AGL-CS-078 — PAPI Alignment Error
The AGL Knowledge Hub | Engineering Case Study Series. A PAPI alignment investigation where post-construction geometry errors caused systematic glide-path indication errors — from pilot reports through survey verification, foundation assessment, repositioning, and operational validation.
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 resurfacing and shoulder rehabilitation, pilots reported that the PAPI appeared consistently high, with aircraft receiving three white and one red indication while flying the published glide path.
The PAPI units were operational, beam intensities were acceptable, and previous maintenance records indicated that beam-angle calibration had been completed.
Because the reported error suggested a systematic alignment problem, a detailed investigation was initiated covering horizontal alignment, vertical alignment, runway reference geometry, foundation levels, unit spacing, survey control points, and beam-angle verification.
The investigation identified PAPI alignment errors introduced during post-construction reinstallation, causing the entire PAPI geometry to shift relative to the runway approach path.
This case demonstrates that PAPI units can be electrically and photometrically healthy while providing incorrect glide-path guidance due to alignment errors in the overall installation geometry.
2Learning Objectives
After completing this case study, the learner should be able to:
- Investigate PAPI alignment errors.
- Verify runway reference geometry.
- Assess horizontal and vertical alignment.
- Evaluate unit spacing accuracy.
- Identify survey-related installation errors.
- Restore ICAO-compliant PAPI alignment.
- Validate glide-path geometry after construction activities.
3System Background
A PAPI installation depends on:
- Runway reference line.
- Threshold reference point.
- Unit spacing.
- Foundation elevation.
- Horizontal alignment.
- Vertical alignment.
- Beam-angle calibration.
- Survey control accuracy.
Common alignment-error causes include:
- Incorrect reinstallation.
- Survey reference errors.
- Foundation settlement.
- Pavement reconstruction.
- Unit relocation.
- Incorrect runway offsets.
- Construction reference mistakes.
PAPI performance depends on geometric accuracy relative to the runway.
4Problem Statement
The reported condition was:
- All four PAPI units ON.
- Correct intensity.
- Beam angles apparently acceptable.
- Pilots consistently observing 3W/1R.
- Error began after runway rehabilitation.
Has the PAPI installation geometry shifted relative to the runway reference system?
5Initial Observations
| PAPI operation | Normal |
| Electrical status | Healthy |
| Beam intensity | Normal |
| Pilot reports | Consistent |
| Recent construction | Yes |
The timing suggested an installation-related issue.
6Initial Engineering Hypotheses
The investigation considered:
Individual beam-angle error.
Intensity imbalance.
Runway reference error.
PAPI alignment error.
Flight inspection error.
7PAPI Alignment Principle
PAPI accuracy depends on survey accuracy.
8Test Equipment
The investigation used:
- Total station.
- Digital level.
- GNSS survey equipment.
- Optical alignment telescope.
- Beam-angle measurement equipment.
- Laser alignment system.
- ICAO Annex 14 PAPI reference data.
9Investigation
Step 1: Verify Beam-Angle Calibration
Measured beam angles:
| Unit | Measured |
| 1 | Correct |
| 2 | Correct |
| 3 | Correct |
| 4 | Correct |
Individual beam angles were acceptable.
Step 2: Survey Horizontal Alignment
Measured PAPI offset from runway reference line:
Required offset:
15.0 m
Measured offset:
16.1 m
Lateral displacement:
1.1 m
The PAPI installation had shifted laterally.
Step 3: Survey Longitudinal Position
Measured distance from threshold reference point:
Required:
300.0 m
Measured:
297.8 m
Longitudinal error:
2.2 m
The installation was closer to the threshold than designed.
Step 4: Foundation Elevation Survey
Measured foundation elevation:
Required:
100.000 m
Measured:
100.142 m
Elevation error:
+142 mm
Foundation height had changed after construction.
Step 5: Unit Spacing Verification
Measured spacing:
| Unit Pair | Required | Measured |
| 1-2 | Correct | Correct |
| 2-3 | Correct | Correct |
| 3-4 | Correct | Correct |
Unit spacing was acceptable.
Step 6: Operational Geometry Analysis
Simulation showed:
- Higher apparent glide path.
- Earlier white indication.
- Reduced transition accuracy.
- Systematic 3W/1R indication.
The geometric displacement reproduced the pilot reports.
Step 7: Controlled Realignment
The PAPI installation was repositioned:
- Correct runway offset.
- Correct longitudinal position.
- Correct foundation elevation.
- Verified beam-angle geometry.
Operational verification restored 2W/2R on the published glide path.
10Root Cause
The PAPI alignment error was caused by incorrect installation geometry following runway rehabilitation, including:
- Lateral displacement.
- Longitudinal displacement.
- Foundation elevation error.
- Survey reference error.
The optical, electrical, and beam-angle systems remained healthy.
The primary failure mechanism was incorrect geometric relationship between the PAPI installation and the runway reference system.
11Corrective Action
The engineering team:
- Re-established survey control points.
- Corrected runway reference geometry.
- Repositioned the PAPI installation.
- Corrected foundation elevations.
- Verified horizontal alignment.
- Verified longitudinal position.
- Recalibrated beam angles.
- Conducted operational verification.
- Updated survey documentation.
- Completed post-construction certification.
12Post-Repair Verification
| Parameter | Before | After |
| Runway offset | 16.1 m | 15.0 m |
| Longitudinal position | 297.8 m | 300.0 m |
| Foundation elevation | +142 mm | Within tolerance |
| Glide-path indication | 3W/1R | 2W/2R |
| ICAO compliance | Fail | Pass |
Correct alignment restored compliant PAPI geometry.
13Root-Cause Analysis
Incorrect glide-path indication
Correct
Geometry error found
Incorrect
Operational verification — correct indication restored
PAPI alignment error
Restore installation geometry
14Why This Case Was Important
Alignment errors can cause:
- Systematic glide-path errors.
- Incorrect visual guidance.
- Increased pilot workload.
- ICAO non-compliance.
- Flight inspection failures.
- Post-construction operational problems.
PAPI alignment is fundamentally a surveying and geometry discipline.
15Engineering Lessons
Correct beam angles do not guarantee correct installation geometry.
Correct beam angles do not guarantee correct installation geometry.
Survey control is critical during PAPI installation.
Survey control is critical during PAPI installation.
Construction activities can alter PAPI alignment.
Construction activities can alter PAPI alignment.
Foundation elevation directly affects glide-path geometry.
Foundation elevation directly affects glide-path geometry.
Post-construction survey verification is essential.
Post-construction survey verification is essential.
16Engineer's Decision Point
You observe: PAPI units ON. Beam angles correct. Intensities normal. Pilots consistently report 3W/1R after runway rehabilitation. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about PAPI alignment errors after runway rehabilitation.
Q1What is AGL-CS-078?
AGL-CS-078 is an engineering training case study. It covers a PAPI that showed three white and one red on the glide path after runway resurfacing, caused by installation geometry errors. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Why did the PAPI appear consistently high after resurfacing?
The whole installation had shifted relative to the runway reference system during post-construction reinstallation. This gave a systematic 3W/1R indication even though units, intensity and beam angles were healthy.
Q3What did the survey measure in AGL-CS-078?
Lateral offset was 16.1 m against 15.0 m required (1.1 m error). Distance from the threshold was 297.8 m against 300.0 m (2.2 m closer), and foundation elevation was +142 mm too high.
Q4Were the individual units or beam angles faulty?
No. Beam angles were acceptable, intensities were correct and unit spacing was within requirements. The error was in the overall installation geometry.
Q5Why does PAPI accuracy depend on survey accuracy?
The red/white transitions are only correct relative to the runway reference. A lateral, longitudinal or elevation error shifts the beam reference and creates a systematic glide-path error.
Q6How was the cause confirmed?
Geometry simulation reproduced the pilot reports: higher apparent glide path, earlier white indication and reduced transition accuracy. After repositioning, operational verification restored 2W/2R.
Q7What was the root cause?
Incorrect installation geometry after runway rehabilitation: lateral and longitudinal displacement, foundation elevation error and survey reference error.
Q8How was the PAPI corrected?
The team re-established survey control points, corrected runway reference geometry, repositioned the installation and corrected foundation elevations. They verified horizontal and longitudinal position, recalibrated beam angles, ran operational verification and completed post-construction certification.
Q9What is the key lesson from this case?
After runway works, verify PAPI position and elevation by survey, not only unit calibration. Perfect beam angles cannot compensate for a shifted installation.
Engineering & Expert Notes
Re-Survey After Runway or Civil Works
SurveyRehabilitation can disturb reference points. Verify position against the design after works.
Check Foundation Elevation Against the Design
ElevationHeight errors alter the geometry even if the angle is set correctly.
Confirm the Reference Used for Setting Out
ReferenceMake sure the survey reference is the correct one and has not moved.
Include Geometry in the Handback Checks
HandbackAfter civil works, require a position check as well as a function test.
20Case Study Conclusion
PAPI alignment errors are installation-geometry failures caused by incorrect runway reference positioning, foundation elevation changes, and survey inaccuracies, and a PAPI system can be electrically and photometrically healthy while providing systematically incorrect glide-path guidance due to geometric misalignment.
Systematic survey verification, runway-reference measurement, installation-coordinate confirmation, foundation assessment, beam-angle validation, and operational glide-path verification can identify alignment errors and restore ICAO-compliant PAPI performance.
In AGL engineering, PAPI is ultimately a surveying instrument built from lights, and geometric accuracy is the foundation of every correct glide-path indication.
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