AGL-CS-039 — CCR Local/Remote Control Failure
The AGL Knowledge Hub | Engineering Case Study Series. A control-system investigation where the CCR operated locally but failed to respond to remote commands — from SCADA verification through PLC diagnostics, selector-switch inspection, and permissive-circuit 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 runway lighting operation, the Airfield Lighting Control and Monitoring System (ALCMS) was unable to operate one runway edge lighting circuit remotely.
The Constant Current Regulator (CCR) was healthy and could be started successfully from the local control panel, but all remote start, stop, and brightness-step commands from the control room failed.
The CCR generated no power-system alarms, and current regulation remained normal during local operation.
A systematic investigation identified failure of the local/remote control selector switch combined with loss of a remote control permissive signal, preventing remote commands from reaching the CCR control logic.
This case demonstrates that a CCR may be electrically healthy while the control system is unable to operate it remotely.
2Learning Objectives
After completing this case study, the learner should be able to:
- Understand CCR local and remote control architecture.
- Diagnose remote-control failures systematically.
- Differentiate control failures from power failures.
- Investigate selector switches, relays, PLCs, and SCADA interfaces.
- Verify control permissive signals.
- Test remote command transmission and feedback.
- Restore reliable local and remote CCR operation.
3System Background
A typical CCR control system includes:
- Local control panel.
- Local/remote selector switch.
- PLC or control logic.
- Remote I/O interface.
- SCADA or ALCMS.
- Control relays.
- Status feedback circuits.
- Permissive interlocks.
Normal remote operation:
Failure of any control element can prevent remote operation.
4Problem Statement
The reported condition was:
- CCR healthy.
- Local control operational.
- Remote control failed.
- No power-system alarms.
- No communication alarm.
- Remote status indication incorrect.
Why can the CCR operate locally but not remotely?
5Initial Observations
| CCR health | Normal |
| Local operation | Successful |
| Remote operation | Failed |
| Current regulation | Normal |
| Output voltage | Normal |
| SCADA command | Not executed |
The ability to operate locally proved that the power section of the CCR was healthy.
6Initial Engineering Hypotheses
The maintenance team considered:
SCADA communication failure.
PLC output failure.
Local/remote selector switch fault.
Remote control relay failure.
Missing permissive interlock signal.
7Control System Principle
Local mode:
Local controls directly command the CCR.
Remote mode:
A selector-switch failure can isolate the remote control path while leaving local control operational.
8Test Equipment
The investigation used:
- Digital multimeter.
- PLC diagnostic software.
- SCADA monitoring interface.
- Relay test equipment.
- Continuity tester.
- Control-circuit drawings.
- I/O status monitoring tools.
- Airport-approved PPE.
9Investigation
Step 1: Verify CCR Power Operation
The CCR successfully:
- Started locally.
- Changed brightness steps.
- Maintained current.
- Stopped normally.
The CCR power section was healthy.
Step 2: Verify SCADA Command
The control room issued:
- Start command.
- Stop command.
- Step-change command.
SCADA confirmed that commands were transmitted successfully.
Step 3: PLC Input Verification
PLC diagnostics showed:
| Remote start | Received |
| Remote stop | Received |
| Step command | Received |
The PLC was receiving remote commands.
Step 4: PLC Output Verification
The remote start output was not energizing the CCR control relay.
The output was being blocked by a permissive condition.
Step 5: Selector Switch Inspection
Testing revealed:
- Mechanical wear.
- Contact oxidation.
- Unstable remote-position continuity.
- Incorrect feedback indication.
The selector switch did not reliably transfer control to the remote circuit.
Step 6: Permissive Interlock Testing
A remote-enable permissive signal was missing.
Investigation found:
- Loose control terminal.
- Intermittent control wiring.
- PLC permissive input inactive.
The CCR control logic was intentionally blocking remote operation.
10Root Cause
The CCR local/remote control failure was caused by a defective local/remote selector switch and loss of the remote control permissive signal.
Remote commands reached the PLC, but the control logic did not energize the CCR start relay because:
- The selector switch did not establish a reliable remote-control path.
- The permissive circuit remained open.
The CCR itself was healthy.
11Corrective Action
The maintenance team:
- Replaced the local/remote selector switch.
- Repaired the permissive control wiring.
- Tightened control terminals.
- Verified PLC input status.
- Verified PLC output operation.
- Tested remote control relays.
- Confirmed SCADA communication.
- Performed local/remote functional testing.
- Restored remote operation.
- Updated maintenance records.
12Post-Repair Verification
The CCR operated correctly from both local and remote control stations.
13Root-Cause Analysis
Remote control failed
Healthy
Commands transmitted
Commands received
Blocked
Defective
Open
Selector-switch failure and missing remote permissive
Replace switch and repair control wiring
14Why This Case Was Important
A CCR can be fully functional electrically while remaining unavailable operationally because of:
- Selector-switch faults.
- PLC logic conditions.
- Control relays.
- Communication interfaces.
- Permissive interlocks.
- Control power failures.
Control diagnostics require a different approach from power diagnostics.
15Engineering Lessons
Local operation proves power health, not remote-control health.
Local operation proves power health, not remote-control health.
Always verify the control path step by step.
Always verify the control path step by step.
PLC diagnostics can quickly identify where commands stop.
PLC diagnostics can quickly identify where commands stop.
Selector switches are common control-system failure points.
Selector switches are common control-system failure points.
Permissive interlocks must be verified before replacing major equipment.
Permissive interlocks must be verified before replacing major equipment.
16Engineer's Decision Point
You observe: CCR healthy. Local control works. Remote control fails. SCADA commands transmitted. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about CCR local/remote control failure, based on this case study.
Q1What is AGL-CS-039?
AGL-CS-039 is an engineering training case study on a CCR that worked from its local panel but failed to respond to remote commands from the control room ALCMS. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2What proves the CCR power section is healthy in this case?
The CCR started successfully from the local control panel, with normal current regulation and output voltage and no power-system alarms. The ability to operate locally proved the power section was healthy.
Q3What was the root cause?
A defective local/remote selector switch and loss of the remote control permissive signal. Remote commands reached the PLC, but the control logic did not energize the CCR start relay.
Q4What should you investigate first when local control works but remote fails?
Verify the local/remote selector switch, PLC outputs, control relays and permissive circuits step by step. Replacing the CCR, increasing the current setting or replacing field transformers is not the right first step.
Q5Which tools were used?
The team used a digital multimeter, PLC diagnostic software, SCADA monitoring interface, relay test equipment, continuity tester, control-circuit drawings and I/O status monitoring tools.
Q6How was the control path repaired?
The team replaced the local/remote selector switch and repaired the permissive control wiring. They tightened control terminals, verified PLC inputs and outputs, tested remote control relays, confirmed SCADA communication and performed local/remote functional testing.
Q7How was the repair verified?
Local and remote operation were both normal, SCADA control was functional, the PLC permissive was active and status feedback was correct. The CCR operated correctly from both control stations.
Q8Why does this case involve control circuits rather than power circuits?
The fault was in the control path, not the field or power circuit, so measuring current and voltage would not have found it. Following the command from SCADA through the PLC, relays and permissive signal located the break.
Engineering & Expert Notes
Walk the Control Path Step by Step
MethodTrace from the operator command through the PLC, relay and permissive to the CCR. Find where the signal stops.
Check Selector Switches and Permissives
ComponentsMechanical contacts are common weak points. Test them before blaming software.
Use PLC Diagnostics Early
ToolsInput and output status often show the break within minutes.
Restore Local Control Safely First
OperationsAgree with operations how the circuit will be run while remote control is repaired.
20Case Study Conclusion
A CCR may be electrically healthy and fully operational locally while remote operation fails because of defects in the selector switch, PLC control path, control relays, communication interfaces, or permissive interlock circuits.
Systematic control-path verification, PLC diagnostics, relay testing, selector-switch inspection, permissive verification, and functional testing can accurately identify local/remote control failures and restore reliable airfield lighting control.
In AGL engineering, a CCR that runs locally but not remotely is usually a control-system problem, not a power-system problem.
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