AGL-CS-064 — Touchdown Zone Light Photometry Failure
The AGL Knowledge Hub | Engineering Case Study Series. A touchdown zone lighting photometric investigation where optical degradation caused reduced intensity, non‑uniform beam distribution, and chromaticity shift — from electrical verification through photometric measurement, optical inspection, and leveling 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 a scheduled CAT III runway lighting photometric inspection, several touchdown zone inset lights were found to have significantly reduced intensity and non‑uniform beam distribution, despite operating electrically and showing no CCR alarms.
Flight crews reported that the touchdown zone reference pattern appeared weak and inconsistent during low‑visibility landings, particularly in wet runway conditions.
The Constant Current Regulator (CCR) was maintaining the selected 6.6 A current, output voltage was normal, and all affected TDZ fixtures were illuminated.
A detailed photometric investigation was initiated covering light intensity, beam symmetry, longitudinal distribution, transverse pattern uniformity, optical contamination, and LED optical performance.
The investigation identified optical degradation caused by lens contamination, internal moisture staining, reflector deterioration, and fixture leveling errors, resulting in failure to meet ICAO Annex 14 touchdown zone light photometric requirements.
This case demonstrates that touchdown zone lights can be electrically operational while failing photometric compliance and compromising precision landing guidance.
2Learning Objectives
After completing this case study, the learner should be able to:
- Diagnose touchdown zone light photometric failures.
- Differentiate electrical and photometric defects.
- Evaluate TDZ light intensity.
- Assess beam symmetry and pattern uniformity.
- Verify white chromaticity compliance.
- Investigate optical degradation in inset fixtures.
- Restore ICAO‑compliant touchdown zone photometric performance.
3System Background
A touchdown zone inset light consists of:
- High‑intensity LED optical module.
- Precision optical lens.
- Beam‑control optics.
- Reflector assembly.
- Inset housing.
- Leveling and mounting system.
- Electrical driver.
Photometric performance depends on:
- Peak intensity.
- Beam symmetry.
- Longitudinal beam distribution.
- Transverse beam distribution.
- White chromaticity.
- Optical cleanliness.
- Fixture leveling.
- TDZ pattern uniformity.
Common photometric failure causes include:
- Lens contamination.
- Internal condensation.
- Optical scratching.
- LED lumen depreciation.
- Reflector deterioration.
- Seal failure.
- Fixture leveling errors.
- Incorrect optical module replacement.
4Problem Statement
The reported condition was:
- TDZ lights illuminated.
- Reduced apparent brightness.
- Poor touchdown reference visibility.
- CCR healthy.
- No electrical alarms.
- CAT III photometric inspection failed.
Are the touchdown zone lights electrically healthy but photometrically non‑compliant with ICAO Annex 14 requirements?
5Initial Observations
| CCR status | Healthy |
| Circuit operation | Normal |
| Light operation | ON |
| Visual brightness | Reduced |
| Touchdown pattern | Weak |
| Beam symmetry | Distorted |
The lights were electrically operational, but their photometric performance was degraded.
6Initial Engineering Hypotheses
The maintenance team considered:
CCR current reduction.
Transformer secondary voltage reduction.
Lens contamination.
LED optical degradation.
Inset fixture leveling error.
7TDZ Photometric Failure Principle
Touchdown zone lighting requires extremely precise optical control.
8Test Equipment
The investigation used:
- Portable AGL photometer.
- Goniophotometric measurement equipment.
- Chromaticity meter.
- Optical inspection tools.
- Digital level.
- True‑RMS multimeter.
- Thermal imaging camera.
- ICAO Annex 14 photometric reference data.
- Airport‑approved PPE.
9Investigation
Step 1: Verify Electrical Operation
Results:
| CCR operation | Normal |
| Current regulation | Normal |
| Secondary voltage | Normal |
| Fixture operation | Normal |
No electrical fault was identified.
Step 2: Photometric Measurement
Measured peak intensity:
22,000 cd
Required intensity:
≥ 40,000 cd (representative training value)
The fixture failed the photometric requirement.
Step 3: Beam Distribution Measurement
Testing showed:
- Reduced peak intensity.
- Vertical beam depression.
- Horizontal beam asymmetry.
- Non‑uniform touchdown‑zone pattern distribution.
The beam no longer matched the required TDZ photometric envelope.
Step 4: Chromaticity Measurement
Measured chromaticity showed:
- Slight shift from the specified white region.
- Reduced color consistency.
- Optical aging effects.
Color performance had degraded measurably.
Step 5: Optical Inspection
Inspection revealed:
- Dirt accumulation.
- Rubber deposits.
- Surface abrasion.
- Internal moisture staining.
- Lens hazing.
Optical transmission had deteriorated significantly.
Step 6: Internal Optical Inspection
The optical assembly showed:
- LED lens discoloration.
- Reflector degradation.
- Moisture contamination.
- Optical surface deterioration.
Optical efficiency had decreased substantially.
Step 7: Fixture Leveling Verification
Measurements showed:
- Minor tilt.
- Beam‑axis displacement.
- Reduced touchdown‑zone overlap.
- Inconsistent pattern continuity.
Leveling errors further reduced touchdown reference effectiveness.
10Root Cause
The touchdown zone light photometric failure was caused by combined optical degradation, including:
- Lens contamination.
- Internal moisture staining.
- LED optical aging.
- Reflector deterioration.
- Fixture leveling errors.
The electrical system remained healthy, but the fixture no longer produced ICAO‑compliant intensity, beam symmetry, and touchdown‑zone pattern performance.
11Corrective Action
The maintenance team:
- Removed the affected TDZ fixture.
- Replaced the optical lens assembly.
- Replaced the degraded optical module.
- Restored reflector condition.
- Cleaned the optical compartment.
- Re‑leveled the fixture.
- Performed photometric testing.
- Verified beam symmetry.
- Confirmed chromaticity compliance.
- Updated photometric maintenance records.
12Post‑Repair Verification
The touchdown zone light met ICAO photometric requirements after repair.
13Root‑Cause Analysis
Dim touchdown zone light
Healthy
Below requirement
Non‑compliant
Contaminated
Degraded
Optical degradation and leveling error
Replace optical components and restore leveling
14Why This Case Was Important
TDZ photometric failures can cause:
- Reduced touchdown reference visibility.
- Poor low‑visibility landing guidance.
- CAT II/III operational degradation.
- ICAO non‑compliance.
- Reduced runway visual cues.
- Increased operational risk.
Touchdown zone photometric performance is essential for precision landing operations.
15Engineering Lessons
An illuminated TDZ light is not necessarily ICAO compliant.
An illuminated TDZ light is not necessarily ICAO compliant.
Touchdown‑zone pattern uniformity is critical for landing guidance.
Touchdown‑zone pattern uniformity is critical for landing guidance.
Inset‑fixture leveling directly affects photometric performance.
Inset‑fixture leveling directly affects photometric performance.
Internal moisture can significantly reduce optical efficiency.
Internal moisture can significantly reduce optical efficiency.
CAT II/III systems require extremely precise photometric control.
CAT II/III systems require extremely precise photometric control.
16Engineer's Decision Point
You observe: TDZ lights ON. Pilots report poor touchdown reference visibility. CCR healthy. No electrical faults. What should you investigate first?
17Knowledge Check
18Interview Questions
19Frequently Asked Questions
Quick answers about touchdown zone (TDZ) light photometric failure and CAT III compliance.
Q1What is AGL-CS-064?
AGL-CS-064 is an engineering training case study. It covers touchdown zone inset lights that were electrically healthy but failed a CAT III photometric inspection because of optical degradation and leveling errors. It is an engineering simulation created for training, not a report of an actual airport incident.
Q2Can touchdown zone lights work electrically but still fail photometric requirements?
Yes. The CCR held 6.6 A and all affected TDZ fixtures were illuminated, but intensity, beam symmetry and pattern uniformity were below the required ICAO Annex 14 values.
Q3What did the measurements show in AGL-CS-064?
Peak intensity was 22,000 cd against a representative training requirement of at least 40,000 cd. The beam had vertical depression, horizontal asymmetry and a non-uniform touchdown-zone pattern, with a slight chromaticity shift.
Q4Why is TDZ pattern uniformity critical for landing guidance?
Crews use the touchdown-zone pattern as a reference during low-visibility landings. Uneven intensity or inconsistent pattern continuity weakens that reference and makes the touchdown zone harder to acquire.
Q5What caused the failure in AGL-CS-064?
Combined optical degradation: lens contamination, internal moisture staining, LED optical aging, reflector deterioration and fixture leveling errors. The electrical system remained healthy.
Q6How does inset-fixture leveling affect touchdown guidance?
Minor tilt caused beam-axis displacement, reduced touchdown-zone overlap and inconsistent pattern continuity, adding to the loss from the degraded optics.
Q7What role did internal moisture play?
Internal moisture staining and contamination reduced optical efficiency inside the fixture. Cleaning the outside alone would not restore performance, so the optical assembly was replaced.
Q8How was compliance restored?
The team replaced the optical lens assembly and degraded optical module, restored the reflector, cleaned the optical compartment and re-leveled the fixture. Beam symmetry and chromaticity were verified and photometric records updated.
Q9What should you investigate first when TDZ visibility is poor but the CCR is healthy?
Measure photometric intensity, verify beam symmetry, inspect the optical assembly and check fixture leveling. This finds the cause without unnecessary CCR or transformer replacement.
Engineering & Expert Notes
Check Seals After Finding Internal Staining
SealingMoisture inside means a seal has failed. Replace it and check drainage.
Verify Fixture Levelling in the Touchdown Pattern
LevellingSmall tilts alter the beam and the pattern seen by pilots.
Compare Each Light With Its Neighbours
UniformityUniformity across the zone matters. A dim unit among bright ones is easy to miss.
Keep Baseline Photometry for the Zone
RecordsBaseline data makes later comparisons meaningful.
20Case Study Conclusion
Touchdown zone light photometric failure is an optical‑performance problem caused by lens contamination, moisture‑related optical degradation, reflector deterioration, LED aging, and fixture leveling errors, and a TDZ light can be electrically operational while failing ICAO Annex 14 photometric requirements and compromising precision landing guidance.
Systematic photometric measurement, beam‑symmetry verification, pattern‑uniformity assessment, chromaticity testing, optical inspection, LED assessment, and fixture‑leveling verification can identify photometric failures and restore compliant touchdown zone lighting performance.
In AGL engineering, the touchdown zone is where precision becomes reality, and photometric compliance determines whether the pilot receives a clear landing reference or a degraded visual cue at the most critical moment of the landing.
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