AGL-CS-093 — Field Equipment Healthy but ALCMS Shows Fault
The AGL Knowledge Hub | Engineering Case Study Series. An ALCMS false alarm investigation where feedback relay failure caused a fault indication on healthy field equipment — from field verification through PLC diagnostics, feedback circuit testing, and monitoring restoration.
Important: This is an engineering simulation created for training. It is not presented as an actual airport incident. All readings, values, dates and outcomes are illustrative, and no airport, organisation, person or manufacturer is identified or implied.
1Case Study Overview
During routine evening operations, the airport operations center reported that the ALCMS displayed a fault alarm for CCR-3 and Taxiway Circuit TW-5.
The HMI indicated:
- CCR fault.
- Circuit unavailable.
- Remote alarm active.
However, field inspection confirmed that:
- CCR output current was normal.
- Taxiway lights were operating correctly.
- Local CCR controls functioned normally.
- No protection trips were present.
Because the ALCMS indicated a fault while the field equipment remained fully operational, a detailed investigation was initiated covering feedback contacts, PLC input signals, remote I/O modules, communication integrity, status-monitoring circuits, alarm logic, signal conditioning, and ALCMS database interpretation.
The investigation identified failure of the feedback relay contact used for CCR status monitoring, causing the ALCMS to interpret healthy equipment as faulty.
This case demonstrates that ALCMS fault alarms must always be verified against actual field conditions before concluding that equipment has failed.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose false ALCMS fault indications.
- Verify field equipment independently.
- Test feedback circuits.
- Investigate PLC input signals.
- Analyze ALCMS alarm logic.
- Restore monitoring accuracy.
- Validate alarm integrity.
3System Background
An ALCMS monitoring system typically includes:
- Field equipment.
- Status sensors.
- Feedback relays.
- PLC input modules.
- Remote I/O stations.
- Communication network.
- ALCMS server.
- Alarm-processing logic.
- HMI displays.
False alarms may result from:
- Feedback relay failure.
- PLC input failure.
- Broken feedback wiring.
- Remote I/O faults.
- Communication latency.
- Configuration errors.
- Alarm-logic errors.
- Signal-conditioning faults.
Monitoring integrity depends on accurate status feedback.
4Problem Statement
The reported condition was:
- ALCMS showing CCR fault.
- Field equipment operating normally.
- No electrical abnormalities.
- Local control available.
- False operational indication suspected.
Why is the ALCMS indicating a fault when the field equipment is healthy?
5Initial Observations
| ALCMS alarm | Active |
| CCR operation | Normal |
| Circuit operation | Normal |
| Local control | Normal |
| Field inspection | Healthy |
The discrepancy suggested a monitoring-system fault.
6Initial Engineering Hypotheses
The investigation considered:
Actual CCR fault.
Communication failure.
Feedback relay failure.
PLC input failure.
Alarm-logic configuration error.
7Monitoring Principle
Monitoring systems can fail independently of field equipment.
8Test Equipment
The investigation used:
- Digital multimeter.
- PLC diagnostic software.
- Signal analyzer.
- Relay test equipment.
- Network diagnostic tools.
- ALCMS event logs.
- Control schematics.
9Investigation
Step 1: Verify Field Operation
Measurements showed:
| CCR current | Normal |
| CCR voltage | Normal |
| Circuit illumination | Normal |
| Protection status | Healthy |
The field equipment was healthy.
Step 2: Verify ALCMS Communication
Communication testing showed:
- PLC communication healthy.
- Remote I/O healthy.
- Server communication healthy.
- No network errors.
Communication was normal.
Step 3: Check PLC Input Status
PLC diagnostics showed:
CCR running input: OFF
Expected: ON
The PLC was receiving an incorrect status signal.
Step 4: Measure Feedback Circuit
Measurements showed:
Feedback contact: Open
Expected: Closed
The status contact was not changing state.
Step 5: Relay Inspection
Inspection revealed:
- Relay coil energized.
- Mechanical operation normal.
- Auxiliary feedback contact failed.
- High contact resistance.
The feedback contact had deteriorated.
Step 6: Temporary Bypass Test
A temporary bypass of the feedback contact caused:
- ALCMS fault cleared.
- CCR status changed to healthy.
- Monitoring restored.
The false alarm disappeared.
Step 7: Alarm Logic Verification
ALCMS logic was confirmed correct.
The alarm was generated solely because of the failed feedback signal.
10Root Cause
The ALCMS displayed a false CCR fault because of failure of the auxiliary feedback relay contact, resulting in:
- Incorrect PLC input status.
- False equipment-fault indication.
- Incorrect HMI alarm.
- Misleading operational information.
The field equipment remained fully operational.
The primary failure mechanism was feedback-circuit failure.
11Corrective Action
The engineering team:
- Replaced the feedback relay.
- Verified relay operation.
- Tested PLC input signals.
- Verified ALCMS status.
- Tested alarm generation.
- Tested alarm clearing.
- Verified event logging.
- Updated maintenance records.
- Added feedback-relay inspection to preventive maintenance.
- Reviewed alarm-validation procedures.
12Post-Repair Verification
Feedback restoration corrected the false alarm.
13Root-Cause Analysis
ALCMS showed fault
Healthy
Healthy
Incorrect
Failed
Status verified correct
Feedback relay contact failure
Restore monitoring feedback integrity
14Why This Case Was Important
False ALCMS alarms can cause:
- Unnecessary maintenance dispatch.
- Incorrect operational decisions.
- Reduced confidence in the monitoring system.
- Increased troubleshooting time.
- Alarm fatigue.
- Incorrect equipment replacement.
Monitoring integrity is an operational safety issue.
15Engineering Lessons
Always verify field equipment before replacing components.
Always verify field equipment before replacing components.
Monitoring systems can fail independently of field equipment.
Monitoring systems can fail independently of field equipment.
Feedback contacts are critical monitoring components.
Feedback contacts are critical monitoring components.
PLC input verification is a key diagnostic step.
PLC input verification is a key diagnostic step.
Alarm validation should be part of ALCMS maintenance.
Alarm validation should be part of ALCMS maintenance.
16Engineer's Decision Point
You observe: ALCMS showing CCR fault. Field equipment operating normally. Communication healthy. Local operation normal. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about false ALCMS fault alarms and feedback signal failure.
Q1What is AGL-CS-093?
AGL-CS-093 is an engineering training case study. It covers an ALCMS showing a CCR fault and unavailable circuit while the field equipment was working normally. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Why did the ALCMS show a fault on healthy equipment?
The auxiliary feedback contact used for CCR status monitoring failed. The PLC then read the CCR running input as OFF instead of ON and the HMI displayed a false alarm.
Q3How was the field equipment shown to be healthy?
CCR current and voltage were normal, the taxiway lights were operating, protection status was healthy and local controls worked.
Q4Was communication the problem?
No. PLC, remote I/O and server communication were healthy with no network errors.
Q5What did the feedback circuit measurements show?
The feedback contact was open when it should have been closed. The relay coil was energized and mechanically normal, but the auxiliary feedback contact showed high contact resistance.
Q6How was the cause confirmed?
A temporary bypass of the feedback contact cleared the ALCMS fault and restored healthy status. The ALCMS alarm logic itself was confirmed correct.
Q7What was the root cause of AGL-CS-093?
Failure of the auxiliary feedback relay contact, giving incorrect PLC input status, a false equipment-fault indication and a misleading HMI alarm.
Q8How was the issue corrected?
The team replaced the feedback relay and verified relay operation, PLC inputs and ALCMS status. They also tested alarm generation and clearing, verified event logging, added feedback-relay inspection to preventive maintenance and reviewed alarm-validation procedures.
Q9What is the key lesson from this case?
Always verify ALCMS fault alarms against actual field conditions. Monitoring systems can fail independently of the equipment they monitor.
Engineering & Expert Notes
Confirm the Field Condition Before Acting on an Alarm
VerificationCheck equipment locally before replacing anything.
Trace the Signal From the Field Contact to the HMI
MethodFollow the input chain to find where the false signal starts.
Test Feedback Contacts Regularly
MaintenanceAuxiliary contacts wear out. Include them in routine tests.
Log False Alarms and Their Causes
RecordsA record of nuisance alarms guides improvement.
20Case Study Conclusion
An ALCMS fault indication does not necessarily represent an actual field-equipment failure, and systematic verification of field operation, PLC input status, feedback circuits, relay contacts, communication integrity, and alarm logic is essential for distinguishing monitoring-system failures from genuine equipment faults.
Systematic field verification, feedback testing, PLC diagnostics, relay inspection, alarm validation, and post-repair monitoring verification provide a complete engineering methodology for restoring accurate ALCMS status monitoring and maintaining confidence in airport lighting automation systems.
In AGL engineering, the most dangerous alarm is not always the one that indicates a fault, but the one that indicates a fault that does not actually exist.
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