AGL-CS-085 — Approach Lighting Alignment Investigation
The AGL Knowledge Hub | Engineering Case Study Series. An approach-lighting alignment investigation where incorrect survey reference caused lateral centerline displacement — from runway reference verification through ALS survey, crossbar geometry inspection, construction record review, and geometric realignment.
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
Following runway rehabilitation and approach-lighting cable replacement, pilots reported that the approach lighting system appeared offset from the runway centerline during final approach.
The lights were operational, intensity was acceptable, and the sequenced flashing system functioned correctly.
Because the reported problem suggested a geometric alignment error, a comprehensive investigation was initiated covering:
- Runway centerline alignment.
- Threshold reference position.
- Lateral offset.
- Longitudinal positioning.
- Foundation elevations.
- Crossbar alignment.
- Sequenced flashing light alignment.
- Survey control verification.
The investigation identified lateral displacement of the approach-lighting centerline caused by incorrect survey reference during post-construction installation, resulting in misalignment of the visual approach guidance system.
This case demonstrates that approach-lighting systems can be electrically and photometrically healthy while providing incorrect visual guidance due to installation alignment errors.
2Learning Objectives
After completing this case study, the learner should be able to:
- Investigate ALS alignment errors.
- Verify runway reference geometry.
- Assess lateral and longitudinal alignment.
- Evaluate crossbar positioning.
- Verify foundation elevations.
- Conduct survey-based alignment inspections.
- Restore ICAO-compliant ALS geometry.
3System Background
An ALS installation depends on:
- Runway centerline reference.
- Threshold reference point.
- Longitudinal spacing.
- Crossbar alignment.
- Foundation elevations.
- Lateral offset.
- Sequenced flashing light alignment.
- Survey control accuracy.
Common alignment-error causes include:
- Incorrect survey references.
- Construction-related displacement.
- Foundation settlement.
- Crossbar installation errors.
- Centerline offset.
- Elevation changes.
- Pavement rehabilitation.
ALS performance depends on accurate geometric alignment with the runway.
4Problem Statement
The reported condition was:
- Approach lights operational.
- Sequenced flashing operational.
- Pilots reporting centerline offset.
- Runway rehabilitation recently completed.
- No electrical abnormalities.
Has the approach-lighting system shifted relative to the runway reference geometry?
5Initial Observations
| ALS operation | Normal |
| Flashing sequence | Normal |
| Pilot reports | Consistent |
| Recent construction | Yes |
| Visual appearance | Slightly offset |
The problem suggested a geometric installation error.
6Initial Engineering Hypotheses
The investigation considered:
Pilot visual illusion.
Runway centerline survey error.
ALS alignment error.
Crossbar displacement.
Foundation settlement.
7ALS Alignment Principle
ALS accuracy depends on survey accuracy.
8Test Equipment
The investigation used:
- Total station.
- Digital level.
- GNSS survey equipment.
- Laser alignment system.
- Optical alignment telescope.
- ICAO Annex 14 ALS reference coordinates.
- Construction survey records.
9Investigation
Step 1: Verify Runway Reference
Survey verification confirmed:
- Runway centerline correct.
- Threshold reference point correct.
- Runway coordinates accurate.
The runway reference system was healthy.
Step 2: Survey ALS Centerline
Measured lateral offset:
Required:
0.0 m
Measured:
1.35 m right of runway centerline
The ALS centerline was displaced.
Step 3: Longitudinal Position Verification
Measured longitudinal positions:
| Feature | Required | Measured |
| Barrette 1 | Correct | Correct |
| Crossbar 1 | Correct | Correct |
| Crossbar 2 | Correct | Correct |
| SFL units | Correct | Correct |
Longitudinal geometry was acceptable.
Step 4: Crossbar Alignment Inspection
Measurements showed:
- Crossbars parallel.
- Crossbar spacing correct.
- Crossbar orientation correct.
Crossbars were displaced laterally with the centerline.
Step 5: Foundation Survey
Measured foundation elevations:
Variation:
Within ±12 mm
Elevations were acceptable.
Step 6: Construction Record Review
Records revealed:
- Temporary survey control used.
- One reference marker disturbed during pavement work.
- ALS installation referenced to the temporary marker.
A survey-reference error was identified.
Step 7: Operational Geometry Analysis
Simulation showed:
- Apparent runway offset.
- Incorrect centerline guidance.
- Increased lateral correction during final approach.
- Pilot reports reproduced.
The geometric error explained the operational observations.
10Root Cause
The ALS alignment error was caused by incorrect lateral installation geometry resulting from survey-reference displacement, including:
- Disturbed survey marker.
- Incorrect construction reference.
- Lateral centerline displacement.
- Post-construction installation error.
The electrical and photometric systems remained healthy.
The primary failure mechanism was incorrect geometric alignment with the runway centerline.
11Corrective Action
The engineering team:
- Re-established permanent survey control.
- Verified runway reference coordinates.
- Re-surveyed the ALS installation.
- Corrected centerline alignment.
- Repositioned affected foundations.
- Verified crossbar geometry.
- Verified SFL alignment.
- Conducted operational verification.
- Updated survey documentation.
- Completed post-realignment certification.
12Post-Repair Verification
Geometric realignment restored correct ALS guidance.
13Root-Cause Analysis
Pilot reports received
Healthy
Correct
Offset
Reference error found
Operational verification
Survey-related ALS alignment error
Restore installation geometry
14Why This Case Was Important
ALS alignment errors can cause:
- Incorrect runway acquisition.
- Lateral approach guidance errors.
- Increased pilot workload.
- ICAO non-compliance.
- Flight inspection failures.
- Post-construction operational problems.
Approach lighting is fundamentally a surveyed visual-guidance system.
15Engineering Lessons
Operational lights do not guarantee correct alignment.
Operational lights do not guarantee correct alignment.
Survey control is critical during ALS installation.
Survey control is critical during ALS installation.
Construction activities can introduce geometric errors.
Construction activities can introduce geometric errors.
Centerline alignment is a primary acceptance parameter.
Centerline alignment is a primary acceptance parameter.
Post-construction survey verification is essential.
Post-construction survey verification is essential.
16Engineer's Decision Point
You observe: Approach lights ON. Sequenced flashing normal. Pilots report the ALS appears offset from the runway centerline. Recent rehabilitation completed. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about approach lighting alignment and survey errors after runway rehabilitation.
Q1What is AGL-CS-085?
AGL-CS-085 is an engineering training case study. It covers an approach lighting system that appeared offset from the runway centreline after rehabilitation and cable replacement. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Can approach lights be electrically and photometrically healthy but still give wrong guidance?
Yes. The lights were operational, intensity was acceptable and the sequenced flashing worked, but the centreline was displaced, so the visual reference was wrong.
Q3How large was the offset in AGL-CS-085?
The survey measured 1.35 m right of the runway centreline against a required 0.0 m.
Q4What else was checked and found correct?
The runway reference and threshold coordinates were correct, longitudinal positions were correct, crossbars were parallel with correct spacing, and foundation elevations were within ±12 mm.
Q5Where did the error come from?
Construction records showed temporary survey control was used and one reference marker was disturbed during pavement work. The ALS installation was referenced to that marker.
Q6What was the root cause?
Incorrect lateral installation geometry from survey-reference displacement: a disturbed marker, an incorrect construction reference and a post-construction installation error.
Q7Why does ALS accuracy depend on survey accuracy?
The approach pattern only guides pilots correctly if it is aligned with the runway centreline. A survey error shifts the whole visual reference, even when every fixture works.
Q8How was the alignment corrected?
The team re-established permanent survey control, re-surveyed the installation, corrected centreline alignment and repositioned affected foundations. They verified crossbar and SFL geometry, ran operational verification, updated survey documentation and completed post-realignment certification.
Q9What is the key lesson from this case?
After runway works, verify approach lighting geometry by survey. Electrical and photometric tests cannot detect a shifted installation.
Engineering & Expert Notes
Recheck Survey Markers Before Installation
SurveyDisturbed markers cause systematic errors. Verify them first.
Verify the Centreline Reference Independently
ReferenceUse an independent check to confirm the construction reference.
Measure As-Built Positions Against the Design
VerificationRecord offsets for each light to show compliance.
Include Geometry in Acceptance Tests
AcceptanceLighting working is not enough. The pattern must be in the right place.
20Case Study Conclusion
Approach-lighting alignment errors are geometric installation failures caused by incorrect survey references, centerline displacement, and construction-related positioning errors, and an ALS can be electrically and photometrically healthy while still providing incorrect visual approach guidance if its installation geometry is not precisely aligned with the runway reference system.
Systematic survey verification, runway-reference measurement, centerline alignment inspection, crossbar geometry verification, foundation assessment, and operational approach validation provide a complete engineering methodology for restoring ICAO-compliant approach-lighting alignment.
In AGL engineering, the approach lighting system is not simply a row of lights leading to the runway; it is a surveyed geometric extension of the runway centerline, and every meter of alignment matters.
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