PAPI Maintenance Checklist Inspection, Testing & Troubleshooting
A practical field checklist for PAPI lenses, aiming, elevation, transformers, electrical tests, photometry and retesting, with troubleshooting steps and records.
PAPI Maintenance Checklist
PAPI maintenance has to cover much more than confirming that all four units are lit. A proper programme examines the optical units, lenses, colour performance, aiming angles, elevation, alignment, electrical connections, isolation transformers, foundations and overall operation. A PAPI can stay electrically operational while developing an optical or alignment problem.
A unit can be ON but
The process combines visual inspection + mechanical inspection + electrical testing + aiming verification + photometry + functional testing + documentation.
Why PAPI Maintenance Matters
| PAPI indication | General interpretation |
|---|---|
| 4 white | Significantly above the indicated path |
| 3 white / 1 red | Above the indicated path |
| 2 white / 2 red | Approximately on the indicated path |
| 1 white / 3 red | Below the indicated path |
| 4 red | Significantly below the indicated path |
The indication depends on the optical transition angle and precise alignment of the four units, so maintenance has to protect the geometry of the whole system, not only each lamp.
PAPI Maintenance Is a System Activity
Optical
- Lenses
- Filters / optical elements
- Intensity
- Colour
- Beam characteristics
Mechanical
- Housing
- Mounting and foundation
- Horizontal alignment
- Vertical aiming
- Adjustment mechanism
Electrical
- Supply
- Transformer
- Cables and connectors
- Insulation
- Earthing
Operational
- ON/OFF operation
- Brightness stages where applicable
- Pilot indication
- Control and monitoring
Photometric
- Intensity
- Colour
- Beam coverage
- Beam alignment
Field Checklist
Click an item to mark it complete. This is a working aid, and the airport’s approved checklist takes precedence.
1. Visual inspection
2. Alignment and elevation
3. Electrical
4. Photometric
5. Final functional test and records
1. Visual Inspection
Start every job with a visual inspection to find obvious problems before detailed measurements. Inspect all four units, lenses, housing, mounting, foundation, cables, connectors, transformers and the surrounding area. Identify each unit individually so maintenance and retesting stay traceable.
Unit and housing
Look for physical damage, cracks, loose components, deformation, displaced housing, damaged adjustment mechanisms, missing hardware, corrosion, loose covers, damaged seals and missing fasteners. The housing protects both optics and electrics.
Lens
Inspect for cracks, scratches, clouding, discoloration, contamination, surface damage, moisture and deposits. A damaged lens can change intensity, beam shape, colour and coverage, so document the condition rather than writing “OK”.
Dirt and contamination
Dust, oil, insects, environmental deposits, construction dust and pollution. AAI schedules call for lens cleaning and condition checks. Use methods and materials compatible with the manufacturer’s requirements, and avoid abrasives that can damage optical surfaces. A cracked or degraded lens needs replacement, not cleaning.
Water ingress and corrosion
Check for condensation, standing moisture, water marks, corroded internals, wet connectors and damaged seals. Causes include a damaged gasket, poor sealing, cracked housing, blocked drainage or poor installation. Do not just remove water and return the unit to service. Find the cause. Corrosion on housing, brackets, fasteners, adjustment hardware, cable entries, transformer housing and supports can eventually affect alignment and connections.
Foundation
The aiming reference depends on the unit’s physical position. Look for cracking, settlement, movement, loose anchor bolts, damaged concrete, drainage problems and pavement deterioration around the foundation. A PAPI can be electrically healthy while its foundation has shifted.
2. Alignment, Aiming and Elevation
Alignment is one of the most critical areas. Check it against the approved PAPI design, not an arbitrary generic angle.
Horizontal alignment
Verify that units keep the required relationship to the runway reference, the row stays correctly oriented and the optical axes point at the approach area. Causes of displacement: foundation movement, impact, loose mounting, maintenance activity, nearby construction, pavement works.
Vertical aiming
Each unit is aimed at a controlled elevation angle, and a small change moves the white/red transition. Use proper aiming and calibration equipment, not visual estimation. AAI documentation includes PAPI angle checks and calibration with instruments such as sighting sticks, clinometers and spirit levels.
Unit-to-unit relationship
Do not stop at “Unit 1 good, Unit 2 good…”. Verify unit 1 ↔ 2 ↔ 3 ↔ 4. The relative aiming relationship is critical to the indication.
Elevation
Verify reference, foundation and unit elevation and the approved aiming settings. Changes to runway profile, pavement level, threshold position or PAPI foundation can alter the relationship between PAPI and approach path.
Approach slope
The slope is runway-specific. Many AAI aerodrome publications show a 3.00° PAPI angle, but always take the value from the applicable runway design and current aeronautical information. Record the approved PAPI angle and the measured / verified angle, not just “3°”.
MEHT
Minimum Eye Height over Threshold is a runway-specific parameter, not a generic equipment value. Make sure no physical change has altered the PAPI’s relationship with the threshold, runway elevation, approach path or approved visual guidance geometry.
3. Electrical Maintenance
An apparent optical problem can start as an electrical one. Check supply, cable, connectors, transformer, insulation, earthing and the control interface. Take exact electrical values from the PAPI manufacturer and the approved project documents. Do not use a generic voltage as a universal acceptance criterion.
Supply, cable and connectors
Check availability and correct circuit operation, abnormal fluctuations, brightness stages and control response. Inspect primary cable and secondary leads for insulation, damage, moisture and terminations. Cable faults cause intermittent operation, low output, complete failure and insulation problems. Inspect plugs and sockets for damage, contamination, moisture, loose connection and damaged sealing. AAI schedules include the plug/socket connection to the isolation-transformer lead.
Isolation transformer and earthing
Inspect the transformer for damage, overheating, moisture, cable-entry and connector condition and insulation. Test it with the circuit as the procedure requires. Do not blame the light unit before checking the transformer and secondary circuit. For earthing, review the connection, conductor, terminals, corrosion, continuity and recorded test values. AAI’s schedule has dedicated earthing checks.
Insulation testing
Where the approved procedure specifies it, test PAPI-associated cabling and record the result. AAI examples include periodic megohmmeter testing with recorded results. The test voltage and minimum acceptance value come from the applicable airport procedure and equipment specification. A value from one contract is not a universal PAPI requirement.
4. Photometric Maintenance
Photometry gives objective evidence of optical condition, and it matters most when visual inspection cannot explain an abnormal indication.
Intensity
Check whether each unit gives the required output under the prescribed test condition. Low readings can come from a dirty lens, optical deterioration, damaged optics, LED or driver degradation, electrical problems, the wrong operating condition or the measurement setup. First confirm the measurement.
Colour
Verify the required white/red indication. Problems can come from filters, optical components, LED degradation, contamination or incorrect replacement parts. Where formal acceptance needs it, use objective chromaticity measurement.
Beam coverage
The beam must give the required angular coverage. A fitting can have adequate peak intensity but abnormal distribution, so peak intensity ≠ complete beam compliance.
Beam alignment
Alignment determines where the useful output goes. Problems cause an incorrect indication, reduced effective guidance, abnormal transition or apparent intensity loss. Read aiming verification and photometry together.
Functional test vs photometric test
| Functional test | Photometric test |
|---|---|
| Unit turns ON | Measures optical output |
| White/red indication visible | Quantifies optical characteristics |
| Control operates | Measures intensity |
| Circuit responds | Measures colour / chromaticity |
| Brightness stages work | Evaluates beam distribution |
| Basic serviceability | Detailed optical performance |
From a low-intensity finding to a verified fix
Identifies low intensity
Detailed measurement
Investigate geometry and aiming
Confirm the result
5. Final Functional Test, Pilot Indication and Calibration
After physical, electrical and optical maintenance, run a complete functional test: all four units, the correct sequence of indications, correct white/red appearance, local and remote control, brightness stages and alarms where available. The final question: does the PAPI give the intended visual indication from the approach direction? This does not replace any required formal flight inspection or authority-approved calibration.
Maintenance
Routine work to preserve serviceability.
Ground calibration / aiming verification
Verification and adjustment of the installed physical and optical settings.
Formal flight inspection
Where the procedure or authority requires it, an airborne verification of the operational relationship with the approach path and related facilities.
6. PAPI Troubleshooting
Pick the reported problem for a structured diagnostic order.
Check, in order
Avoid
7. Maintenance Frequency
No single frequency applies to every airport. Intervals can come from the approved airport maintenance programme, operational category, authority requirements, equipment manufacturer, contract, local environment and failure history. AAI contracts show daily, weekly, monthly and half-yearly activities. One current AAI schedule includes daily serviceability and physical checks, weekly angle and lens checks and half-yearly calibration, earthing, insulation and transformer-connection checks. Another combines routine lens and equipment checks with angle calibration, electrical checks and periodic earthing and wiring inspections. The airport’s approved schedule takes precedence.
| Activity | Typical planning approach |
|---|---|
| Visual serviceability | Routine / operational |
| Lens inspection and cleaning | Routine, cleaning as required or scheduled |
| Physical damage inspection | Routine |
| Horizontal alignment and aiming | Periodic, and after disturbance |
| Calibration verification | Periodic, and after major maintenance |
| Electrical connections and transformer | Periodic |
| Insulation testing and earthing | Scheduled |
| Photometric testing | According to the approved programme |
| Full functional test | Scheduled, and after intervention |
| Retest after repair | As required |
Related Guides
AGL preventive maintenance guideWhen to Check Immediately, and After Changes
Check immediately after
- Vehicle impact
- Construction near the PAPI
- Runway rehabilitation
- Threshold relocation
- Pavement work
- Severe weather
- Foundation disturbance
- Equipment replacement
- Major electrical work
- An abnormal pilot report
After runway changes
Relocation, extension, resurfacing, pavement-level changes, PAPI relocation or a change in approach geometry need an engineering review. Do not just check that the PAPI still turns on. Decide whether its relationship to the approach path remains correct.
After major repair or lens replacement
Repeat the relevant electrical, mechanical, aiming, photometric and functional checks and retest. After a lens or optical component change, inspect the installation, verify optical alignment, check colour, run photometry where applicable and confirm operation.
After foundation work
Foundation work can change unit height, horizontal alignment, vertical aiming and the relationship between the four units. Recheck the complete PAPI geometry.
After unit replacement
Confirm the replacement is correct
Verify mechanical installation
Verify horizontal alignment
Set vertical angle
Lock the adjustment
Verify optical performance
Perform functional testing
Update asset records
The new unit joins the system’s maintenance baseline.
Records, Trends and the Defect Register
Photometric and condition records
Track intensity, colour, aiming, alignment, defects, repairs and replacement dates. History shows normal → degradation → intervention → recovery instead of isolated inspections. Condition indicators can include optical trend (is intensity declining?), alignment trend (is the setting moving?), failure trend (does one unit keep failing?), electrical trend and environmental trend (moisture or corrosion). This supports predictive rather than reactive maintenance.
A maintenance report should hold
- Asset information: airport, runway, PAPI designation, unit numbers, model and serial numbers
- Physical condition
- Electrical results
- Optical results and aiming
- Functional results
- Defects, corrective actions, replacements, adjustments
- Retest date, result and status
Example defect register
| ID | Unit | Observation | Cause | Action | Retest | Status |
|---|---|---|---|---|---|---|
| P-001 | Unit 1 | Dirty lens | Contamination | Cleaned | Pass | Closed |
| P-002 | Unit 2 | Incorrect angle | Mechanical movement | Realigned | Pass | Closed |
| P-003 | Unit 3 | Low intensity | Optical degradation | Component replaced | Pass | Closed |
| P-004 | Unit 4 | Moisture | Seal failure | Seal repaired | Pending | Open |
Closing the maintenance cycle
Printable Record: Identification and Sign-off
The fields a completed PAPI maintenance record should carry, alongside the checklist above.
PAPI identification
- Airport
- Runway
- PAPI designation
- Date
- Technician
- Weather and conditions
Documentation and sign-off
- Defects recorded: yes / no
- Corrective actions completed: yes / no
- Photometric retest completed: yes / no / N/A
- Final functional test: pass / fail
- Maintenance record updated: yes / no
- Technician signature
- Engineer / inspector
PAPI Within the Wider AGL System
Electrical testing first
A functional failure can be a transformer, cable or connector problem. Sequence: PAPI failure → AGL electrical testing → transformer / cable / connector → optical test → repair → retest. This avoids needless replacement of the light unit.
Lightning and surge
Repeated electronic or transformer failures, especially after severe weather, justify checking lightning protection, grounding, bonding, surge protection and counterpoise where applicable.
Emergency power
Where PAPI relies on secondary power, verify normal power, backup supply, UPS and DG where used, changeover and control status. Assess the complete chain, not an isolated fitting.
CAT II / CAT III airports
PAPI maintenance matters more where it forms part of a precision approach system. Coordinate with runway visual aids, approach lighting, centreline, touchdown-zone lighting, stop bars, AGL secondary power, low-visibility procedures and airport requirements. Serviceability criteria depend on the runway category and airport procedure.
Airport operations
Coordinate with ATC, airport operations, electrical maintenance, AGL control-room staff, flight inspection authorities where applicable and airside safety teams. Never switch, adjust or make a PAPI unavailable without the airport’s approved operational procedure.
Related Guides
Lightning and surge protection for AGL circuits Emergency power and lighting systems Testing and commissioning guideStandards and References
ICAO
Annex 14 Volume I is the principal international framework. ICAO lists the 9th Edition, July 2022, with Amendment 18 dated 4 August 2025 and later corrigenda in 2026. Aerodrome Design Manual Part 4 (Doc 9157), Visual Aids, gives engineering guidance on visual aids including approach lighting, runway and taxiway lighting and visual approach slope indicator systems. The ICAO Store lists its 5th Edition, 2021.
For Indian airports, also follow
AAI contracts are useful examples, but their frequencies and acceptance values are contract- and airport-specific.
- DGCA requirements
- AAI requirements
- Airport maintenance programme
- Project specifications
- PAPI manufacturer’s instructions
- Approved operational procedures
Common PAPI Maintenance Mistakes
Only checking the lights are ON
A working light can still have wrong aiming or low output.
Not checking the foundation
Foundation movement can create an alignment error.
Cleaning without inspecting the lens
A cracked or degraded lens needs replacement.
Adjusting one unit alone
PAPI is a coordinated four-unit system.
Replacing a unit without recalibrating
A replacement needs correct alignment and aiming.
Ignoring electrical components
Transformer, connector and secondary circuit cause apparent PAPI failures.
No photometric baseline
Gradual degradation is hard to see without history.
No retest after corrective action
A repair should be verified, not just recorded.
Treating contract frequencies as universal
The approved local programme governs.
Best practices
Service scope
Quick Knowledge Check
Q1. Can a PAPI be ON and still be defective?
Q2. One PAPI unit is OFF. What is a sensible first order of checks?
Q3. Why record both the approved and the measured PAPI angle?
Q4. What should follow a repair?
Q5. Is a maintenance frequency from one AAI contract a universal PAPI requirement?
Explore More AGL Engineering
PAPI Installation & Aiming Handbook
Install and aim PAPI units.
Open PAPI Installation & Aiming Handbook →PAPI Alignment & Maintenance Field Guide
Alignment, testing and maintenance.
Open PAPI Alignment & Maintenance Field Guide →AGL Photometric Testing Guide
Practical photometric testing.
Open AGL Photometric Testing Guide →AGL Preventive Maintenance Guide
Maintenance cycles and checks.
Open AGL Preventive Maintenance Guide →Testing & Commissioning Guide
Test and commission AGL systems.
Open Testing & Commissioning Guide →Lightning & Surge Protection
Counterpoise, bonding and SPDs.
Open Lightning & Surge Protection →Airport Isolating Transformer
The transformer between circuit and light.
Open Airport Isolating Transformer →How Airfield Lighting Works
The full AGL system explained.
Open How Airfield Lighting Works →Maintain the Geometry, Not Just the Lamps.
From aiming verification and photometry to electrical testing and retesting, explore the complete AGL knowledge base.
Explore the AGL Engineering Knowledge Hub
Fundamentals
Standards & Compliance
Runway, Taxiway & Approach Lighting
Surface Movement & Safety
PAPI
Electrical Engineering & Power
Photometry & Testing
Maintenance, Inspection & Safety
Resources, Documentation & Careers
PAPI Maintenance FAQ
Q1What should a PAPI maintenance checklist include?
Visual condition, lenses, contamination, housing, moisture, corrosion, foundation, horizontal alignment, vertical aiming, elevation, electrical supply, cables, connectors, isolation transformer, earthing, insulation, photometry, colour, beam coverage, functional operation and documentation.
Q2How often should PAPI be maintained?
It depends on the approved airport maintenance programme, regulations, equipment requirements and contract. AAI schedules show that different activities can have different frequencies.
Q3Does PAPI need aiming checks?
Yes. Aiming is fundamental to PAPI operation and should be verified with appropriate survey or calibration equipment, not visual estimation alone.
Q4What causes PAPI misalignment?
Foundation movement, physical impact, loose mounting, construction activity, pavement changes and incorrect maintenance adjustment.
Q5Does cleaning PAPI lenses affect performance?
Yes. Contamination can reduce optical performance, and AAI maintenance schedules include routine lens cleaning.
Q6Does PAPI require photometric testing?
Optical performance can need objective measurement in commissioning, maintenance or acceptance, depending on the programme. Photometry can assess intensity, colour, coverage and beam characteristics.
Q7Can a PAPI be ON and still be defective?
Yes. Electrical operation does not prove that intensity, colour, aiming or beam characteristics are correct.
Q8What electrical components should be checked?
Depending on the installation: supply, CCR or circuit, primary and secondary cable, connector, isolation transformer, insulation and earthing.
Q9Should PAPI be recalibrated after replacement?
Check the replacement for correct installation, alignment and aiming, then apply the functional and photometric verification the system and airport require.
Q10What should be done after major PAPI maintenance?
Perform the applicable mechanical, electrical, aiming, photometric and functional checks, and complete any required retesting or formal calibration.
Q11Does runway resurfacing affect PAPI?
It can. Changes in runway elevation or threshold geometry can alter the relationship between PAPI and the intended approach path, so an engineering review may be needed.
Q12Is PAPI maintenance different for LED systems?
Some lamp-specific tasks change, but LED PAPI still needs inspection of optical surfaces, aiming, alignment, mechanical condition, electrical systems and optical performance.
Q13What should happen after a vehicle strikes a PAPI unit?
Remove it from service as the airport procedure requires, inspect for structural and optical damage, check it electrically, realign it and test it before return to service.
Q14What records should be maintained?
Asset identification, inspection dates, aiming results, photometric data, electrical test results, defects, corrective actions, replacement history and retest results.
Engineering & Expert Notes
Use the Checklist Consistently
MethodFollowing the same sequence builds a comparable history.
Clean Before You Judge
VisualDirt on lenses affects both intensity and colour. Clean and check again.
Check Alignment After Any Disturbance
AlignmentInclude checks after impacts, mowing or ground works.
Inspect Electrical Connections
ElectricalLook for corrosion, water and loose terminals.
Include Photometric Checks at Intervals
PhotometryVisual checks alone will not show gradual degradation.
Keep Records Up to Date
RecordsUse the record sheet to log findings and track recurring issues.
Understand AGL. Build Your Engineering Skills.
You now understand how to inspect, test and maintain PAPI units and prove they still give the intended indication.
Take the next step with Precision AGL Academy.
Learn AGL Step by Step
Build your knowledge through structured technical learning covering:
Move from understanding how AGL works to developing the practical knowledge needed to analyse, operate and maintain complete AGL systems.
Follow the Academy
Important Notice, Disclaimer & Limitation of Liability
Please read before relying on any content on this page or on any record downloaded from it.
- Purpose and intended users. This page is an independent educational and informational reference for competent, suitably trained and authorised airfield ground lighting (AGL) and aerodrome personnel. It is not professional engineering, legal or regulatory advice, and using it creates no professional or client relationship. Learning-path and Academy references describe stages of learning only and are not a certification, accreditation or authorisation.
- Safety. Work on series circuits, constant current regulators (CCRs), isolating transformers, cables, connectors and lighting fixtures can be fatal. Always apply the aerodrome’s permit-to-work and isolation/LOTO procedures, prove dead, use correctly rated instruments and PPE, and follow the manufacturer’s instructions. Nothing here authorises work on live equipment or the bypassing of any safety device or interlock.
- Aviation operations. Nothing on this page authorises return to service, the installation, adjustment or modification of any aviation visual aid, a change of operating category (CAT I/II/III), a NOTAM or any aerodrome certification decision. These rest with the aerodrome operator, the air navigation service provider and the competent regulator.
- Standards, regulations and named sources. References to ICAO, FAA, EASA, DGCA, AAI, IEC, IEEE, ISO, BIS and other standards, regulations, AIP material and published sources, and statements about what a named airport, State or authority has published, are summaries for orientation as understood when the page was prepared. They are not official text, may be incomplete, amended or superseded, and have not been independently verified. Consult the current official editions, including the current AIP, AIRAC cycle and NOTAMs. Following this content does not guarantee compliance with any law, standard or approval, and official documents, regulations, the approved design and manufacturers’ instructions always prevail.
- Generic content; no warranty. Values, tolerances, intervals, sequences, examples, checklists and pass/fail or “compliant” wording are generic and indicative; real installations differ in design, make, model, age and condition. Content is provided “as is” and “as available”, without warranty of any kind, express or implied, including accuracy, completeness, currency or fitness for a particular purpose.
- Diagrams. Diagrams on this page are original schematic illustrations, are not to scale and are not engineering drawings. Do not use them to work on, connect, set or modify equipment.
- Limitation of liability. To the fullest extent permitted by applicable law, Precision AGL Academy and its owners, authors and contributors are not liable for any injury, death, loss, damage, downtime, penalty or regulatory consequence, whether direct or indirect, arising from the use of, reliance on, or inability to use this content. Nothing in this notice excludes or limits any liability that cannot lawfully be excluded or limited.
- Independence and trademarks. This is an independent publication, not affiliated with or endorsed by ICAO, the FAA, EASA, the IEC, DGCA, AAI, any airport, regulator, employer or equipment manufacturer. Third-party names, standard designations and trademarks (including Megger®) belong to their respective owners and are used for identification only.
- Copyright and use. © 2026 Precision AGL Academy. All rights reserved. The text, structure, diagrams and compilation of this page are protected by copyright. You may read it and use it for personal reference and internal training, keeping this notice attached; any other copying, republication, scraping or commercial use needs our prior written permission. Credited third-party photographs remain subject to their owners’ terms.
- Corrections. Content is reviewed periodically, with no obligation to update. To report an error, safety concern, copyright, privacy or other rights issue, write to precisionaglsolutions@gmail.com.