AGL-CS-094 — Incorrect Lighting Status Display
The AGL Knowledge Hub | Engineering Case Study Series. An ALCMS HMI diagnostics case where incorrect PLC-to-HMI tag mapping caused the display to show the opposite of actual field conditions — from field verification through PLC diagnostics, HMI configuration review, and status 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 observed that the ALCMS displayed Runway Edge Circuit RE-2 as ON, while field personnel confirmed that the runway edge lights were actually OFF.
Additional verification revealed that:
- CCR output current was zero.
- The circuit had not been energized.
- No lighting was visible on the runway.
- The ALCMS HMI continued displaying the circuit as ON.
Because the displayed lighting status did not match actual field conditions, a detailed investigation was initiated covering CCR feedback signals, PLC input mapping, HMI tag configuration, communication updates, signal inversion, status logic, database configuration, and event synchronization.
The investigation identified incorrect PLC-to-HMI tag mapping combined with inverted status logic, causing the ALCMS to display the opposite operational status of the runway lighting circuit.
This case demonstrates that incorrect lighting status displays can create operational hazards even when communication and field equipment are functioning correctly.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose incorrect ALCMS status displays.
- Verify actual field equipment operation.
- Test PLC input mapping.
- Investigate HMI tag configuration.
- Analyze signal inversion.
- Restore status-display accuracy.
- Validate monitoring-system integrity.
3System Background
A typical ALCMS status-monitoring chain includes:
- Field equipment.
- Status contacts.
- PLC input modules.
- PLC logic.
- Communication network.
- SCADA server.
- HMI database.
- Graphical status display.
- Alarm-processing logic.
Incorrect status displays may result from:
- PLC tag mapping errors.
- Signal inversion.
- HMI configuration errors.
- Database synchronization problems.
- Communication update failures.
- PLC logic errors.
- Incorrect addressing.
- Software modification errors.
Accurate HMI status depends on correct signal interpretation.
4Problem Statement
The reported condition was:
- ALCMS showed circuit ON.
- Field lights OFF.
- CCR output OFF.
- No communication alarms.
- Status display incorrect.
Why is the ALCMS displaying an incorrect operational status despite healthy communication and correct field conditions?
5Initial Observations
| ALCMS display | ON |
| Field lights | OFF |
| CCR output | OFF |
| Communication | Healthy |
| Alarm status | Normal |
The discrepancy indicated a monitoring interpretation problem.
6Initial Engineering Hypotheses
The investigation considered:
Field-equipment failure.
Feedback-contact failure.
PLC input mapping error.
HMI tag inversion.
Database synchronization error.
7Status-Display Principle
Display accuracy depends on correct signal mapping.
8Test Equipment
The investigation used:
- PLC programming software.
- SCADA diagnostic tools.
- Signal analyzer.
- Digital multimeter.
- HMI tag-monitoring software.
- Communication diagnostics.
- Event logs.
- Control schematics.
9Investigation
Step 1: Verify Field Operation
Measurements showed:
| CCR output current | 0 A |
| CCR output voltage | 0 V |
| Circuit illumination | OFF |
| Local status | OFF |
The field equipment was OFF.
Step 2: Verify Feedback Signal
Feedback-contact measurements showed:
- Feedback contact OPEN.
- OFF status correctly transmitted.
- No wiring fault.
The feedback signal was correct.
Step 3: Verify PLC Input
PLC diagnostics showed:
Input I3.2: OFF
The PLC correctly received the OFF signal.
Step 4: Verify PLC Logic
Logic monitoring showed:
Internal status bit: OFF
The PLC logic was correct.
Step 5: Verify HMI Tag
The HMI displayed:
ON
Tag monitoring showed:
- HMI tag linked to the wrong PLC bit.
- Logic inversion applied.
- Incorrect Boolean interpretation.
The HMI configuration was incorrect.
Step 6: Database Review
Configuration review revealed:
- Recent software modification.
- Tag remapping.
- Incorrect address assignment.
- Inverted display logic.
The HMI database contained a configuration error.
Step 7: Corrected Configuration Test
After correcting the tag mapping:
Field status: OFF
PLC status: OFF
HMI display: OFF
The display matched actual field conditions.
10Root Cause
The incorrect lighting status display was caused by incorrect PLC-to-HMI tag mapping combined with inverted status logic, resulting in:
- Incorrect status interpretation.
- Incorrect graphical display.
- Misleading operational information.
- Incorrect operator awareness.
Field equipment and communication remained healthy.
The primary failure mechanism was HMI configuration error.
11Corrective Action
The engineering team:
- Corrected HMI tag mapping.
- Removed signal inversion.
- Verified PLC addressing.
- Tested status changes.
- Verified ON/OFF transitions.
- Verified alarm logic.
- Confirmed event synchronization.
- Updated configuration documentation.
- Implemented configuration review procedures.
- Added HMI validation testing.
12Post-Repair Verification
Configuration correction restored accurate status display.
13Root-Cause Analysis
HMI showed incorrect status
Correct
Correct
Correct
Correct
Incorrect
Display verified correct
Incorrect HMI tag mapping
Restore monitoring configuration integrity
14Why This Case Was Important
Incorrect status displays can cause:
- Incorrect operational decisions.
- Delayed lighting activation.
- Incorrect runway availability assessment.
- Increased controller workload.
- Reduced confidence in the ALCMS.
- Potential operational safety risks.
HMI accuracy is an operational safety requirement.
15Engineering Lessons
Always verify field conditions before trusting the HMI.
Always verify field conditions before trusting the HMI.
PLC and HMI must be tested independently.
PLC and HMI must be tested independently.
Configuration errors can create false operational information.
Configuration errors can create false operational information.
Signal inversion should be carefully validated.
Signal inversion should be carefully validated.
HMI validation testing is essential after software modifications.
HMI validation testing is essential after software modifications.
16Engineer's Decision Point
You observe: ALCMS shows runway lights ON. Field lights OFF. CCR output OFF. Communication healthy. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about incorrect lighting status displays and PLC-to-HMI tag mapping.
Q1What is AGL-CS-094?
AGL-CS-094 is an engineering training case study. It covers an ALCMS that showed a runway edge circuit as ON while it was actually OFF. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2What was the mismatch in AGL-CS-094?
The ALCMS displayed Runway Edge Circuit RE-2 as ON, but CCR output current and voltage were zero, the lights were off and local status was OFF.
Q3Why is an incorrect status display a hazard?
Operators rely on the display to make decisions. A false ON indication creates operational risk even when communication and field equipment are functioning correctly.
Q4Where in the signal chain was the error found?
The feedback contact was open and correctly transmitted OFF. PLC input I3.2 was OFF and the PLC logic status bit was OFF. The HMI tag, however, showed ON.
Q5What was wrong with the HMI configuration?
The HMI tag was linked to the wrong PLC bit, with logic inversion and incorrect Boolean interpretation applied.
Q6What caused the configuration error?
A recent software modification had remapped tags and introduced an incorrect address assignment and inverted display logic.
Q7What was the root cause of AGL-CS-094?
Incorrect PLC-to-HMI tag mapping combined with inverted status logic, so the ALCMS displayed the opposite of the actual circuit status.
Q8How was it corrected?
The team corrected the tag mapping, removed the signal inversion and verified PLC addressing. They tested status changes and ON/OFF transitions, verified alarm logic and event synchronization, updated configuration documentation and added configuration review and HMI validation testing.
Q9What is the key lesson from this case?
Display accuracy depends on correct signal mapping. Validate HMI status against field conditions after any software change.
Engineering & Expert Notes
Compare the Field Status With Each Screen State
MethodWalk through each state and check the display matches.
Validate Tag Mapping During Commissioning
ConfigurationWrong tags cause wrong displays. Check each tag.
Test Both States of Every Signal
LogicInverted logic shows only when you test both on and off.
Keep Configuration Under Change Control
RecordsRecord changes so the display stays aligned with the system.
20Case Study Conclusion
An incorrect ALCMS lighting status display is often a monitoring and HMI configuration failure rather than a field-equipment failure, and systematic verification of field conditions, feedback signals, PLC inputs, PLC logic, HMI tag mapping, and status-display configuration is essential for maintaining accurate operational awareness.
Systematic field verification, PLC diagnostics, HMI configuration review, tag validation, signal-logic verification, and post-repair display testing provide a complete engineering methodology for restoring accurate lighting-status representation and maintaining confidence in airport lighting automation systems.
In AGL engineering, operators do not control the airport based on the lights they can see; they often control it based on the lights the ALCMS tells them are operating, which makes display accuracy a safety-critical engineering responsibility.
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