AGL-CS-091 — ALCMS Communication Failure
The AGL Knowledge Hub | Engineering Case Study Series. An ALCMS communication investigation where a core fiber-optic switch failure caused loss of remote monitoring and control — from local equipment verification through network diagnostics, fiber-optic testing, and redundancy 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 night operations, the airport operations center reported that multiple CCRs and taxiway lighting circuits had become unavailable on the ALCMS workstation.
The system displayed communication loss alarms, remote control commands were not being executed, and status indications from several field devices had disappeared.
However, local inspection confirmed that the CCRs remained operational in local mode, and the affected lighting circuits continued operating normally.
Because the failure affected communication rather than field equipment operation, a detailed investigation was initiated covering ALCMS server, network switches, fiber-optic communication, PLC communication, remote I/O stations, Ethernet infrastructure, communication protocols, and redundancy systems.
The investigation identified failure of a core fiber-optic communication switch, resulting in loss of communication between the ALCMS control network and multiple field equipment groups.
This case demonstrates that ALCMS communication failures are network and control-system problems that must be distinguished from actual airfield lighting equipment failures.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose ALCMS communication failures.
- Interpret communication-loss alarms.
- Verify network architecture.
- Investigate fiber-optic communication.
- Test PLC and remote I/O communication.
- Restore ALCMS connectivity.
- Verify post-repair control functionality.
3System Background
An ALCMS communication system typically includes:
- ALCMS servers.
- Operator workstations.
- PLC controllers.
- Remote I/O stations.
- Ethernet switches.
- Fiber-optic backbone.
- Communication gateways.
- CCR interfaces.
- Lighting control panels.
- Redundant communication paths.
Communication failures may result from:
- Fiber break.
- Switch failure.
- PLC communication failure.
- IP addressing problems.
- Gateway failure.
- Power-supply failure.
- Network loop.
- Configuration errors.
ALCMS reliability depends on communication infrastructure integrity.
4Problem Statement
The reported condition was:
- Multiple devices unavailable on ALCMS.
- Remote control not functioning.
- Communication alarms active.
- Local equipment operation normal.
- Field lighting operational.
Has the field equipment failed, or has the communication network connecting the ALCMS to the field equipment failed?
5Initial Observations
| ALCMS alarms | Communication loss |
| CCR local operation | Normal |
| Remote control | Failed |
| Field lighting | Operational |
| Multiple devices affected | Yes |
The symptoms suggested a communication infrastructure problem.
6Initial Engineering Hypotheses
The investigation considered:
ALCMS server failure.
Fiber-optic communication failure.
PLC communication failure.
Network switch failure.
Field equipment failure.
7ALCMS Communication Principle
Communication loss is not necessarily equipment failure.
8Test Equipment
The investigation used:
- Network analyzer.
- Ethernet tester.
- Fiber-optic OTDR.
- Managed-switch diagnostics.
- PLC programming software.
- Protocol analyzer.
- Ping and network diagnostic tools.
- ALCMS diagnostic software.
9Investigation
Step 1: Verify Field Equipment Operation
Local testing confirmed:
| CCR-1 | Normal |
| CCR-2 | Normal |
| CCR-3 | Normal |
| Taxiway panel | Normal |
Field equipment was healthy.
Step 2: Verify ALCMS Server
Server diagnostics showed:
- Server operational.
- Database operational.
- HMI operational.
- No server hardware alarms.
The server was healthy.
Step 3: Network Communication Test
Ping testing showed:
| Device | Result |
| Server | Reachable |
| Core switch | Reachable |
| Field PLC network | Not reachable |
Communication failed beyond the core network.
Step 4: Switch Diagnostics
Managed-switch inspection revealed:
- Fiber uplink failure.
- Port communication loss.
- Link status inactive.
- Redundancy path unavailable.
The communication backbone was interrupted.
Step 5: Fiber-Optic Testing
OTDR testing identified:
- No fiber break.
- Excessive optical loss.
- Link interruption at the core switch.
The fiber cable remained intact.
Step 6: Core Switch Inspection
Inspection revealed:
- Switch power normal.
- Internal switching failure.
- Optical interface fault.
- Communication processing failure.
The switch had failed internally.
Step 7: Redundancy Verification
Investigation showed:
- Redundant switch bypass disabled.
- Automatic failover not functioning.
- Redundancy configuration incomplete.
The backup communication path was unavailable.
10Root Cause
The ALCMS communication failure was caused by failure of the core fiber-optic Ethernet switch, resulting in:
- Loss of communication with field PLCs.
- Loss of remote I/O communication.
- Loss of CCR monitoring.
- Loss of remote control functionality.
Field equipment remained operational locally.
The primary failure mechanism was network infrastructure failure.
11Corrective Action
The engineering team:
- Replaced the failed core switch.
- Restored fiber communication.
- Verified PLC communication.
- Restored remote I/O connectivity.
- Verified CCR monitoring.
- Tested remote control commands.
- Corrected redundancy configuration.
- Tested automatic failover.
- Updated network documentation.
- Implemented switch-health monitoring.
12Post-Repair Verification
Switch replacement restored full ALCMS communication.
13Root-Cause Analysis
Multiple devices unavailable
Healthy
Healthy
Field network unreachable
Failed
Communication restored
Core fiber-optic switch failure
Restore network infrastructure
14Why This Case Was Important
ALCMS communication failures can cause:
- Loss of remote lighting control.
- Loss of CCR monitoring.
- Increased maintenance response time.
- Reduced operational visibility.
- SCADA alarm failures.
- Reduced automation capability.
Communication infrastructure is critical airport operational infrastructure.
15Engineering Lessons
Communication loss does not necessarily indicate field-equipment failure.
Communication loss does not necessarily indicate field-equipment failure.
Network architecture must be understood before troubleshooting.
Network architecture must be understood before troubleshooting.
Fiber-optic switches are critical infrastructure components.
Fiber-optic switches are critical infrastructure components.
Redundancy systems must be periodically tested.
Redundancy systems must be periodically tested.
Communication diagnostics should begin with network segmentation.
Communication diagnostics should begin with network segmentation.
16Engineer's Decision Point
You observe: Multiple CCRs unavailable on ALCMS. Remote control not working. Field lighting operating normally. Communication alarms active. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about ALCMS communication failure and network diagnosis.
Q1What is AGL-CS-091?
AGL-CS-091 is an engineering training case study. It covers an ALCMS that lost communication with multiple CCRs and circuits while the field equipment kept working locally. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2How do you tell communication loss from equipment failure?
Test the field equipment locally first. Here the CCRs and taxiway panel worked normally in local mode and the lighting stayed on, so the failure was in the network, not the equipment.
Q3Was the ALCMS server at fault?
No. The server, database and HMI were operational with no hardware alarms.
Q4Where did the communication fail?
Ping tests reached the server and core switch but not the field PLC network, so communication failed beyond the core network.
Q5What did switch and fibre testing show?
The managed switch showed a fibre uplink failure, port communication loss and an inactive link, with the redundancy path unavailable. OTDR testing found no fibre break.
Q6What was the root cause of AGL-CS-091?
Failure of a core fibre-optic Ethernet switch, which cut communication with field PLCs and remote I/O, and with it CCR monitoring and remote control.
Q7How was communication restored?
The team replaced the failed core switch, restored fibre communication, verified PLC, remote I/O and CCR monitoring, and tested remote control commands. They also corrected the redundancy configuration and tested automatic failover.
Q8What preventive measures followed?
The team updated network documentation and implemented switch-health monitoring.
Q9What is the key lesson from this case?
Communication loss is not necessarily equipment failure. Verify the field locally before troubleshooting the lighting circuits.
Engineering & Expert Notes
Draw the Network Before Testing
ArchitectureUnderstand the topology to see what each failure affects.
Confirm Field Equipment Works Locally
VerificationLocal operation shows the field is healthy and narrows the search.
Test Redundant Paths Regularly
RedundancyA standby path that has never been tested may not work.
Keep Spare Network Switches
SparesCore switches are single points. Have replacements configured and ready.
20Case Study Conclusion
ALCMS communication failures are network and automation-system failures caused by communication infrastructure defects such as fiber-optic switch failures, PLC network interruptions, and redundancy-system problems, and effective diagnosis requires systematic verification of field equipment, server operation, network connectivity, communication architecture, and control-system redundancy.
Systematic network segmentation, switch diagnostics, fiber-optic testing, PLC communication verification, redundancy validation, and post-repair control testing provide a complete engineering methodology for restoring reliable ALCMS communication and airport lighting automation.
In AGL engineering, the most important question during an ALCMS alarm is not whether the lights are off, but whether the control system has simply lost the ability to see them.
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