PAPI Alignment, Testing & Maintenance Field Guide
How to install, level, align, aim, test and maintain a PAPI: site survey, foundation, unit spacing, aiming angles, electrical installation, photometry, maintenance checklists, records and troubleshooting. A PAPI that is electrically operational can still be aimed wrongly, and one that passed commissioning can later move.
What a Complete PAPI Installation Involves
A Precision Approach Path Indicator (PAPI) gives pilots visual information about their position relative to the desired approach path. Correct performance depends on far more than installing and switching on the units: the system must be sited, positioned, levelled, aligned, aimed, connected, tested and maintained.
Site Survey, Siting and Threshold Crossing Height
Site survey should establish
- Threshold, runway centreline and edge
- Longitudinal and transverse runway profile
- Proposed PAPI location, ground and PAPI elevation
- Aircraft group, TCH and desired approach angle
- Obstacles, ILS or other approach guidance
- Utilities, cable routes, drainage, maintenance access
Siting and TCH
Location comes from an approved siting calculation, never a convenient distance from the threshold. PAPI location, PAPI elevation, threshold elevation, approach angle and aircraft characteristics together give the approved geometry, and the intended TCH must suit the aircraft and runway operation.
Do not use one universal distance. Aircraft, runway geometry, ILS relationships and obstacles change the answer.
Foundation, Elevation and Drainage
Stable reference
The foundation provides stable support, accurate position, mechanical rigidity, protection against settlement and maintenance access. Build it from controlled survey information, not estimates from ground conditions.
Elevation matters
Beam geometry depends on physical unit elevation. A wrong foundation elevation gives incorrect TCH, effective approach angle and beam geometry, plus excess adjustment at the mounts.
Drainage
Standing water causes deterioration, corrosion, cable and connector problems, movement and electrical faults. Provide drainage suited to local conditions.
Unit Position, Spacing and Runway Offset
A standard four-unit PAPI is four units in a horizontal row, installed to the approved layout. Check sequence, spacing, runway offset, elevation, azimuth, orientation, mounting stability, adjustment access and frangibility where applicable, then survey the final positions.
| Parameter | FAA AC 150/5340-30J value (as cited) | Note |
|---|---|---|
| Unit spacing (L-880/L-881) | 20 to 30 ft (6 to 9 m) centre-to-centre | Variation limited to ±1 ft (0.3 m) |
| Inboard unit from runway edge | At least 50 ft (15 m) | 30 ft (10 m) allowed for small non-jet general-aviation runways |
| Horizontal aiming | Outward, parallel to runway centreline within ±0.5° | Check with survey or alignment equipment |
| Beam-centre elevation | All unit beam centres within ±1 in (25 mm) of a horizontal plane | Common reference plane |
Record actual spacing
Installation and frangibility
Where frangibility requirements apply, install the approved breakaway arrangement. Do not introduce unapproved rigid supports, excessive structure, improper foundations or non-approved mounting modifications. Consider the unit, mounting arrangement and foundation together; the applicable airport design standard controls the final arrangement.
Levelling, Mounting and Horizontal Alignment
Levelling
The aim is not for each unit to "look level" but to establish the required physical relationship between all four optical systems. Check unit elevation and tilt, housing level, mounting-leg adjustment, foundation settlement and unit-to-unit difference.
Mounting legs
FAA AC 150/5345-28H requires at least three adjustable mounting legs for levelling, unless an alternative shows equivalent rigidity and levelling capability. Take the final measurement after the hardware is secured.
Position the unit
Place it per the approved layout.
Establish the reference plane
Common plane for all four units.
Adjust mounting legs
Bring the unit level.
Verify housing stability
Check for movement under hand load.
Lock the adjustment
Tighten all hardware.
Recheck after tightening
Tightening can shift the setting.
PAPI Aiming Angles and Verification
The units are not all aimed at the same angle. Their different angles create the sequence 4 White, 3 White + 1 Red, 2 White + 2 Red, 1 White + 3 Red and 4 Red as the aircraft position changes.
| Unit | Standard FAA L-880 offset | HG4 aircraft with ILS (FAA) |
|---|---|---|
| Nearest runway | 30′ above glide path | 35′ above |
| Next unit | 10′ above | 15′ above |
| Next unit | 10′ below | 15′ below |
| Farthest unit | 30′ below | 35′ below |
Establish approved glide path
Establish physical reference plane
Set unit elevation
Set horizontal alignment
Set individual vertical angle
Lock adjustment
Verify
Precision inclinometer, optical or survey equipment, manufacturer-approved aiming device or calibrated digital angle equipment, as the required accuracy demands.
Record actual values, not just "PAPI aligned"
| Unit | Design angle | Actual angle | Difference | Status |
|---|---|---|---|---|
| Unit 1 | Approved value | Measured | Calculated | Pass/Fail |
| Unit 2 | Approved value | Measured | Calculated | Pass/Fail |
| Unit 3 | Approved value | Measured | Calculated | Pass/Fail |
| Unit 4 | Approved value | Measured | Calculated | Pass/Fail |
PAPI Aiming Angle Calculator
Choose a glide path and configuration to see the four unit angles. Add these as the "design angle" column in your aiming record.
Electrical Installation, Power Supply and Wiring
Installation
Follow the approved electrical design and equipment documents. Equipment can include AC supply, power control unit, CCR/series circuit, isolation transformer, secondary connections, local and remote control, monitoring and photocell brightness control.
Before energizing
Verify supply voltage, frequency, circuit identification, protective devices, grounding, cable insulation, connections, transformer rating and control wiring. Never assume a replacement unit can be connected to an existing circuit without checking its electrical characteristics.
Wiring inspection
Inspect supply cable, primary and secondary connections, control and monitoring wiring, grounding, cable glands and connectors. Look for loose terminals, damaged insulation, moisture, corrosion, mechanical strain and incorrect connections.
Functional Test and Light Indication Test
Power and operation
All units receive the required supply and operate correctly.
Colour and brightness
Red/white indication appears correctly and the intended intensity setting operates.
Controls, monitoring, interlocks
Local and remote control, alarms, status signals and any runway-lighting interlock work as designed.
| Aircraft position | Expected indication |
|---|---|
| Too high | 4 White |
| Slightly high | 3 White + 1 Red |
| On path | 2 White + 2 Red |
| Slightly low | 1 White + 3 Red |
| Too low | 4 Red |
Photometric Testing and Equipment Calibration
PAPI photometry is broader than brightness. Depending on the requirement it verifies intensity, beam pattern, colour/chromaticity, red/white transition, horizontal and vertical distribution and full-intensity performance. FAA AC 150/5345-28H contains the photometric and chromaticity requirements and requires calibrated test equipment.
Equipment
PAPI photometer, calibrated photometric sensor, angular measurement system, alignment equipment, data acquisition, and colour/chromaticity equipment where required. Check identification, certificate, validity, condition, power, settings and reference configuration before testing, and retain a traceable certificate with the records.
Test conditions
The procedure may specify full intensity, measurement distance and angles, beam position, horizontal coverage setting, vertical reference, temperature and test sequence. Keep conditions controlled.
Colour and red/white transition
A unit can look bright yet have a colour problem from filter degradation, optical damage, LED performance, contamination, incorrect optical assembly or temperature. A poor transition gives an unclear indication, inconsistent unit-to-unit behaviour and a wrong perceived approach position, so test the whole optical system, not just centre-beam intensity.
Alignment vs Photometry, Visual Inspection vs Measurement
Alignment asks
Is the unit physically aimed in the correct direction?
Photometry asks
Does the optical system produce the required light output, colour and beam pattern?
A unit can have correct aiming with poor photometry, good photometry with wrong aiming, or both faults at once. Commissioning verifies both.
| Check | Visual inspection | Measurement |
|---|---|---|
| Unit on/off | Yes | Yes |
| Physical damage | Yes | — |
| Contamination | Yes | Yes |
| Level | Limited | Yes |
| Aiming angle | Limited | Yes |
| Intensity | Subjective | Yes |
| Beam pattern | Limited | Yes |
| Colour | Visual indication | Yes |
| Documentation | Limited | Yes |
PAPI Maintenance: Daily, Monthly and Optical, Mechanical, Electrical Checks
Base maintenance on the airport programme, manufacturer instructions, authority requirements, operational experience, environment and defect history. FAA AC 150/5340-26C gives PAPI preventive-maintenance procedures.
Daily (FAA 150/5340-26C)
Confirm the PAPI is operating with equal brightness, and keep spare lamps and, where applicable, spare bypass fuses. For LED systems follow the manufacturer procedure rather than incandescent practice. Also check for visible damage, obstruction, contamination and signs of misalignment.
Monthly (FAA 150/5340-26C)
Control operation, photocell brightness control, runway-lighting interlock, radio and remote control, damage from mowing or snowplows, cleaning lamps and filters, mechanical parts, burned wires or connections, and cracked insulators, lamps or filters. Adapt to the installed equipment.
Optical
Clean lens, filter, protective window, housing, heater, ventilation; check moisture. Dirt, dust, insects, salt and oil reduce output, and the equipment standard addresses rain, snow, dew, frost and ice on lens surfaces.
Mechanical
Housing, mounting legs, fasteners, foundation, level, alignment, frangible parts, corrosion. Ask: has anything changed the geometry since commissioning?
Electrical
Supply voltage, connections, fuses and protection, cable, transformer, control circuit, remote control, monitoring, grounding. Look for burned terminals, water ingress and heating.
When to Re-Check Alignment and Photometry
| Event | What to do |
|---|---|
| Light-unit replacement | Check position, level, azimuth, vertical aiming, electrical connection, function and photometry where required, returning the unit to the approved geometric and optical condition |
| Physical impact (vehicles, mowers, snowplows, wind, ground movement) | Do not rely on visual inspection: check foundation, mounting, level, azimuth, aiming, optics, electrics and photometry |
| Optical change (lamp, LED module, filter, lens, beam adjustment) | Photometric verification, because optical performance may have changed |
| Heavy weather, flooding | Check position, housing, foundation, level, aiming, connections, water ingress and optics; consider alignment and photometry before relying on it operationally |
| Nearby civil works (resurfacing, shoulders, drainage, trenching, grading) | Reassess elevation, relative runway elevation, drainage, line of sight and stability, even if the units were never touched |
| Repeated aiming drift | Find the physical cause (loose hardware, foundation movement, deformation, impact, vibration, wrong method) instead of re-adjusting |
Coastal
Corrosion, salt contamination, connectors.
Snow and ice
Snowplow damage, ice, lens heating, exposed foundations.
Dusty
Lens contamination, filter condition, cleanliness.
High rainfall
Drainage, water ingress, connector sealing.
Common PAPI Faults and Troubleshooting Sequence
Click a fault to see where to investigate first.
One unit dark
Possible Cause
- Failed lamp or led
- Fuse or protection
- Power supply
- Connection
- Control fault
- Internal electronics
One unit dimmer
Possible Cause
- Light-source degradation
- Optical contamination
- Power problem
- Internal optical fault
- Ccr or current problem
Wrong red/white indication
Possible Cause
- Incorrect aiming
- Wrong unit sequence
- Optical problem
- Incorrect elevation
- Physical movement
All units misaligned
Possible Cause
- Foundation movement
- Incorrect survey reference
- Runway geometry change
- Common installation error
One unit misaligned
Possible Cause
- Mechanical impact
- Loose mounting
- Damaged adjustment mechanism
- Incorrect maintenance
Repeated aiming drift
Possible Cause
- Foundation
- Loose hardware
- Deformation
- Impact
- Vibration
- Wrong adjustment method
Remote control failure
Possible Cause
- Control wiring
- Interface
- Control equipment
Post-maintenance failure
Possible Cause
- Installation
- Aiming
- Electrical connection
- Photometry
All four units wrong
Check common conditions first: observation position, siting geometry, reference measurement, common supply or control setting, overall PAPI elevation and location. Do not adjust four units individually before eliminating a common-system problem.
One unit wrong or unequal brightness
Check that unit's supply, connector, light source, optics, mounting, level and aiming angle. For brightness, split electrical, optical, mechanical and photometric causes, and follow visual observation with measurement.
Troubleshooting quick reference
| Problem | First areas to check |
|---|---|
| One unit dark | Power, fuse, light source, connector |
| One unit dim | Supply, optics, light source, photometry |
| Wrong colour | Optics, filter, light source |
| Wrong red/white transition | Optical system, aiming, photometry |
| All units wrong | Siting, reference, common supply, geometry |
| One unit misaligned | Mounting, level, aiming mechanism |
| Repeated aiming drift | Foundation, hardware, mechanical stability |
| Remote control failure | Control wiring, interface, control equipment |
| Unequal brightness | Electrical supply, light source, optics |
| Post-maintenance failure | Installation, aiming, electrical connection, photometry |
Installation and Maintenance Workflows
Routine maintenance combined with objective performance verification.
Installation: 15 steps
Maintenance cycle
Commissioning, Maintenance and Fault Records
Commissioning report
Project (airport, runway, threshold, designation, date); equipment (manufacturer, model, serial, type); survey (coordinates, elevation, spacing, runway offset, foundation elevation); alignment (horizontal, levels, aiming angles); electrical; functional tests; photometry (equipment, calibration, geometry, intensity, colour, beam pattern, pass/fail); supporting records.
Maintenance report
Date, runway, unit, work performed, fault, parts replaced, alignment adjustment, electrical test, photometry, retest, technician, equipment used, remarks.
Fault register example
| Date | Unit | Fault | Cause | Action | Retest |
|---|---|---|---|---|---|
| Date | Unit 1 | Low intensity | Optical contamination | Cleaned | Pass |
| Date | Unit 3 | Misalignment | Mechanical movement | Realigned | Pass |
| Date | Unit 2 | No output | Electrical fault | Repaired | Pass |
Alignment history: keep previous angle, current angle, adjustment date, technician, reason, test equipment and calibration status per unit. A pattern of small corrections can reveal foundation or mechanical movement.
Installation and Maintenance Quality Checklists
Click an item to mark it complete.
Installation Acceptance
Alignment
Maintenance Quality
Photometry
Common PAPI Installation Mistakes
Using a generic PAPI distance
Distance comes from the approved siting process.
Aiming all four units at one angle
The units need different aiming angles.
Ignoring unit elevation
Small elevation differences change PAPI geometry.
Checking only visual appearance
Visual inspection cannot replace measurement.
Forgetting foundation movement
An unstable foundation causes recurring aiming problems.
Ignoring horizontal alignment
Correct vertical aiming is not enough if azimuth is wrong.
No photometric retest
Aiming alone does not prove optical compliance.
No records
Without baseline aiming and photometry records, maintenance gets much harder.
Standards, References and Numerical Values
Design and Installation Details for Airport Visual Aids: PAPI siting, spacing, alignment, mounting.
PAPI Systems equipment standard: light units, photometric characteristics.
Maintenance of Airport Visual Aid Facilities, including PAPI preventive maintenance.
International SARPs for aerodrome design and operation, including visual aids.
Quick Knowledge Check
Q1. Why can a PAPI that passed commissioning later give wrong guidance?
Q2. After replacing a PAPI light unit, what should be re-checked?
Q3. What should come first when all four units give a wrong indication?
Q4. Why is photometry needed in addition to an alignment check?
Q5. What should be recorded for each unit at commissioning?
Explore More AGL Engineering
PAPI Engineering Guide
Working principle, geometry and siting.
Open PAPI Engineering Guide →PAPI Installation & Aiming Handbook
Installation procedure, aiming and the indication simulator.
Open PAPI Installation & Aiming Handbook →PAPI/APAPI Maintenance
Preventive and corrective maintenance.
Open PAPI/APAPI Maintenance →PAPI Testing & Commissioning
Test and commission PAPI and APAPI.
Open PAPI Testing & Commissioning →AGL Isolating Transformers
Transformer ratings, construction and testing.
Open AGL Isolating Transformers →ICAO Annex 14 AGL Requirements
SARPs covering visual aids.
Open ICAO Annex 14 AGL Requirements →Airport Isolating Transformer
AGL transformers, 6.6 A and 20 A systems, ratings and testing.
Open Airport Isolating Transformer →Airport Junction Box & Transformer Housing
Light bases, transformer housings, cable entry and drainage.
Open Airport Junction Box & Transformer Housing →CCR Working Principle
How the CCR supplies and controls the AGL series circuit.
Open CCR Working Principle →Install Accurately. Maintain Systematically.
From survey to aiming records, photometry and maintenance history, explore the complete AGL knowledge base for PAPI engineering.
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 Installation and Maintenance FAQ
Q1How is a PAPI installed?
Site survey, approved siting, foundation construction, unit positioning, spacing, levelling, horizontal alignment, vertical aiming, electrical connection, functional testing and photometric verification.
Q2How do you align PAPI lights?
Establish the runway reference, position the units, level them, set azimuth and adjust each unit to its approved vertical aiming angle.
Q3Are all PAPI units aimed at the same angle?
No. Each unit is aimed at a different elevation relative to the selected approach path.
Q4What are the standard PAPI aiming angles?
They depend on the selected approach path and the applicable design standard. FAA guidance gives four-unit offsets of 30 minutes above, 10 above, 10 below and 30 below the pre-selected glide path for the standard installation.
Q5What is the spacing between PAPI units?
For FAA L-880/L-881 installations the cited spacing is 20 to 30 ft (6 to 9 m) centre-to-centre with a maximum variation of ±1 ft (0.3 m). Other authorities may specify different values.
Q6How far should PAPI be from the runway?
It depends on the approved siting calculation, runway geometry, TCH, aircraft characteristics and applicable standards. FAA guidance gives specific lateral and longitudinal siting requirements.
Q7What is PAPI elevation?
The physical elevation of the light units relative to the runway and the approved reference geometry.
Q8Why is PAPI levelling important?
The vertical orientation of each unit determines its effective beam angle and therefore the guidance given to pilots.
Q9What is PAPI photometry?
Measurement of optical performance, including applicable intensity, beam distribution and colour characteristics.
Q10When should PAPI photometry be performed?
During commissioning and whenever the inspection, maintenance, authority or project procedure requires it, especially after optical adjustment, unit replacement or major maintenance.
Q11What equipment is used for PAPI photometry?
A calibrated PAPI photometer, angular measurement equipment and, where required, colour or chromaticity measurement equipment.
Q12Does PAPI maintenance require alignment checks?
Per the approved maintenance programme and whenever there is evidence of movement, impact, civil work or an abnormal visual indication.
Q13What should be checked daily on PAPI?
FAA maintenance guidance includes confirming the PAPI is operating with equal brightness. For LED systems follow the airport and manufacturer procedure.
Q14What should be checked during periodic PAPI maintenance?
Controls, optical cleanliness, mechanical condition, wiring, connectors, mounting, alignment and applicable photometric performance.
Q15What causes PAPI misalignment?
Mechanical impact, loose mounting, foundation movement, incorrect installation, maintenance error or nearby civil works.
Q16What causes unequal PAPI brightness?
Light-source degradation, optical contamination, electrical faults, incorrect intensity control or internal equipment problems.
Q17Can PAPI be visually inspected without photometry?
Visual inspection is useful for routine maintenance but does not replace objective photometric verification where it is required.
Q18What should be done after replacing a PAPI light unit?
Verify position, level, horizontal alignment, vertical aiming, electrical connection and the required operational and photometric performance.
Engineering & Expert Notes
Start With the Survey Data
SitingVerify distances and elevations against the design before touching the units.
Record Angles Before and After Adjustment
AlignmentKeeping both sets of readings shows what changed and why.
Check Levelling and Mounting Stability
MountingLoose fixings or settling foundations can shift the beam over time.
Verify Colour Transition Sharpness
PhotometryThe change between red and white should be crisp, which is easy to miss without measurement.
Re-Check After Any Disturbance
MaintenanceImpacts, mowing damage and civil work may move units even if nothing looks wrong.
Update the Maintenance Record
RecordsLog checks and adjustments to maintain a history of the installation.
Understand AGL. Build Your Engineering Skills.
You now understand how PAPI provides visual approach slope guidance — from unit arrangement and angular settings to siting, commissioning and troubleshooting.
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.
- Calculators and interactive tools. Sliders, calculators, selectors and animated demonstrations on this page are teaching illustrations. Their starting values are examples, they give approximate results from simplified models, and they must not be used to design, size, set, test, accept or operate real equipment, or to support a financial or procurement decision. Values you enter are processed in your browser and are not sent to us.
- 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.