy Test Procedure), SOP-LVO-08 (Redundancy System Maintenance Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5340-30, ICAO Annex 6

Diagram: Redundancy system wiring and transfer logic diagram

Case Study: Division 01 — CCR Troubleshooting

Rev 0 — Initial issue

LVO-502 Standby Circuit Failure

Symptom — A standby lighting circuit — a duplicate or parallel circuit intended to maintain lighting continuity if the primary circuit fails — does not energise correctly when the primary circuit fails or is taken out of service, leaving the associated fixtures dark despite the standby provision existing.

Equipment / System

Standby circuit wiring, its dedicated or shared CCR, and the switching arrangement that transfers fixtures from the primary to the standby circuit.

Possible Causes

HighStandby circuit itself has an independent fault (open, short, or earth fault) that has gone undetected because the circuit is normally de-energised

HighSwitching arrangement (contactor, changeover switch) failing to transfer fixtures to the standby circuit

MedThe standby CCR not configured or not commanded to energise when the transfer occurs

MedThe standby circuit wiring not properly maintained/tested since it sees little to no normal operating hours

LowA labelling or documentation error causing confusion about which circuit is primary versus standby during a transfer attempt

LowThe standby circuit sized or configured differently from the primary, causing a partial or incorrect illumination pattern even if it does energise

Required Tools

True-RMS multimeter1000V insulation resistance tester (megger)Standby circuit wiring diagramChangeover switch/contactor diagnostic tool

Safety Precautions

LOTO before testing.
Be aware the standby circuit may carry latent faults given its infrequent use, and test cautiously.
Coordinate any live transfer test with ATC/airfield operations.

Diagnostic Procedure

ObservationConfirm the standby circuit failed to energise during an actual primary failure or a scheduled test.
MeasurementTest the standby circuit's own integrity (insulation resistance, continuity) independent of the transfer mechanism.
MeasurementTest the changeover switch/contactor and standby CCR configuration/command sequence.
DecisionStandby circuit itself faulted → repair per the applicable cable/circuit fault procedure (Division 13). Switching/CCR configuration fault → repair the changeover mechanism or correct the CCR command sequence.
Fault LocationConfirm whether the fault lies in the standby circuit itself or the transfer/energising mechanism.
Root CauseConfirm a full, successful transfer test with the standby circuit correctly energising and illuminating all associated fixtures.

Test Points

Standby circuit insulation resistance and continuity

Changeover switch/contactor function

Standby CCR configuration and command sequence

Fixture-by-fixture illumination check on standby circuit energisation

Root Cause Verification

Conduct a full transfer test from primary to standby and confirm correct illumination of all associated fixtures before considering the standby circuit restored to service.

Corrective Action

Repair the standby circuit fault per the applicable cable/circuit procedure.
Repair or replace the changeover switch/contactor.
Correct the standby CCR configuration.

Preventive Action

Test the standby circuit on a defined regular interval to catch latent faults before they are needed.
Maintain clear, unambiguous labelling of primary vs. standby circuits.
Include standby circuit insulation resistance testing in the periodic PM cycle alongside primary circuits.

Related References

SOP: SOP-LVO-08 (Redundancy System Maintenance Procedure), Division 13 procedures

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

Diagram: Standby circuit wiring and changeover diagram

Case Study: Division 13 — AGL Cable Troubleshooting

Rev 0 — Initial issue

LVO-503 Backup Power Failure

Symptom — The backup power system — generator, UPS, or battery bank — intended to maintain AGL operation during a loss of primary utility power fails to start, fails to carry the load, or does not provide power within the required transfer time.

Equipment / System

Backup generator, automatic transfer switch, UPS/battery bank, and the fuel/starting system supporting the backup power source for AGL-critical circuits.

Possible Causes

HighGenerator fails to start due to a battery, starter motor, fuel supply, or control system fault

HighAutomatic transfer switch fails to detect the primary power loss or fails to complete the transfer

MedGenerator starts but fails to reach rated output or trips under load due to a mechanical or electrical fault

MedUPS/battery bank depleted or degraded, unable to bridge the transfer period even if the generator eventually starts

LowFuel supply insufficient or contaminated, affecting generator run time

LowA maintenance or testing gap meaning a latent fault went undetected since the last actual power loss event

Required Tools

Generator diagnostic toolsBattery/starter test equipmentAutomatic transfer switch diagnostic toolFuel quality test kitUPS/battery bank load test equipment

Safety Precautions

LOTO before hands-on generator/UPS work.
Follow manufacturer safety procedures for fuel systems and battery banks.
Coordinate any live test with ATC/airfield operations given the operational impact of a backup power test.

Diagnostic Procedure

ObservationConfirm the specific failure mode: generator fails to start, transfer switch fails to transfer, generator starts but fails under load, or UPS/battery bank fails to bridge.
MeasurementTest the generator starting system (battery, starter, fuel delivery) independently.
MeasurementTest the automatic transfer switch's detection and switching function.
MeasurementLoad-test the generator and/or UPS/battery bank under a simulated outage.
DecisionStarting system fault → repair the battery, starter, or fuel delivery issue. Transfer switch fault → repair or replace the switch. Generator/UPS fails under load → investigate the specific mechanical/electrical component responsible.
Fault LocationConfirm the specific component within the backup power chain responsible for the failure.
Root CauseConfirm a full, successful backup power test, including load-carrying capability for the required duration, before relying on the system again.

Test Points

Generator starting system function

Automatic transfer switch detection and switching time

Generator output under rated load

UPS/battery bank state of charge and load capacity

Fuel quality and quantity

Root Cause Verification

Conduct a full simulated power-loss test, confirming automatic start, transfer, and sustained rated-load operation for the required duration, before considering backup power restored to service.

Corrective Action

Repair or replace the generator starting system component.
Repair or replace the automatic transfer switch.
Repair the generator's mechanical/electrical fault.
Replace a depleted or degraded UPS/battery bank.
Address fuel supply issues.

Preventive Action

Test backup power under load on a defined regular interval, not just a no-load start test.
Maintain fuel quality through regular testing and turnover.
Track battery bank health and replace proactively before end of service life.

Related References

SOP: SOP-LVO-09 (Backup Power Test Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5340-30, NFPA 110 (emergency power systems) where applicable

Diagram: Backup power system single-line diagram

Case Study: Division 01 — CCR Troubleshooting

Rev 0 — Initial issue

LVO-504 Lighting System Availability Failure

Symptom — The calculated availability (serviceability percentage over a defined period, or current serviceable-unit percentage) of a lighting system falls below the threshold required for its declared operating category, even if no single fault is individually catastrophic.

Equipment / System

The full lighting system(s) subject to an availability/serviceability requirement, evaluated over the applicable measurement period or in real time against the category-specific threshold.

Possible Causes

HighA cluster of individually minor faults accumulating to breach the availability threshold even though each fault alone would be acceptable

HighA systemic issue, such as an ageing cable circuit or a batch of failing lamps, causing an elevated fault rate across the system

MedMaintenance response times not keeping pace with the fault occurrence rate, allowing the availability calculation to drift downward

MedA monitoring/reporting error overstating the actual unserviceable count

LowA single major event, such as a circuit failure, causing a sudden availability drop rather than gradual erosion

LowThe availability calculation itself using an incorrect measurement period or threshold reference

Required Tools

ALCMS availability/serviceability trend reportMaintenance response time logCategory-specific availability threshold referenceFault history log

Safety Precautions

Coordinate with ATC/airfield operations regarding the operational implications of an availability shortfall.
Treat an availability failure as requiring the same urgency as a direct serviceability failure for the affected category.

Diagnostic Procedure

ObservationReview the availability trend report to determine whether the shortfall is gradual erosion or a sudden drop from a specific event.
ObservationConfirm the calculation itself is using the correct measurement period and threshold for the declared category.
MeasurementIdentify the specific faults contributing to the current shortfall and their individual status.
MeasurementReview maintenance response time data to determine whether response speed is a contributing factor.
DecisionShortfall driven by a cluster of specific faults → prioritise and clear the outstanding faults per their respective division procedures. Shortfall driven by a systemic issue → investigate the underlying systemic cause in addition to clearing individual faults.
Fault LocationIdentify all specific faults contributing to the current availability shortfall.
Root CauseConfirm availability recalculates above the required threshold after all contributing faults are cleared.

Test Points

Current availability/serviceability percentage vs. category threshold

Individual fault status contributing to the shortfall

Maintenance response time trend

Measurement period and threshold configuration accuracy

Root Cause Verification

After clearing contributing faults, recalculate availability and confirm it is restored above the required threshold, and continue monitoring the trend to confirm stability.

Corrective Action

Clear the specific outstanding faults contributing to the shortfall via their respective division procedures.
Address a systemic cause such as an ageing circuit section or a maintenance response time gap.
Correct a calculation/configuration error if found.

Preventive Action

Monitor availability trend proactively with an early-warning threshold below the hard operational minimum.
Prioritise maintenance response for faults on systems with a tight availability margin.
Review systemic causes periodically rather than only after a threshold breach.

Related References

SOP: SOP-LVO-01 (Category Serviceability Assessment Procedure), SOP-LVO-10 (Availability Trend Review Procedure)

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

Diagram: Availability trend report and category threshold reference table

Case Study: Division 24 — Complete Runway Lighting Failure

Rev 0 — Initial issue

LVO-505 Critical AGL Equipment Failure

Symptom — A piece of AGL equipment classified as flight-critical — a single point of failure for the declared operating category, or an item whose failure has an outsized impact on system availability — fails, requiring an immediate, prioritised response distinct from routine fault handling.

Equipment / System

Any AGL equipment item designated on the critical equipment list for the runway's declared operating category, e.g. a primary CCR with no functioning standby, the sole ALCMS server, or a critical single circuit feeding multiple LVP-mandatory systems.

Possible Causes

HighThe critical equipment item failed outright due to an internal fault (electrical, mechanical, or thermal) unrelated to redundancy

HighThe critical equipment item's designated backup/redundancy was itself already unavailable, converting a normally non-critical failure into a critical one

MedA maintenance action inadvertently affected the critical item while working on an adjacent, non-critical system

MedThe critical equipment list itself was outdated, failing to flag the item's criticality before the failure occurred

LowAn environmental event (power surge, extreme temperature, flooding) affecting the critical item specifically

LowEnd-of-life failure of a critical item that was not proactively replaced despite known ageing

Required Tools

Critical equipment list/registerEquipment-specific diagnostic tools per the applicable division procedureSpare parts/critical spares inventoryEmergency response/escalation procedure

Safety Precautions

Treat any critical equipment failure as an immediate flight-safety priority.
Coordinate immediately with ATC/airfield operations regarding potential NOTAM or category downgrade.
LOTO before any corrective work.
Escalate per the emergency response procedure given the criticality designation.

Diagnostic Procedure

ObservationConfirm the failed item's status on the critical equipment list and its associated operational impact (category downgrade, NOTAM requirement, etc.).
MeasurementApply the equipment-specific diagnostic procedure from the applicable division to identify the specific fault.
MeasurementConfirm whether any redundancy/backup for this item is available and functional.
DecisionRedundancy available and functional → transfer to backup while repairing the primary under standard priority. No redundancy available → treat as an immediate, all-hands repair priority with NOTAM/category action as required.
Fault LocationConfirm the specific component or subsystem responsible for the critical item's failure.
Root CauseConfirm full restoration of the critical item, and its redundancy where applicable, before closing out the critical response.

Test Points

Critical equipment list status

Equipment-specific diagnostic test points per the applicable division procedure

Redundancy/backup availability and function for the failed item

Root Cause Verification

After repair, confirm the critical item is fully restored to normal function, confirm redundancy is available and tested where applicable, and formally close out the critical response per the emergency procedure.

Corrective Action

Apply the equipment-specific corrective action per the applicable division procedure.
Expedite spare parts or specialist support given the critical designation.
Restore redundancy where it was found unavailable.

Preventive Action

Maintain and regularly review the critical equipment list to ensure it reflects current system configuration.
Hold critical spares inventory for designated items.
Proactively replace critical items approaching end of life rather than waiting for failure.
Brief maintenance staff on the critical equipment list so routine work near critical items is handled with appropriate caution.

Related References

SOP: SOP-LVO-11 (Critical Equipment Emergency Response Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5340-30, ICAO Annex 6

Diagram: Critical equipment list/register and system criticality map

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

Rev 0 — Initial issue

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Frequently Asked Questions

Quick answers about CAT I / CAT II / CAT III troubleshooting in the AGL Troubleshooting Atlas.

Q1What does Division 20 of the AGL Troubleshooting Atlas cover?

Division 20 is the CAT I / CAT II / CAT III Troubleshooting reference. It publishes 15 diagnostic topics, LVO-491 to LVO-505, for CAT I / CAT II / CAT III 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 CAT I / CAT II / CAT III faults are covered?

The 15 topics are LVO-491 to LVO-505: CAT I Lighting System Failure; CAT II Lighting System Failure; CAT III Lighting System Failure; Low Visibility Lighting Failure; Runway Centreline Failure During LVO; TDZ Lighting Failure During LVO; Stop Bar Failure During LVO; Approach Lighting Failure During LVO; PAPI Failure During LVO; AGL Monitoring Failure During LVO; Redundancy System Failure; Standby Circuit Failure; Backup Power Failure; Lighting System Availability Failure; Critical AGL Equipment Failure.

Q4How does a diagnostic procedure begin in Division 20?

Each procedure starts with observation before any intervention. For example, LVO-491 (CAT I Lighting System Failure) begins: Pull the current ALCMS/CCR serviceability report for all CAT I-required systems on the affected runway.

Q5What safety precautions apply when troubleshooting CAT I / CAT II / CAT III?

Each topic lists its own precautions. For example, LVO-491 (CAT I Lighting System Failure) starts with: Coordinate with ATC/airfield operations before any NOTAM action.

Q6Which SOPs are referenced in Division 20?

Related references in the topics include SOP-LVO-01 (Category Serviceability Assessment Procedure), SOP-LVO-02 (Standby Power Verification Procedure), SOP-LVO-03 (CAT III Redundancy Test Procedure), SOP-LVO-04 (LVP Activation and Verification Procedure), SOP-LVO-05 (LVO-Priority Fault Response Procedure), SOP-LVO-06 (Stop Bar Verification Procedure), SOP-LVO-07 (Monitoring System Failure Fallback Procedure) and SOP-LVO-08 (Redundancy System Maintenance Procedure), and 3 more.

Q7Which standards are referenced in the CAT I / CAT II / CAT III 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, ICAO Annex 6, FAA AC 120-118, ICAO Doc 9365, ICAO Annex 14 Vol I Attachment A, FAA AC 150/5345-28 and NFPA 110 where applicable.

Q8How can I find a specific fault in Division 20 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.