What does Division 18 of the AGL Troubleshooting Atlas cover?

Division 18 is the AGL Photometry Troubleshooting reference. It publishes 20 diagnostic topics, PHM-451 to PHM-470, for AGL Photometry faults on airfield ground lighting systems.

What information does each topic include?

Every topic follows the same diagnostic record: the symptom, equipment/system, possible causes ranked by likelihood (high, medium or low), required tools, safety precautions, diagnostic procedure, test points, root cause verification, corrective action, preventive action and related references (SOPs, standards, diagrams and case studies).

Which AGL Photometry faults are covered?

The 20 topics are PHM-451 to PHM-470: Low Light Intensity; Excessive Light Intensity; Uneven Light Intensity; Beam Misalignment; Beam Spread Incorrect; Incorrect Photometric Distribution; Incorrect Chromaticity; Wrong Light Color; Photometry Measurement Instability; Photometer Calibration Problem; Photometer Zero Error; Photometer Reference Error; Frame Alignment Error; Measurement Distance Error; Measurement Height Error; Instrument Mounting Error; Incorrect Measurement Orientation; Repeated Photometry Failure; Fixture Passes Visual Inspection but Fails Photometry; Fixture Passes Photometry but Field Appearance Abnormal.

How does a diagnostic procedure begin in Division 18?

Each procedure starts with observation before any intervention. For example, PHM-451 (Low Light Intensity) begins: Confirm the fixture is otherwise operating normally (no flicker, correct colour) and re-verify the isocandela specification and measurement angle applicable to this fixture type and category.

What safety precautions apply when troubleshooting AGL Photometry?

Each topic lists its own precautions. For example, PHM-451 (Low Light Intensity) starts with: Coordinate runway/taxiway closure or NOTAM before conducting field photometry that requires close approach to in-pavement fixtures.

Which SOPs are referenced in Division 18?

Related references in the topics include SOP-PHM-01 (Field Photometric Test Procedure), SOP-PHM-04 (Lamp/LED Module Replacement Procedure), SOP-CCR-06 (CCR Output Calibration Procedure), SOP-PHM-02 (Circuit Photometric Survey Procedure), SOP-PHM-03 (Fixture Aiming and Alignment Procedure), SOP-PHM-05 (Optical Assembly Inspection and Replacement Procedure), SOP-PHM-06 (Full Distribution Verification Procedure) and SOP-PHM-07 (Chromaticity Verification Procedure), and 7 more.

Which standards are referenced in the AGL Photometry topics?

Where a standard applies, it appears in the topic's Related References. Those cited in this division include ICAO Annex 14 Vol I Attachment A, FAA AC 150/5340-30, FAA AC 150/5345-46, ICAO Annex 14 Vol I and ISO/IEC 17025.

How can I find a specific fault in Division 18 quickly?

Use the search box at the top of the page to filter the 20 published topics by code, fault name or keyword; matching topics open automatically. Each diagnostic record can also be downloaded as a Word (.doc) file.

Division 18 — AGL Photometry Troubleshooting

PHM-451 Low Light Intensity

Symptom — A fixture measures below the minimum specified luminous intensity (candela) at the design aiming angle during scheduled or acceptance photometric testing, even though the fixture illuminates and appears visually normal.

Equipment / System

Runway/taxiway edge, centreline, or approach light fixture undergoing field or bench photometric verification against ICAO Annex 14 Attachment A / FAA AC 150/5340-30 isocandela curves.

Possible Causes

HighLamp/LED module degradation (lumen depreciation) below rated output, especially in aged incandescent or early-generation LED fixtures

HighUndervoltage at the fixture from a low CCR step setting, high circuit resistance, or transformer tap mismatch

MediumLens/prism contamination (dust, dirt, insect residue, oxidation) attenuating output

MediumPartial beam misalignment shifting the peak intensity away from the measurement angle rather than a true output loss

LowInternal reflector degradation, discolouration or delamination

LowLED driver operating in a reduced current/dimming mode due to a configuration error

Required Tools

Calibrated field photometer/luminance meterTrue-RMS clamp ammeterDigital inclinometerLens cleaning kitReference fixture or NIST-traceable calibration source

Safety Precautions

Coordinate runway/taxiway closure or NOTAM before conducting field photometry that requires close approach to in-pavement fixtures.
Use appropriate PPE and reflective/lighted equipment for night runway measurements.

Diagnostic Procedure

ObservationConfirm the fixture is otherwise operating normally (no flicker, correct colour) and re-verify the isocandela specification and measurement angle applicable to this fixture type and category.
MeasurementRecord circuit current/voltage at the fixture during the test to rule out an undervoltage condition before attributing the result to the fixture itself.
ObservationInspect and clean the lens/prism, then re-measure to rule out contamination as the cause.
MeasurementRe-check beam alignment (aiming angle) against the design orientation, since a misaimed fixture can show low intensity at the test angle while still producing full total output.
DecisionLow intensity persists after cleaning and with correct circuit voltage and alignment → lamp/LED module or driver degradation. Intensity recovers after cleaning/alignment correction → contamination or misalignment was the root cause.
Fault LocationIsolate to lamp/LED module, driver/transformer, alignment, or lens condition.
Root CauseBench-test the removed lamp/LED module against a reference standard to confirm true output degradation.

Test Points

Luminous intensity at 0°/specified test angle

Fixture supply voltage/current

Lens transmittance (visual/cleaned comparison)

Aiming angle vs. design

Root Cause Verification

After correction (module replacement, cleaning, or realignment), re-measure intensity at the full set of specified angles and confirm results meet or exceed the ICAO/FAA minimum curve, then log values into the fixture’s photometric history.

Corrective Action

Replace a degraded lamp/LED module.
Correct an undervoltage condition at the CCR/transformer.
Clean or replace a degraded lens/prism.

Preventive Action

Trend photometric readings per fixture over successive maintenance cycles to catch gradual degradation before it drops below minimum.
Include lens cleaning in the routine PM schedule.
Verify CCR output calibration periodically.

Related References

SOP: SOP-PHM-01 (Field Photometric Test Procedure), SOP-PHM-04 (Lamp/LED Module Replacement Procedure)

Standards: ICAO Annex 14 Vol I Attachment A, FAA AC 150/5340-30 (Design and Installation of Airport Visual Aids), FAA AC 150/5345-46 (Specification for Runway and Taxiway Light Fixtures)

Diagram: Isocandela Curve Reference Set

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue

PHM-452 Excessive Light Intensity

Symptom — A fixture measures luminous intensity meaningfully above the specified maximum (or noticeably above sibling fixtures on the same circuit), producing glare complaints from pilots or unexpectedly high photometric readings during verification.

Equipment / System

Approach, runway edge, or PAPI/VASI-adjacent AGL fixture under photometric test.

Possible Causes

HighFixture set to an incorrect (higher) intensity step in a multi-step CCR or ALCMS configuration

HighIncorrect lamp/LED module wattage or a higher-output replacement fitted in error

MediumOvervoltage at the fixture from CCR calibration drift or a wrong transformer tap

MediumReflector or lens replaced with a non-matching optical assembly increasing peak candela

LowPhotometer calibration drift reading high (cross-reference PHM-460)

Required Tools

Calibrated field photometerTrue-RMS multimeterCCR step/output verification toolFixture nameplate/spec referenceReplacement correctly-rated module

Safety Precautions

Coordinate runway/taxiway access and NOTAM as required.
Treat the circuit as live during observation unless it is isolated for module work, even if the fixture is confirmed over-intensity.

Diagnostic Procedure

ObservationConfirm the ALCMS/CCR commanded intensity step matches the step under which the fixture is being tested.
MeasurementRecord fixture supply voltage/current and compare to the nameplate rating for the commanded step.
ObservationVerify the fitted lamp/LED module part number and optical assembly against the fixture’s design specification.
DecisionVoltage/current within spec and correct module fitted → verify photometer calibration (cross-reference PHM-460) before concluding a true fixture fault. Wrong module or overvoltage confirmed → fixture-level fault.
Fault LocationIsolate to CCR step/calibration, wrong module, or optical mismatch.
Root CauseConfirm by substituting a known-correct module or retesting after CCR calibration correction.

Test Points

Luminous intensity at test angle

Fixture supply voltage

Module part number vs. spec

CCR commanded step vs. actual output

Root Cause Verification

After correcting the module, CCR step, or calibration, re-measure and confirm intensity falls within the specified range across all commanded steps.

Corrective Action

Fit the correct-rated lamp/LED module.
Recalibrate CCR output.
Correct ALCMS step assignment.
Replace a mismatched optical assembly with the design-specified part.

Preventive Action

Maintain strict part-number control for lamp/LED modules and optical assemblies during maintenance.
Verify ALCMS step-to-fixture mapping after any configuration change.
Include CCR output calibration in the periodic PM programme.

Related References

SOP: SOP-PHM-01, SOP-CCR-06 (CCR Output Calibration Procedure)

Standards: ICAO Annex 14 Vol I Attachment A, FAA AC 150/5340-30

Diagram: CCR Step/Intensity Reference Table

Case Study: Division 01 — CCR Troubleshooting

Rev 0 — Initial issue

PHM-453 Uneven Light Intensity

Symptom — Fixtures along the same run or circuit show significant variation in measured intensity between adjacent lights, even though each individually may pass or be near the specified minimum, producing a visually inconsistent runway/taxiway edge line.

Equipment / System

A run/series of runway or taxiway edge, centreline, or approach lights on the same circuit or design section.

Possible Causes

HighMixed lamp/LED module ages or part numbers across the run from piecemeal replacement over time

HighVoltage drop variation along a long series or parallel circuit affecting fixtures differently by position

MediumInconsistent lens/prism cleaning or contamination levels across fixtures

MediumIndividual fixtures with minor alignment variance

LowMixed manufacturer batches with slightly different photometric output

Required Tools

Calibrated field photometerTrue-RMS clamp ammeterCircuit layout drawingFixture maintenance/replacement history log

Safety Precautions

Coordinate extended runway/taxiway occupancy for a multi-fixture survey.
Use appropriate lighting and PPE for night measurement runs.

Diagnostic Procedure

ObservationConduct a sequential intensity survey across the full run and plot results by fixture position to identify a pattern (gradual, isolated, or grouped variance).
MeasurementRecord voltage at several points along the circuit to check for position-dependent drop.
ObservationCross-reference each fixture’s maintenance history for module replacement date/part number.
DecisionVariance correlates with circuit position → voltage drop/circuit design issue. Variance correlates with replacement history → mixed module ages/part numbers. Random variance → cleaning/alignment inconsistency.
Fault LocationIsolate to circuit voltage profile, module consistency, or fixture-level maintenance quality.
Root CauseConfirm by standardising module part numbers or correcting circuit voltage profile and re-surveying.

Test Points

Intensity per fixture position along the run

Voltage at multiple circuit points

Module part number/age per fixture

Lens condition per fixture

Root Cause Verification

After standardising modules and/or correcting circuit voltage, re-survey the full run and confirm intensity variance falls within an acceptable band across all fixtures.

Corrective Action

Standardise lamp/LED module part numbers across the run.
Correct circuit design/voltage drop where applicable.
Bring cleaning and alignment to a consistent standard across all fixtures.

Preventive Action

Maintain a single approved part number per fixture type/circuit for replacements.
Include full-run photometric surveys (not just spot checks) in the periodic PM programme.
Track module installation dates for planned batch replacement.

Related References

SOP: SOP-PHM-02 (Circuit Photometric Survey Procedure)

Standards: ICAO Annex 14 Vol I Attachment A, FAA AC 150/5340-30

Diagram: Circuit Layout and Voltage Profile Drawing

Case Study: Division 05 — Runway Centreline Light Troubleshooting

Rev 0 — Initial issue

PHM-454 Beam Misalignment

Symptom — Photometric testing shows the peak luminous intensity occurring at an angle offset from the design aiming point, even though total light output may be within specification.

Equipment / System

Any directional AGL fixture (runway/taxiway edge, approach, PAPI) with a defined design aiming angle (elevation and azimuth).

Possible Causes

HighFixture physically bumped or shifted by vehicle contact, snow removal equipment, or mowing operations

HighIncorrect aiming during a prior installation or maintenance visit

MediumMounting base or stake loosened, allowing gradual drift over time from wind/vibration

MediumFrost heave or ground settlement shifting an in-pavement or semi-flush fixture base

LowIncorrect aiming template or reference used during original installation

Required Tools

Digital inclinometer/aiming toolAzimuth reference (compass or runway centreline reference line)Calibrated field photometerTorque wrench for mounting hardwareAiming template per fixture type

Safety Precautions

Coordinate runway/taxiway closure for aiming correction work requiring access to in-pavement fixtures.
Secure the work area from vehicle/aircraft movement during adjustment.

Diagnostic Procedure

ObservationMeasure luminous intensity across a spread of angles around the design aiming point to map the actual peak location.
MeasurementRecord the fixture’s current elevation and azimuth angle using the aiming tool and compare to the design specification.
ObservationInspect the mounting base/stake for looseness, damage, or signs of impact.
DecisionMeasured peak angle offset matches a physical aiming discrepancy → realign fixture. Base loose/damaged → repair mounting before realigning, since realignment will not hold on an unstable base.
Fault LocationIsolate to aiming error alone versus a mounting/foundation issue causing drift.
Root CauseConfirm by re-aiming to design specification and verifying the peak intensity now occurs at the correct angle.

Test Points

Peak intensity angle vs. design aiming angle

Elevation angle

Azimuth angle

Mounting base condition/torque

Root Cause Verification

After realignment (and mounting repair if applicable), re-survey the beam pattern and confirm the peak intensity occurs within the allowable angular tolerance of the design aiming point.

Corrective Action

Re-aim the fixture to the design elevation/azimuth using the correct template/tool.
Repair or replace a loose or damaged mounting base.
Re-torque mounting hardware to specification.

Preventive Action

Include aiming verification in the periodic PM programme, particularly after any known vehicle contact or ground disturbance near the fixture.
Use witness marks or aiming templates during installation to speed future verification.

Related References

SOP: SOP-PHM-03 (Fixture Aiming and Alignment Procedure)

Standards: ICAO Annex 14 Vol I Attachment A, FAA AC 150/5340-30

Diagram: Fixture Aiming Template/Isocandela Reference

Case Study: Division 09 — Approach Lighting System Troubleshooting

Rev 0 — Initial issue

PHM-455 Beam Spread Incorrect

Symptom — The measured beam width (horizontal and/or vertical spread) is narrower or wider than the specification for the fixture type, even when peak intensity and aiming angle are correct.

Equipment / System

Directional AGL fixture with a defined isocandela beam-spread envelope (e.g., elliptical or circular beam optics).

Possible Causes

HighIncorrect lens or reflector/optic assembly fitted, not matching the fixture’s design beam-spread class

HighLens cracked, chipped, or partially obstructed, distorting the beam pattern

MediumLamp/LED module not correctly seated/centred in the optical assembly (off the focal point)

MediumInternal reflector deformed from heat damage or physical impact

LowManufacturing tolerance issue with the specific optical assembly

Required Tools

Calibrated field photometer with angular scan capabilityInspection light/borescopeReplacement optic assembly correctly rated for the fixtureLamp/LED seating gauge

Safety Precautions

LOTO the fixture before any lens/optic disassembly.
Handle glass/plastic lens components carefully to avoid injury from sharp edges.

Diagnostic Procedure

ObservationPerform an angular intensity scan across both horizontal and vertical planes and compare the resulting envelope to the specified isocandela beam-spread curve.
ObservationVisually inspect the lens/reflector for damage, cracking, or an incorrect part number against the fixture’s design specification.
MeasurementVerify the lamp/LED module is correctly seated at the designed focal point within the optical assembly.
DecisionWrong or damaged optic confirmed → replace with correct-spec assembly. Optic correct but module miscentred → reseat/adjust module.
Fault LocationIsolate to optic assembly, lens damage, or module seating.
Root CauseConfirm by re-scanning the beam pattern after correction and comparing against the specified envelope.

Test Points

Horizontal beam spread angle

Vertical beam spread angle

Lens/reflector condition and part number

Module seating position

Root Cause Verification

After replacing the optic or correcting module seating, re-scan the beam pattern and confirm the horizontal and vertical spread fall within the specified isocandela envelope.

Corrective Action

Replace the lens/reflector with the correct-spec optical assembly.
Reseat or replace the lamp/LED module at the correct focal position.
Replace a damaged or cracked lens.

Preventive Action

Maintain strict part-number control for optical assemblies by fixture type.
Inspect lenses for damage during every routine visit.
Verify module seating whenever a module is replaced.

Related References

SOP: SOP-PHM-05 (Optical Assembly Inspection and Replacement Procedure)

Standards: ICAO Annex 14 Vol I Attachment A, FAA AC 150/5345-46

Diagram: Fixture Beam-Spread Isocandela Envelope

Case Study: Division 09 — Approach Lighting System Troubleshooting

Rev 0 — Initial issue

PHM-456 Incorrect Photometric Distribution

Symptom — The overall isocandela pattern (shape of the intensity distribution across angles) does not match the specified curve for the fixture category, even though peak intensity, aiming, and beam spread individually may appear close to normal.

Equipment / System

Any AGL fixture undergoing a full photometric distribution verification against its ICAO/FAA category curve.

Possible Causes

HighWrong fixture category/model installed for the application (e.g., a taxiway-rated fixture in a runway-rated location)

HighCombination of multiple minor faults (slight misalignment, minor optic damage, module miscentring) compounding into an overall distribution error

MediumNon-approved or counterfeit replacement parts with a different photometric profile

LowFixture design revision mismatch (older fixture generation with a superseded photometric curve)

Required Tools

Calibrated field photometer with full angular scan capabilityFixture category/model reference documentationIsocandela curve overlay tool or software

Safety Precautions

Standard fixture-access safety precautions apply; no unique hazard beyond routine photometric survey work.

Diagnostic Procedure

ObservationConfirm the installed fixture model/category against the design drawing for that location.
MeasurementPerform a full angular scan and overlay the resulting distribution against the specified isocandela curve to identify where and how it deviates.
ObservationCross-reference the deviation pattern against other single-fault topics (alignment, beam spread, intensity) to determine whether one dominant fault explains the distribution error.
DecisionWrong fixture category installed → replace with correct model. Distribution deviation explained by a combination of minor faults → resolve each contributing fault individually and re-scan.
Fault LocationIsolate to fixture category mismatch versus a compounded set of individually minor faults.
Root CauseConfirm by re-scanning after correction and comparing the full distribution to the specified curve.

Test Points

Full angular intensity scan (horizontal and vertical)

Fixture model/category vs. design drawing

Deviation pattern vs. specified isocandela curve

Contributing single-parameter faults

Root Cause Verification

After replacing the fixture or correcting the contributing faults, re-scan the full distribution and confirm it matches the specified isocandela curve within tolerance across all angles.

Corrective Action

Replace an incorrect fixture category/model with the design-specified unit.
Resolve each contributing minor fault (alignment, optics, module) individually.

Preventive Action

Verify fixture model/category against design drawings during installation and at every major maintenance visit.
Use only approved, traceable replacement parts.
Retain isocandela reference curves for every fixture category in use.

Related References

SOP: SOP-PHM-01, SOP-PHM-06 (Full Distribution Verification Procedure)

Standards: ICAO Annex 14 Vol I Attachment A, FAA AC 150/5340-30

Diagram: Isocandela Curve Reference Set

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue

PHM-457 Incorrect Chromaticity

Symptom — The fixture’s measured chromaticity coordinates fall outside the specified CIE colour box for its designated colour (e.g., white, red, yellow/amber, blue, green), even though the light appears approximately the correct colour to the eye.

Equipment / System

Any colored AGL fixture undergoing chromaticity verification per ICAO Annex 14 Attachment A colour boxes.

Possible Causes

HighLens or filter degradation/fading from long-term UV exposure shifting the transmitted colour

HighIncorrect lens/filter colour fitted (close but not exact match) during a prior replacement

MediumLED module colour shift from thermal ageing or driver current drift (especially phosphor-converted white LEDs)

MediumLens contamination (yellowing film, residue) subtly shifting apparent colour

LowPhotometer/colorimeter calibration drift (cross-reference PHM-460)

Required Tools

Calibrated field colorimeter/spectroradiometerCIE 1931 colour box reference chart for the fixture’s designated colourReplacement lens/filter of correct specification

Safety Precautions

Standard fixture-access precautions; no unique hazard.

Diagnostic Procedure

ObservationConfirm the fixture’s designated colour per the design drawing and the corresponding ICAO Annex 14 colour box coordinates.
MeasurementRecord the chromaticity coordinates (x, y) using a calibrated colorimeter and plot against the specified colour box.
ObservationInspect the lens/filter for fading, yellowing, or an incorrect part number.
DecisionCoordinates fall outside the box and lens shows visible fading/wrong part → replace lens/filter. Lens appears correct and clean but coordinates still off → suspect LED module colour shift or photometer calibration (cross-reference PHM-460).
Fault LocationIsolate to lens/filter degradation, wrong part, LED module shift, or instrument error.
Root CauseConfirm by re-measuring with a verified-calibrated instrument after lens/filter replacement.

Test Points

Chromaticity coordinates (x, y) vs. specified colour box

Lens/filter condition and part number

LED module colour temperature/drift where applicable

Colorimeter calibration status

Root Cause Verification

After replacing the lens/filter or module, re-measure chromaticity and confirm the coordinates fall within the specified ICAO Annex 14 colour box.

Corrective Action

Replace a faded or incorrect lens/filter with the correct-spec part.
Replace an LED module exhibiting colour shift.
Recalibrate the colorimeter if instrument error is confirmed.

Preventive Action

Track lens/filter age and include chromaticity spot checks in the periodic PM programme, particularly for fixtures with high UV exposure.
Maintain strict part-number control for coloured lenses/filters.

Related References

SOP: SOP-PHM-07 (Chromaticity Verification Procedure)

Standards: ICAO Annex 14 Vol I Attachment A (Colours for Aeronautical Ground Lights), FAA AC 150/5345-46

Diagram: CIE Colour Box Reference Chart

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue

PHM-458 Wrong Light Color

Symptom — The fixture displays an obviously incorrect colour to visual inspection (e.g., showing white where yellow/amber is required, or red where green is required), distinct from a subtle chromaticity shift.

Equipment / System

Any coloured AGL fixture, typically at a runway/taxiway intersection, threshold, or caution zone where colour coding is safety-critical.

Possible Causes

HighWrong-colour lens/filter fitted during installation or a prior maintenance visit

HighFixture installed in the wrong location relative to the lighting design (e.g., a white edge fixture installed where a yellow caution fixture belongs)

MediumLED module of the wrong colour (e.g., a white module fitted to a fixture requiring in-line colour filtering, without the filter installed)

LowLens completely missing or a clear lens fitted in place of a coloured one

Required Tools

Design lighting layout drawingCorrect-colour replacement lens/filter or LED moduleVisual colour reference standard

Safety Precautions

This is a safety-critical, often reportable discrepancy (e.g., at runway/taxiway hold points) — flag and prioritise for immediate correction or fixture de-energisation until fixed.

Diagnostic Procedure

ObservationConfirm the fixture’s location against the design lighting layout drawing to verify the colour that should be displayed there.
ObservationInspect the fixture for the fitted lens/filter colour and part number, or confirm whether a lens is missing entirely.
DecisionWrong lens/filter or module fitted → replace with correct colour. Fixture in wrong location per design → relocate or reassign per the design layout.
Fault LocationIsolate to a parts/fitting error versus a location/design-assignment error.
Root CauseConfirm by cross-referencing the as-built layout against the original design and any recent modification records.

Test Points

Fixture location vs. design layout

Lens/filter/module colour vs. design specification

Chromaticity coordinates after correction (cross-reference PHM-457)

Root Cause Verification

After fitting the correct-colour lens/filter/module, visually confirm correct colour display and verify chromaticity coordinates fall within the specified colour box.

Corrective Action

Fit the correct-colour lens/filter or LED module.
Relocate or reassign the fixture if it was installed at the wrong design position.
Update as-built records to reflect the correction.

Preventive Action

Verify colour-coding against the design layout during final commissioning and after any fixture replacement.
Maintain strict, physically distinct part numbering/packaging for different coloured lenses to prevent mix-ups in the field.

Related References

SOP: SOP-PHM-08 (Fixture Colour Verification Procedure), SOP-PHM-07

Standards: ICAO Annex 14 Vol I Attachment A, FAA AC 150/5340-30

Diagram: Airfield Lighting Colour-Coding Layout

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue

PHM-459 Photometry Measurement Instability

Symptom — Repeated photometric readings on the same fixture, under apparently stable conditions, fluctuate significantly between successive measurements rather than settling to a consistent value.

Equipment / System

Fixture under test together with the field photometer/measurement setup.

Possible Causes

HighLED driver operating in a pulse-width-modulated (PWM) dimming mode not properly synchronised with the photometer’s sampling/integration time

HighUnstable circuit voltage (CCR output fluctuation) during the measurement window

MediumPhotometer battery/power instability or a loose sensor cable connection

MediumAmbient light or reflective interference at the measurement site varying between readings (e.g., passing vehicle headlights, changing cloud cover for daylight-adjacent tests)

LowPhotometer sensor thermal drift during an extended test session

Required Tools

Calibrated field photometerOscilloscope or PWM-capable meter for driver waveform checkTrue-RMS multimeter for circuit voltage stabilityStable/regulated power source for the photometer

Safety Precautions

Standard fixture-access precautions; ensure the measurement site is clear of other light sources or reflective interference during testing.

Diagnostic Procedure

ObservationRepeat the measurement multiple times in immediate succession and record the spread of readings.
MeasurementCheck the LED driver output waveform for PWM dimming and confirm the photometer’s integration time is appropriate for the modulation frequency.
MeasurementMonitor circuit voltage at the fixture throughout the test window for fluctuation.
DecisionPWM/integration mismatch confirmed → adjust photometer integration settings or use an averaging/RMS-capable instrument. Circuit voltage unstable → investigate CCR output stability. Neither → check photometer power/cable/ambient conditions.
Fault LocationIsolate to driver modulation/instrument mismatch, circuit voltage instability, or measurement environment/equipment issue.
Root CauseConfirm by re-testing with corrected integration settings, a stabilised circuit, or a controlled measurement environment.

Test Points

Spread of repeated intensity readings

LED driver PWM frequency and duty cycle

Circuit voltage stability during test window

Photometer battery/cable condition

Root Cause Verification

After correcting the identified cause, repeat the measurement series and confirm readings converge to a consistent value within the photometer’s stated accuracy.

Corrective Action

Adjust or upgrade the photometer’s integration/averaging settings to suit PWM-dimmed LED drivers.
Address CCR output instability.
Repair or replace a faulty photometer cable/battery.
Relocate the test to minimise ambient interference.

Preventive Action

Standardise on photometry equipment and settings appropriate for the fixture types in use (particularly PWM-dimmed LED).
Include a stability check (multiple readings) as standard practice in the field test procedure.

Related References

SOP: SOP-PHM-01, SOP-PHM-09 (Photometer Setup and Stability Verification Procedure)

Standards: ICAO Annex 14 Vol I Attachment A, IES LM-79 (Electrical and Photometric Measurements of LED Products)

Diagram: Field Photometer Setup Reference

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue

PHM-460 Photometer Calibration Problem

Symptom — A field photometer produces readings that are suspected or confirmed to be inaccurate — consistently high, low, or inconsistent with a reference standard — potentially affecting the validity of fixture acceptance test results.

Equipment / System

Field/portable photometer or luminance meter used for AGL photometric verification.

Possible Causes

HighCalibration interval exceeded — instrument used past its certified calibration due date

HighSensor degradation or damage from physical shock, moisture ingress, or UV exposure over its service life

MediumFirmware/software calibration coefficients corrupted or reset

MediumIncorrect calibration reference or method used at the last calibration cycle

LowEnvironmental factors (temperature, humidity) outside the instrument’s rated operating range affecting the current reading set

Required Tools

NIST-traceable reference light source or calibrated reference fixtureCalibration certificate recordsManufacturer calibration/service procedureSpare calibrated photometer for cross-check

Safety Precautions

No unique hazard; standard equipment handling precautions apply.

Diagnostic Procedure

ObservationCheck the instrument’s calibration due date against its current certificate.
MeasurementCross-check the suspect photometer’s reading against a known reference source or a second calibrated instrument.
ObservationInspect the instrument for physical damage, moisture ingress, or sensor discolouration.
DecisionCalibration overdue or reading deviates from reference beyond stated accuracy → send for recalibration/service, do not use for acceptance testing until corrected. Reading within stated accuracy of reference → instrument cleared, investigate other causes for the original suspect result.
Fault LocationIsolate to expired calibration, physical/sensor damage, or a valid instrument with a different root cause elsewhere.
Root CauseConfirm via the calibration service report identifying the specific out-of-tolerance parameter.

Test Points

Reading vs. NIST-traceable reference source

Calibration certificate due date

Cross-check against a second calibrated instrument

Physical/sensor condition

Root Cause Verification

After recalibration or repair, re-verify the instrument against the reference source and confirm readings fall within the manufacturer’s stated accuracy before returning it to service.

Corrective Action

Send the photometer for manufacturer recalibration or repair.
Retire and replace an instrument with confirmed sensor damage.
Correct any calibration record/interval tracking error.

Preventive Action

Maintain a strict calibration interval tracking system with due-date alerts for all field photometry instruments.
Store and transport instruments per manufacturer guidance to minimise damage risk.
Perform a quick reference-source cross-check before major acceptance test campaigns.

Related References

SOP: SOP-PHM-10 (Photometer Calibration and Verification Procedure)

Standards: ICAO Annex 14 Vol I, IES LM-79, ISO/IEC 17025 (calibration laboratory competence, where applicable)

Diagram: Instrument Calibration Tracking Log

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue

PHM-461 Photometer Zero Error

Symptom — The photometer reads a non-zero value with no light source present (dark/zero reading check), indicating a baseline offset that will bias every subsequent measurement.

Equipment / System

Field photometer/luminance meter sensor and readout unit.

Possible Causes

HighSensor dark-current drift from thermal effects or long continuous use

HighStray/ambient light leaking into the sensor housing or optical path

MediumZero/dark calibration not performed or performed incorrectly at the start of the session

MediumInternal electronic offset drift from instrument ageing

LowPhysical damage to the sensor housing allowing light leakage

Required Tools

Light-tight cap or dark enclosure for zero-checkManufacturer zero/dark calibration procedureSpare calibrated instrument for cross-check

Safety Precautions

No unique hazard.

Diagnostic Procedure

ObservationPerform the manufacturer’s dark/zero check with the sensor fully covered or in a light-tight enclosure and record the reading.
ObservationInspect the sensor housing and optical path for damage or light leakage.
MeasurementRepeat the zero check after allowing the instrument to thermally stabilise.
DecisionZero offset persists after proper dark check and thermal stabilisation → instrument requires service/recalibration. Zero offset resolves with correct dark-check procedure → prior readings may have used an incorrect zero reference; re-zero and repeat affected measurements.
Fault LocationIsolate to a procedural zero-check error versus a genuine instrument fault.
Root CauseConfirm via the calibration service report if the instrument is sent for service.

Test Points

Zero/dark reading value

Sensor housing integrity

Thermal stabilisation time

Repeatability of zero check

Root Cause Verification

After correcting the zero-check procedure or servicing the instrument, confirm a stable near-zero dark reading before proceeding with fixture measurements.

Corrective Action

Re-perform the correct zero/dark calibration procedure before each test session.
Send the instrument for service if the offset persists after correct procedure.
Repeat any fixture measurements taken with an unconfirmed zero reference.

Preventive Action

Make a documented dark/zero check a mandatory first step of every field photometry session.
Include sensor housing inspection in the routine equipment check.

Related References

SOP: SOP-PHM-09, SOP-PHM-10

Standards: ICAO Annex 14 Vol I, IES LM-79

Diagram: Field Photometer Setup Reference

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue

PHM-462 Photometer Reference Error

Symptom — Photometric results are inconsistent with expected values because the reference standard, reference fixture, or reference dataset used for comparison is itself incorrect, outdated, or mismatched to the fixture under test.

Equipment / System

Reference light source, reference fixture, or reference isocandela dataset used during acceptance or comparative photometric testing.

Possible Causes

HighWrong reference isocandela curve selected (mismatched fixture category, revision, or manufacturer)

HighReference light source itself out of calibration or degraded

MediumOutdated reference documentation used instead of the current applicable ICAO/FAA specification revision

MediumReference fixture used for comparison has drifted from its own certified output over time

LowData entry or transcription error when recording the reference values for comparison

Required Tools

Current ICAO Annex 14 / FAA AC 150/5340-30 reference documentationCalibrated reference light source with current certificateFixture design drawings

Safety Precautions

No unique hazard.

Diagnostic Procedure

ObservationConfirm the exact fixture model, category, and applicable specification revision, then verify the reference curve/source used matches it precisely.
ObservationCheck the calibration status and certificate of any physical reference light source used.
MeasurementCross-check the reference values against the current published ICAO/FAA specification directly.
DecisionReference mismatch or outdated documentation confirmed → correct the reference and re-evaluate the original test result. Reference confirmed correct → re-examine the fixture itself as the source of the discrepancy.
Fault LocationIsolate to a wrong/outdated reference versus a genuine fixture-level fault.
Root CauseConfirm by re-running the comparison against the verified-correct reference.

Test Points

Reference curve/source identity vs. fixture model and category

Reference source calibration status

Specification revision date

Root Cause Verification

After correcting the reference used, re-evaluate the original fixture measurement against the verified-correct reference and confirm whether a genuine discrepancy remains.

Corrective Action

Replace the incorrect reference curve/source with the correct one for the fixture’s exact model, category, and applicable specification revision.
Recalibrate or replace a degraded reference light source.

Preventive Action

Maintain a controlled, version-tracked library of reference isocandela curves by fixture model and specification revision.
Verify the applicable specification revision before every acceptance test campaign.

Related References

SOP: SOP-PHM-01, SOP-PHM-11 (Reference Standard Control Procedure)

Standards: ICAO Annex 14 Vol I Attachment A, FAA AC 150/5340-30

Diagram: Isocandela Curve Reference Set

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue

PHM-463 Frame Alignment Error

Symptom — Photometric readings are inconsistent or biased because the photometer’s own measurement frame/mounting rig is not correctly aligned relative to the fixture’s optical axis or the runway/taxiway reference plane.

Equipment / System

Photometer mounting frame, tripod, or measurement rig used to position the sensor relative to the fixture under test.

Possible Causes

HighMounting frame not levelled to the pavement/reference plane before measurement

HighFrame azimuth not aligned to the fixture’s design reference axis (e.g., runway centreline direction)

MediumFrame mechanically loose or drifted during the test session

MediumIncorrect frame height setting relative to the fixture’s optical centre (cross-reference PHM-465)

LowFrame damaged or warped, preventing true alignment

Required Tools

Digital level/inclinometerAzimuth reference toolFrame alignment checklist per fixture typeTorque tools for frame hardware

Safety Precautions

Ensure the frame/rig is stable and will not tip or be struck by vehicle/aircraft movement during a runway/taxiway-adjacent test.

Diagnostic Procedure

ObservationVerify the frame is level using a digital level before beginning measurements.
MeasurementConfirm the frame’s azimuth alignment against the fixture’s design reference axis using an azimuth reference tool.
ObservationCheck all frame mounting hardware for looseness or drift during the session.
DecisionFrame found out of level or misaligned in azimuth → correct alignment and repeat affected measurements. Frame confirmed correctly aligned → investigate other causes for the original suspect readings.
Fault LocationIsolate to a levelling error, azimuth misalignment, or mechanical drift during the session.
Root CauseConfirm by repeating the measurement with verified frame alignment and comparing results.

Test Points

Frame level (pitch/roll)

Frame azimuth vs. design reference axis

Mounting hardware tightness

Frame height vs. fixture optical centre

Root Cause Verification

After correcting frame alignment, repeat the affected measurements and confirm results are consistent with a properly aligned setup.

Corrective Action

Re-level and re-align the frame before resuming testing.
Tighten or repair loose/drifted mounting hardware.
Replace a damaged or warped frame.

Preventive Action

Include a documented frame alignment check (level, azimuth, height) as a mandatory first step before every photometric test session.
Inspect the frame for mechanical condition as part of routine equipment maintenance.

Related References

SOP: SOP-PHM-09, SOP-PHM-12 (Measurement Frame Setup and Alignment Procedure)

Standards: ICAO Annex 14 Vol I, IES LM-79

Diagram: Field Photometer Setup Reference

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue

PHM-464 Measurement Distance Error

Symptom — Photometric readings are biased high or low because the actual distance between the photometer sensor and the fixture under test differs from the distance assumed in the test procedure or intensity calculation.

Equipment / System

Field photometer and the distance-measurement method (tape, laser rangefinder, or fixed test-station geometry) used during fixture testing.

Possible Causes

HighIncorrect distance entered into the photometer/calculation software, not matching the actual measured distance

HighTest conducted from an unmarked or estimated position rather than the specified/surveyed test distance

MediumLaser rangefinder or tape measure error/miscalibration

MediumFixture or photometer position shifted slightly after the distance was measured but before the reading was taken

LowConfusion between near-field and far-field measurement conventions for the fixture type being tested

Required Tools

Calibrated laser rangefinder or surveyed test-station markersPhotometer with distance-input/inverse-square correction capabilityFixture test procedure specifying the required distance

Safety Precautions

Standard fixture-access and runway/taxiway coordination precautions apply.

Diagnostic Procedure

ObservationConfirm the required test distance for the fixture type/category per the applicable test procedure.
MeasurementRe-measure the actual sensor-to-fixture distance using a calibrated rangefinder and compare to the value used in the original test.
ObservationVerify neither the fixture nor the photometer moved between the distance measurement and the intensity reading.
DecisionDistance discrepancy found → recalculate or repeat the measurement using the correct, verified distance. Distance confirmed correct → investigate other causes for the original suspect reading.
Fault LocationIsolate to a distance measurement/input error versus a genuine fixture-level fault.
Root CauseConfirm by repeating the test at the verified-correct distance and comparing results.

Test Points

Actual sensor-to-fixture distance vs. specified test distance

Rangefinder/tape calibration status

Distance value entered into calculation software

Root Cause Verification

After correcting the distance and repeating the measurement, confirm the resulting intensity value is consistent with a correctly-distanced test.

Corrective Action

Repeat the measurement at the correct, verified test distance.
Correct the distance value in any calculation software before re-evaluating stored results.
Recalibrate or replace a faulty rangefinder/tape.

Preventive Action

Use fixed, surveyed test-station markers where practical rather than relying on a fresh distance measurement each time.
Include a distance verification step in the standard field test checklist.

Related References

SOP: SOP-PHM-01, SOP-PHM-13 (Measurement Distance and Geometry Verification Procedure)

Standards: ICAO Annex 14 Vol I, IES LM-79

Diagram: Field Photometer Setup Reference

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue

PHM-465 Measurement Height Error

Symptom — Photometric readings are biased because the photometer sensor height above the pavement (or above the fixture’s optical centre) differs from the height specified in the test procedure.

Equipment / System

Photometer mounting frame/tripod height setting relative to the fixture under test.

Possible Causes

HighFrame/tripod height not set or verified against the specified test height before measurement

HighHeight specification confused between different fixture types (e.g., in-pavement flush fixture vs. elevated approach light) leading to a wrong reference height

MediumFrame height drifted or was bumped during the test session

MediumUneven pavement/ground at the test location affecting the true sensor height even with a correctly set frame

LowHeight measurement tool (ruler/rod) miscalibrated or misread

Required Tools

Calibrated height gauge or measuring rodFixture test procedure specifying the required heightDigital level to confirm frame stability

Safety Precautions

Standard fixture-access and runway/taxiway coordination precautions apply.

Diagnostic Procedure

ObservationConfirm the required sensor height for the specific fixture type/category per the applicable test procedure.
MeasurementRe-measure the actual sensor height above the reference plane (pavement or fixture optical centre, as specified) using a calibrated gauge.
ObservationCheck for uneven pavement/ground at the test location that could affect true height despite a correctly set frame.
DecisionHeight discrepancy found → correct frame height and repeat the measurement. Height confirmed correct → investigate other causes for the original suspect reading.
Fault LocationIsolate to a height-setting error versus ground/pavement unevenness versus a genuine fixture fault.
Root CauseConfirm by repeating the test at the verified-correct height and comparing results.

Test Points

Actual sensor height vs. specified test height

Ground/pavement levelness at test location

Height gauge calibration status

Root Cause Verification

After correcting the height and repeating the measurement, confirm the resulting intensity value is consistent with a correctly-heighted test.

Corrective Action

Reset the frame to the correct, verified test height and repeat the measurement.
Select a level test location or compensate for ground unevenness per procedure.
Recalibrate or replace a faulty height gauge.

Preventive Action

Mark and use fixed, surveyed test-station heights where practical.
Include a height verification step in the standard field test checklist alongside distance and alignment checks.

Related References

SOP: SOP-PHM-09, SOP-PHM-13

Standards: ICAO Annex 14 Vol I, IES LM-79

Diagram: Field Photometer Setup Reference

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue

PHM-466 Instrument Mounting Error

Symptom — Photometric readings are unreliable or inconsistent because the photometer itself is not securely or correctly mounted, independent of frame alignment, distance, or height issues already ruled out.

Equipment / System

Photometer body-to-frame mounting hardware and connection.

Possible Causes

HighPhotometer not fully seated or locked into its frame mounting bracket

HighMounting bracket damaged, worn, or incompatible with the specific photometer model in use

MediumVibration (from wind, nearby vehicle/aircraft movement, or an unstable frame) causing the instrument to shift during the reading

MediumSensor cable tension or routing pulling the instrument slightly out of its set position

LowIncorrect mounting orientation without being a full alignment/orientation fault (cross-reference PHM-467)

Required Tools

Manufacturer mounting hardware and bracketTorque tool if specifiedVisual/physical seating check procedure

Safety Precautions

Ensure the photometer cannot fall or be struck during measurement, particularly near active pavement.

Diagnostic Procedure

ObservationVisually and physically check that the photometer is fully seated and locked into its mounting bracket.
ObservationInspect the mounting bracket for damage, wear, or incompatibility with the instrument model.
MeasurementTake repeated readings while gently checking for any movement or play in the mounting, to detect vibration-induced shift.
DecisionInstrument not fully seated or bracket damaged → correct seating or replace bracket, then repeat measurement. Mounting confirmed secure → investigate other causes (frame alignment, vibration source) for the original suspect readings.
Fault LocationIsolate to a seating/locking error, damaged bracket, or external vibration source.
Root CauseConfirm by repeating the measurement with a verified-secure mounting and comparing results.

Test Points

Instrument seating/lock engagement

Mounting bracket condition

Repeatability of readings with mounting re-verified

Cable tension/routing

Root Cause Verification

After correcting the mounting, repeat the affected measurements and confirm results are consistent and stable with a securely mounted instrument.

Corrective Action

Fully seat and lock the photometer into its bracket before measurement.
Replace a damaged or incompatible mounting bracket.
Re-route the sensor cable to remove tension on the instrument.

Preventive Action

Include an instrument seating check as a mandatory step in the field test checklist.
Inspect mounting hardware for wear as part of routine equipment maintenance.

Related References

SOP: SOP-PHM-09, SOP-PHM-12

Standards: ICAO Annex 14 Vol I, IES LM-79

Diagram: Field Photometer Setup Reference

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue

PHM-467 Incorrect Measurement Orientation

Symptom — Readings are inconsistent with expected values because the photometer sensor’s rotational orientation (relative to the fixture’s designed horizontal/vertical reference axes) is incorrect, distinct from frame azimuth or mounting seating issues already ruled out.

Equipment / System

Photometer sensor orientation relative to the fixture’s defined horizontal and vertical measurement axes.

Possible Causes

HighSensor rotated from its intended reference orientation (e.g., horizontal/vertical axes swapped) when set up

HighFixture’s own horizontal/vertical reference axes misunderstood or misidentified for an asymmetric beam-spread fixture (cross-reference PHM-455)

MediumOrientation marking on the sensor or frame worn away or illegible, leading to a guessed setup

LowOrientation correct at setup but drifted during the test session without being caught by the mounting check (cross-reference PHM-466)

Required Tools

Fixture design drawing showing horizontal/vertical reference axesOrientation reference marks or jig for the photometerDigital level/inclinometer

Safety Precautions

Standard fixture-access precautions apply.

Diagnostic Procedure

ObservationConfirm the fixture’s defined horizontal and vertical measurement axes from its design drawing, particularly for elliptical or asymmetric beam fixtures.
ObservationVerify the photometer sensor’s actual orientation against its own reference marks and against the fixture’s axes.
MeasurementRepeat the measurement with orientation corrected and compare to the original suspect reading.
DecisionOrientation error confirmed and corrected reading differs meaningfully from the original → orientation was the root cause. Corrected reading matches the original → orientation was not the issue; investigate elsewhere.
Fault LocationIsolate to a sensor orientation error versus a misidentified fixture reference axis.
Root CauseConfirm by re-running the full angular scan with verified-correct orientation and comparing the resulting distribution to the specified isocandela curve.

Test Points

Sensor orientation vs. reference marks

Fixture horizontal/vertical axes per design drawing

Comparison of readings before/after orientation correction

Root Cause Verification

After correcting orientation, repeat the measurement or full scan and confirm results align with the expected distribution for the fixture’s design axes.

Corrective Action

Correct the sensor orientation to match the fixture’s defined reference axes before measurement.
Re-mark or replace worn orientation indicators on the sensor/frame.

Preventive Action

Include an explicit orientation verification step in the field test checklist, especially for asymmetric or elliptical-beam fixture types.
Maintain clear, durable orientation markings on all field photometry equipment.

Related References

SOP: SOP-PHM-09, SOP-PHM-12

Standards: ICAO Annex 14 Vol I Attachment A, IES LM-79

Diagram: Field Photometer Setup Reference

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue

PHM-468 Repeated Photometry Failure

Symptom — A specific fixture or fixture type repeatedly fails photometric acceptance testing across multiple visits or replacement attempts, despite individual corrective actions appearing successful at the time.

Equipment / System

A recurring-failure fixture, its circuit, and its maintenance/test history.

Possible Causes

HighUnderlying circuit-level issue (chronic undervoltage, poor connection) masked as a series of individual fixture-level fixes

HighSystematic use of an incorrect or marginal replacement part across multiple repair attempts

MediumEnvironmental factor specific to the fixture’s location (e.g., persistent debris/vegetation shading, drainage issue causing repeat water ingress) not addressed by prior corrective actions

MediumTest procedure or equipment issue specific to that test station being overlooked as a contributing factor

LowFixture design/category genuinely marginal for its application, requiring an engineering review rather than routine maintenance

Required Tools

Full fixture maintenance and test history logCircuit layout drawingRoot-cause analysis worksheetEngineering escalation contact/process

Safety Precautions

Standard fixture-access precautions apply; treat a chronic failure location as a priority for full diagnostic depth rather than repeat quick fixes.

Diagnostic Procedure

ObservationCompile the fixture’s full history of photometric failures, corrective actions taken, and subsequent results across all prior visits.
MeasurementRe-verify circuit voltage/current, environmental conditions, and test setup/equipment at this specific location, rather than assuming the fixture itself is again at fault.
ObservationCross-reference the location for any recurring environmental or drainage issue.
DecisionPattern points to a circuit-level or environmental root cause common across all failures → address that systemic issue rather than repeating fixture-level fixes. No common pattern found → escalate for engineering review of the fixture design/application suitability.
Fault LocationIsolate to a systemic circuit/environmental cause, a repeated parts/procedure error, or a genuine design/application issue requiring escalation.
Root CauseConfirm by addressing the identified systemic cause and monitoring the fixture through at least one additional full maintenance cycle without recurrence.

Test Points

Full failure history and corrective-action log

Circuit voltage/current at this fixture over time

Environmental/drainage condition at the location

Root Cause Verification

After addressing the identified systemic cause, monitor the fixture’s photometric performance over the following one to two maintenance cycles and confirm no recurrence of the failure pattern.

Corrective Action

Address the underlying circuit-level or environmental root cause rather than repeating isolated fixture-level fixes.
Escalate to engineering review if no systemic cause is found.

Preventive Action

Maintain fixture-level failure history logs and review them for repeat-failure patterns as part of the periodic PM programme.
Flag chronic-failure locations for a deeper root-cause investigation rather than routine repair.

Related References

SOP: SOP-PHM-01, SOP-PHM-14 (Chronic Fault Escalation Procedure)

Standards: ICAO Annex 14 Vol I, FAA AC 150/5340-30

Diagram: Fixture Maintenance History Log Template

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue

PHM-469 Fixture Passes Visual Inspection but Fails Photometry

Symptom — A fixture appears completely normal on visual inspection (correct colour, no visible damage, illuminates as expected) yet fails one or more parameters during formal photometric testing.

Equipment / System

Any AGL fixture where visual inspection alone was previously used as the primary field assessment.

Possible Causes

HighBeam misalignment or beam spread error not detectable to the naked eye without instrumentation (cross-reference PHM-454, PHM-455)

HighIntensity below specification but still visually bright enough to appear normal, especially in daylight or under casual observation

MediumSubtle chromaticity shift not perceptible to the eye but outside the specified colour box (cross-reference PHM-457)

MediumPhotometric distribution shape error that does not change the fixture’s overall visual brightness or apparent colour

LowA genuine photometer/measurement error producing a false failure on an actually-compliant fixture

Required Tools

Calibrated field photometer/colorimeterFull angular scan capabilityReference isocandela and colour box curves

Safety Precautions

Standard fixture-access precautions apply.

Diagnostic Procedure

ObservationConfirm the specific parameter(s) that failed in the photometric test report (intensity, alignment, spread, distribution, or chromaticity).
MeasurementRepeat the failed measurement(s) to rule out a one-off instrument or setup error before proceeding to fixture-level correction.
DecisionFailure confirmed on repeat measurement → proceed to the relevant fixture-level diagnostic topic (PHM-451 through PHM-458) for the specific failed parameter. Failure does not repeat → treat as a measurement error and cross-reference PHM-459 through PHM-467.
Fault LocationIsolate to the specific confirmed photometric parameter for targeted correction.
Root CauseFollow the diagnostic procedure of the corresponding specific-parameter topic to establish and correct the true root cause.

Test Points

All standard photometric parameters (intensity, alignment, spread, distribution, chromaticity)

Repeatability of the original failed result

Root Cause Verification

Once the specific failed parameter is corrected per its dedicated diagnostic procedure, re-run the full photometric test and confirm the fixture passes on all parameters.

Corrective Action

Address the confirmed failed parameter via the corresponding specific corrective action (module replacement, realignment, optic replacement, lens/filter correction, etc.).

Preventive Action

Do not rely on visual inspection alone as a substitute for scheduled instrumented photometric testing, particularly for alignment, spread, distribution, and chromaticity parameters that are not reliably visible to the eye.

Related References

SOP: SOP-PHM-01; cross-reference PHM-451 through PHM-458 and PHM-459 through PHM-467

Standards: ICAO Annex 14 Vol I Attachment A, FAA AC 150/5340-30

Diagram: Isocandela and Colour Box Reference Set

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue

PHM-470 Fixture Passes Photometry but Field Appearance Abnormal

Symptom — A fixture measures fully within specification on formal photometric testing, yet appears visually abnormal in the field — for example, looking dim, oddly coloured, or unevenly lit compared to neighbouring fixtures under night operating conditions.

Equipment / System

A fixture with a passing photometric test result but a reported or observed field appearance discrepancy.

Possible Causes

HighTest conducted at a different intensity step than the one typically used operationally, or at a different time from the visual observation

HighPerceptual effect from surrounding fixtures, ambient lighting, weather (fog, rain), or pilot/observer viewing angle differing from the fixed test geometry

MediumA parameter within specification but at the edge of its tolerance band, visually distinguishable from neighbouring fixtures comfortably mid-band

MediumPhotometric test measured a single angle/condition that does not capture the specific viewing condition where the abnormality was reported

LowThe photometer itself passed a marginal fixture that a more sensitive or differently-calibrated instrument would have flagged

Required Tools

Calibrated field photometer with full angular scan capabilityALCMS step/log recordsWeather/visibility records for the reported observation timeComparison readings from neighbouring fixtures

Safety Precautions

Standard fixture-access precautions apply; if the report originated from a pilot, treat with appropriate priority pending investigation.

Diagnostic Procedure

ObservationConfirm the exact intensity step, time, and conditions (weather, viewing angle) under which the abnormality was reported, and compare to the conditions under which the passing photometric test was conducted.
MeasurementRe-test the fixture at the specific intensity step and, where feasible, from a viewing angle closer to the reported condition.
ObservationCompare the fixture’s readings directly against several neighbouring fixtures to check for a within-tolerance but relatively weak position in the run (cross-reference PHM-453).
DecisionRe-test under matched conditions reveals a genuine parameter discrepancy → proceed to the relevant specific-parameter corrective action. Re-test confirms full compliance under matched conditions → likely a perceptual/environmental effect; document and monitor rather than replace a compliant fixture.
Fault LocationIsolate to a test-condition mismatch, a marginal-but-compliant fixture, or a genuine parameter fault only detectable under specific conditions.
Root CauseConfirm by correlating multiple reports (if any) against consistent conditions to distinguish a real trend from an isolated perceptual observation.

Test Points

Intensity/colour at the reported operational step

Comparison against neighbouring fixtures

Viewing-angle-specific readings if feasible

Weather/visibility conditions at time of report

Root Cause Verification

If a genuine discrepancy is found and corrected, re-test under the matched condition and confirm the appearance concern is resolved; if no discrepancy is found, document the investigation and monitor for recurrence.

Corrective Action

Where a genuine parameter issue is confirmed, apply the relevant specific corrective action (module, alignment, lens).
Where the fixture is confirmed compliant, consider tightening its position within an uneven run (cross-reference PHM-453) as a preventive step even without an outright failure.

Preventive Action

Include the operational intensity step (not only the acceptance-test step) in routine photometric checks.
Log field appearance reports with condition details to help distinguish genuine trends from one-off perceptual effects.

Related References

SOP: SOP-PHM-01; cross-reference PHM-453, PHM-451 through PHM-458

Standards: ICAO Annex 14 Vol I Attachment A, FAA AC 150/5340-30

Diagram: Isocandela Curve Reference Set

Case Study: Division 25 — Field Case Study Troubleshooting

Rev 0 — Initial issue

AGL Photometry Troubleshooting

01 · AGL Photometry Faults

Equipment / System

Possible Causes

Required Tools

Safety Precautions

Diagnostic Procedure

Test Points

Root Cause Verification

Corrective Action

Preventive Action

Related References

Equipment / System

Possible Causes

Required Tools

Safety Precautions

Diagnostic Procedure

Test Points

Root Cause Verification

Corrective Action

Preventive Action

Related References

Equipment / System

Possible Causes

Required Tools

Safety Precautions

Diagnostic Procedure

Test Points

Root Cause Verification

Corrective Action

Preventive Action

Related References

Equipment / System

Possible Causes

Required Tools

Safety Precautions

Diagnostic Procedure

Test Points

Root Cause Verification

Corrective Action

Preventive Action

Related References

Equipment / System

Possible Causes

Required Tools

Safety Precautions

Diagnostic Procedure

Test Points

Root Cause Verification

Corrective Action

Preventive Action

Related References

Equipment / System

Possible Causes

Required Tools

Safety Precautions

Diagnostic Procedure

Test Points

Root Cause Verification

Corrective Action

Preventive Action

Related References

Equipment / System

Possible Causes

Required Tools

Safety Precautions

Diagnostic Procedure

Test Points

Root Cause Verification

Corrective Action

Preventive Action

Related References

Equipment / System

Possible Causes

Required Tools

Safety Precautions

Diagnostic Procedure

Test Points

Root Cause Verification

Corrective Action