PAPI ENGINEERING GUIDE
PAPI is one of the most critical visual approach aids at an airport. A properly engineered PAPI system depends not only on the light units themselves, but on siting, elevation, approach geometry, optical aiming, foundation stability, electrical supply, control, environmental conditions, photometric performance and commissioning.
What Is PAPI?
Precision Approach Path Indicator (PAPI) is a visual approach slope guidance system consisting of a horizontal array of light units positioned beside the runway. The units produce combinations of white and red light depending on the aircraft's vertical position relative to the designed approach path.
PAPI is therefore both a lighting system and an approach guidance system.
Engineering Objective of PAPI
The engineering objective is not simply to install four light boxes. It is to establish a reliable visual reference.
Correct location + correct elevation + correct aiming + correct optical performance + reliable power + stable structure.
A PAPI installation can be electrically functional and still provide incorrect approach guidance if its geometry or optical alignment is incorrect.
PAPI System at a Glance
| Element | Engineering Function |
|---|---|
| PAPI light units | Produce the visual guidance signal |
| Optical system | Generates defined white/red transition |
| Mounting structure | Maintains unit position and alignment |
| Foundation | Provides stable mounting reference |
| Power supply | Provides electrical energy |
| Power & Control Unit | Controls system operation where applicable |
| CCR | Provides constant-current supply for applicable installations |
| Isolation transformers | Electrical isolation in series-circuit configurations |
| Connectors | Field electrical connection |
| Control system | Enables operational control |
| Monitoring | Detects equipment/system conditions |
| Cabling | Transfers power and control |
| Frangible mounting | Reduces hazard from aircraft impact |
| Photometry equipment | Verifies optical performance |
The Complete PAPI Engineering Chain
A problem at any stage can affect the final visual guidance provided to the pilot.
Runway Requirement
Approach Geometry
PAPI Location
PAPI Elevation
Light-Unit Arrangement
Foundation and Mounting
Optical Aiming
Power and Control
Electrical Installation
Commissioning
Photometric Verification
Operational Monitoring
Maintenance
PAPI Siting & Location Engineering
PAPI siting is one of the most important engineering activities. PAPI location should never be selected simply because there is convenient space beside the runway.
Location Must Be Established Against
Location Engineering Should Establish
Runway Reference
Threshold Reference
Runway Centreline
PAPI Lateral / Longitudinal Position
PAPI Elevation & Approach Slope
Required Aiming Angles, Clearance & Foundation Elevation
Location is fixed relative to runway centreline, threshold and the intended approach path — not by convenient space beside the runway
Light Unit Arrangement & Visual Indications
A conventional PAPI installation consists of multiple light units arranged in a horizontal line. The physical arrangement is part of the optical system.
| Pilot Sees | General Indication |
|---|---|
| More White | Aircraft above desired approach path |
| Balanced Transition | Aircraft near desired approach path |
| More Red | Aircraft below desired approach path |
On the desired approach path, the pilot sees an even split — 2 red, 2 white — across the four units
PAPI Working Principle, Light Colours & Pilot Indications
The PAPI working principle is simple: each light unit projects a beam that is white in its upper part and red in its lower part, and the units are aimed at slightly different elevation angles. The pilot's height relative to the approach slope therefore decides how many units look red and how many look white.
| Pilot position | PAPI 4 unit system | PAPI 2 unit system (APAPI) |
|---|---|---|
| Above slope | 4 white | 2 white |
| Slightly above slope | 3 white, 1 red | — |
| On slope | 2 white, 2 red | 1 white, 1 red |
| Slightly below slope | 1 white, 3 red | — |
| Below slope | 4 red | 2 red |
How Does PAPI Work? Why Does PAPI Show Red and White?
Inside each unit a lamp or LED, a lens and a red filter split the beam vertically. The PAPI red signal means the aircraft is below that unit's aiming angle; the PAPI white signal means it is above. With the units aimed in small angular steps, the PAPI pilot indication changes step by step as the aircraft climbs or descends through the approach slope. The PAPI red white lights use only two PAPI light colors, red and white, so the signal reads at a glance.
PAPI 4-unit vs APAPI 2-unit PAPI optical principlePAPI Optical System, Transition Sector, Sensitivity & Safe Zone
The PAPI optical system (source, lens, filter and, depending on the design, reflector or PAPI prism elements) must give a sharp PAPI transition sector — the narrow band where a unit changes from white to red. PAPI sensitivity, meaning how quickly the PAPI signal changes for a small height error, follows from the angular spacing of the unit aiming angles. A pilot who keeps an on-slope or above-slope indication stays inside the obstacle-protected PAPI safe zone, within the coverage defined by the applicable standard.
PAPI Light Colors
PAPI light colors are red and white only. Each unit's beam is white in its upper part and red in its lower part, so the colour a pilot sees from a unit depends on whether the pilot is above or below that unit's aiming angle.
PAPI Red Signal & PAPI White Signal
A PAPI red signal from a unit means the aircraft is below that unit's aiming angle. A PAPI white signal means it is above. Read the red and white units together to judge the position on the slope.
PAPI Pilot Indication
The PAPI pilot indication runs from four white lights (well above the slope), through two white and two red (on slope), to four red (well below). An APAPI shows one white and one red on slope.
PAPI Prism
Some PAPI designs use a prism or reflector element in the optical system, alongside the lens and red filter, to shape the beam and give a sharp white-to-red transition. The design differs by manufacturer, so follow the datasheet and keep the optical elements clean and undamaged.
PAPI Sensitivity
PAPI sensitivity is how quickly the indication changes for a small height error. It follows from the angular spacing of the unit aiming angles and the sharpness of each transition, which is why aiming has to be accurate.
PAPI Safe Zone
The PAPI safe zone is the part of the approach where a pilot seeing an on-slope or above-slope indication is protected from obstacles, within the coverage defined by the applicable standard. A below-slope indication means the aircraft is leaving it.
Approach Geometry & Glide Path
PAPI engineering is fundamentally a geometry problem. This is why a small change in elevation, foundation settlement or optical aiming can affect the perceived approach guidance.
The Glide Path Engineering Process Considers
The final values must be established using the applicable design standard and project requirements.
PAPI elevation, aiming angle and threshold crossing height are all defined relative to the same approach geometry
PAPI Angles, Distance from Runway & Threshold Crossing Height
Every PAPI angle and distance comes from one approach geometry. Take values from the approved design and the applicable standard, never from memory.
| Parameter | What it means | How it is checked |
|---|---|---|
| PAPI angle / PAPI aiming angle | The vertical angle each unit's beam is set to; the units differ by small fixed steps around the approach slope. | Calibrated inclinometer at the unit, then photometric check and flight inspection where required. |
| PAPI elevation angle | The elevation of each beam above horizontal, together with the installed height of the unit relative to the threshold. | Foundation level survey plus inclinometer reading. |
| PAPI distance from runway | Lateral offset of the nearest unit from the runway edge, kept clear of the runway strip and mounted frangibly. | Survey against runway edge and centreline. |
| PAPI threshold crossing height | Height of the aircraft's wheel path above the threshold when flying the PAPI slope; it depends on the unit's longitudinal position, slope and elevation. | Geometry calculation against the design, then survey. |
| PAPI tolerance & accuracy | Aiming angles are held to tight angular tolerances, so PAPI accuracy depends on rigid foundations and a repeatable procedure. | Measured angles compared with design values. |
PAPI Site Survey & PAPI Layout
A PAPI site survey records runway centreline, threshold, ground profile, obstacles and cable routes. The PAPI layout drawing then fixes unit positions, spacing, offset, elevations, transformer locations and the cable route before installation begins.
PAPI glide path PAPI alignment PAPI geometry calculatorPAPI Threshold Crossing Height
PAPI threshold crossing height is the height of the aircraft's wheel path over the threshold when the pilot flies the on-slope indication. It depends on the units' distance from the threshold, the slope angle and the unit elevation, so re-confirm it by calculation and survey after any change. Use the PAPI geometry calculator for the geometry.
PAPI Accuracy & PAPI Tolerance
PAPI accuracy depends on holding each unit's aiming angle within the angular tolerance in the applicable standard. PAPI tolerance is tight, and it is met only with rigid foundations, a calibrated inclinometer and a repeatable procedure. Record measured angles against design values.
Optical Aiming & Alignment
Optical aiming is one of the most critical PAPI activities. Alignment should be treated as an engineering measurement rather than a visual guess. A PAPI can appear visually normal while being incorrectly aimed.
Important Aiming Controls
Alignment Process
Foundation Engineering & Frangible Mounting
Foundation Engineering
- Structural stability
- Correct elevation
- Resistance to settlement
- Accurate mounting
- Drainage
- Environmental durability
Foundation Settlement
Even when light units remain operational, foundation movement can change unit elevation, beam angle, relative unit geometry and visual transition.
Mechanical stability = optical stability.
Frangible Mounting
The mounting arrangement should satisfy the applicable frangibility requirements, reducing the hazard created by a fixed installation if an aircraft impacts the equipment.
PAPI Electrical System
PAPI electrical engineering depends on the selected system configuration — a direct/utility-type arrangement, or a constant-current arrangement supplied through a CCR.
Power Supply Should Be Evaluated For
Cable Engineering Should Consider
Isolation Transformers, Connectors, Earthing & Surge Protection
Isolation Transformers
- Transformer rating
- Primary current
- Secondary voltage
- Insulation
- Water ingress
Connectors
- Correct connector type
- Correct mating
- Contact condition
- Water ingress
- Strain relief
Earthing & Bonding
- Light-unit grounding
- Equipment bonding
- Control enclosure grounding
- Ground continuity
Lightning & Surge Protection
- Power-line surge protection
- Control-line protection
- Cable routing
- Protection coordination
PAPI Electrical Supply, Isolating Transformers, Connectors & Wiring Diagram
In a conventional installation the PAPI electrical supply is a series circuit from a constant-current regulator. Each unit is fed through its own PAPI isolating transformer, and L-823 connectors join the primary and secondary sides. Some LED designs use a local driver instead, so always follow the manufacturer's PAPI wiring diagram.
PAPI Circuit Diagram & PAPI Wiring Diagram
The PAPI circuit diagram below shows conventional wiring: a CCR feeds a primary series loop and each light unit is fed through its own isolating transformer. The PAPI wiring diagram for a project must follow the manufacturer's drawing, because LED designs may differ.
PAPI circuit diagram — series-circuit wiring overview (conventional installation)
Why the PAPI Isolating Transformer Matters
The isolating transformer lets a lamp or LED failure on the secondary side happen without breaking the primary series circuit, and it must be rated for the unit load. A faulty PAPI connector is among the commonest causes of dark or flickering units.
PAPI transformer arrangement PAPI cable connectionPAPI System Equipment
Light units, frangible mounting and foundations, isolating transformers, connectors, primary and secondary cable, junction box, CCR, control and monitoring, earthing and surge protection, plus the alignment and photometry tools used for verification.
PAPI component pagePAPI Control & Intensity
Possible Control Functions
Verify
PAPI Photometry, Colour & Verification Methods
Photometry verifies the actual optical performance of the PAPI — intensity, beam characteristics, colour, optical transition, unit-to-unit consistency, alignment and performance across operating conditions. A complete acceptance process draws on several verification methods together, electrical and optical alike.
| Verification | What It Tells You |
|---|---|
| Voltage measurement | Supply condition |
| Current measurement | Electrical loading |
| Insulation test | Cable/equipment insulation condition |
| Continuity test | Electrical path |
| Earth test | Grounding condition |
| Visual inspection | Physical condition |
| Alignment measurement | Optical aiming geometry |
| Photometry | Actual optical performance |
| Functional test | System operation |
Colour Performance Should Be Considered During
PAPI Test Procedure, Photometric Testing & Reports
A PAPI test procedure follows a fixed order: visual, electrical, alignment, photometric, functional, then documentation.
| Step | PAPI test procedure | Record |
|---|---|---|
| 1. Visual & mechanical | Check optical windows, fasteners, levelling and frangible mounting. | Inspection record |
| 2. Electrical | Insulation, continuity, earth and series-circuit current checks. | Measured values |
| 3. Alignment | Verify azimuth and elevation angle of each unit (PAPI alignment procedure) with a calibrated inclinometer. | Measured vs design angles |
| 4. Photometric | PAPI intensity testing, colour and transition checks using a PAPI photometer. | PAPI photometric report |
| 5. Functional | Intensity steps, control and monitoring operation. | Pass / fail |
| 6. Acceptance | PAPI acceptance test sign-off, or PAPI commissioning test on a new system. | PAPI test report |
What Is PAPI Photometric Testing? How Is PAPI Calibrated?
PAPI photometric testing is the PAPI photometry procedure: PAPI light measurement of PAPI light intensity and colour with a calibrated PAPI photometer, checked against the PAPI photometric requirements of the applicable standard. PAPI calibration procedure: set each unit's elevation angle against the surveyed reference, lock the adjustment, verify the transition photometrically, record before and after values and confirm by flight inspection where required.
PAPI testing & commissioningPAPI Checklists
PAPI installation procedure: survey, foundation, levelling, cabling. PAPI inspection checklist: visual, mechanical, optical. PAPI commissioning checklist and PAPI testing checklist: the six steps above. PAPI maintenance checklist: cleaning, alignment re-check, connectors, intensity.
PAPI SOP PAPI inspection PAPI maintenancePAPI Light Measurement & PAPI Light Intensity
PAPI light measurement uses a calibrated PAPI photometer to measure the intensity and colour of each unit and to check the red-white transition. PAPI light intensity is checked at each brilliancy step against the photometric requirements of the applicable standard.
How Is PAPI Calibrated?
PAPI calibration sets each unit's elevation angle against the surveyed reference with a calibrated inclinometer, locks the adjustment, then verifies the transition photometrically. Record values before and after, and confirm by flight inspection where required.
PAPI Acceptance Test & PAPI Commissioning Test
A PAPI acceptance test is the formal sign-off that the installed system meets the design and the standard. On a new system it forms part of the PAPI commissioning test. Both follow the six steps above and end in a signed report.
PAPI Test Report
A PAPI test report records the measured electrical, alignment and photometric results against design values, with the instruments and their calibration, date, location and a pass or fail result for each unit.
PAPI Commissioning Checklist
The PAPI commissioning checklist follows the six test steps above and adds survey, foundation, cabling and documentation checks. The full design review, installation and commissioning checklists appear later on this page.
PAPI Commissioning — A Structured Process
Stage 1 · Documentation
- Approved drawings
- Manufacturer documentation
- Equipment certificates
- Installation records
- Electrical diagrams
- PAPI location data
Stage 2 · Mechanical
- Foundation
- Mounting
- Frangible fittings
- Unit spacing
- Level
- Structural condition
Stage 3 · Electrical
- Cable continuity
- Insulation resistance
- Earthing
- Supply voltage
- Current
- Transformer & connector condition
Stage 4 · Optical
- Aiming
- Alignment
- Colour
- Beam performance
- Unit consistency
Stage 5 · Operational
- ON/OFF
- Brightness
- Control
- Monitoring
- Fault indication
Stage 6 · Final Verification
- Photometry
- Engineering measurements
- Documentation
- As-built information
- Commissioning record
PAPI Installation Inspection
| Inspection Area | Check |
|---|---|
| Location | Correct PAPI position |
| Foundation | Stable and correctly constructed |
| Mounting | Secure and correctly aligned |
| Frangibility | Correct arrangement |
| Unit spacing | As approved |
| Elevation | Correct reference |
| Cabling | Correct routing |
| Connectors | Correct and secure |
| Earthing | Continuous and secure |
| Power | Correct supply |
| Optics | Clean and undamaged |
| Control | Functional |
| Environment | No interfering obstruction/light |
PAPI Alignment Verification
Establish Survey Reference
Verify PAPI Elevation
Verify Unit Positions
Verify Mechanical Level
Check Individual Aiming
Adjust If Required
Lock Adjustment
Re-measure
Perform Optical Verification
Record Final Values
PAPI Troubleshooting
| Symptom | Possible Cause | First Investigation |
|---|---|---|
| One unit completely dark | Power, connector, fuse, lamp/LED or internal fault | Check supply and connection |
| Multiple units dark | CCR, feeder, control or common supply problem | Check circuit supply |
| Incorrect colour indication | Optical/lamp/LED problem | Inspect optical output |
| All units appear misaligned | Foundation/reference movement or incorrect adjustment | Survey installation |
| One unit differs from others | Individual optical/mechanical fault | Compare unit alignment |
| Flickering | Loose connection, power instability or internal fault | Check supply and connectors |
| Low brightness | Supply, optical contamination or source degradation | Check electrical and optical output |
| PAPI indication inconsistent | Alignment/geometry problem | Verify unit aiming |
| Repeated unit failure | Environmental, electrical or mechanical issue | Analyse failure history |
| Photometry fails | Optical alignment, source, contamination or equipment fault | Compare measured data with requirements |
Troubleshooting Philosophy
Observe → Measure → Compare → Isolate → Correct → Re-test. Do not immediately replace components — first determine whether the problem is electrical, mechanical, optical, geometrical, environmental, or control-related.
Open the AGL Troubleshooting EnginePAPI Not Working? Common Faults, Causes of Failure & Replacement
For a PAPI not working, first establish which units are affected, then work from supply to optics to alignment. This PAPI troubleshooting guide lists the PAPI common faults by group.
| Cause group | What causes PAPI failure |
|---|---|
| Electrical | Lamp or LED failure, isolating transformer fault, loose or corroded connector, CCR or series-circuit problem |
| Optical | Dirty lens or optical window, condensation or water ingress, damaged filter |
| Mechanical | Foundation settlement, ground movement, impact or loose fasteners causing misalignment |
| Environmental / control | Lightning or surge damage, vegetation or snow cover, intensity-control or monitoring faults |
PAPI Replacement
After any PAPI replacement of a unit, lamp or LED, transformer or connector, re-verify alignment and photometry before returning the system to service.
PAPI troubleshooting PAPI failure databasePAPI Troubleshooting Guide & PAPI Common Faults
Start with which units are affected. One dark unit points to its lamp or LED, transformer or connector. All units dark points to the series circuit or CCR. Wrong colours or a shifted slope point to alignment or the foundation.
What Causes PAPI Failure?
PAPI failure usually comes from electrical faults (lamp or LED, isolating transformer, connector, CCR), dirty optics or water ingress, misalignment from foundation settlement or impact, and lightning or surge damage. The cause table above groups them.
PAPI Maintenance, Heavy Weather & LED Systems
PAPI maintenance should combine visual inspection, mechanical inspection, electrical testing and optical verification.
Routine Maintenance
- Cleaning optical surfaces
- Checking light sources
- Checking connectors & cables
- Checking grounding
- Checking foundation & mounting
- Checking alignment & brightness
After Heavy Weather
- Water ingress
- Foundation movement
- Structural / lightning damage
- Loose mounting
- Optical contamination
- Alignment changes
PAPI LED Systems
- LED module condition
- Driver electronics
- Thermal management
- Colour consistency
- Manufacturer-specific requirements
PAPI, Threshold, Approach Lighting & Obstructions
PAPI & Runway Threshold
PAPI → threshold → runway elevation → approach slope → aircraft geometry. A change to the threshold can trigger reassessment.
PAPI & Approach Lighting
Works alongside approach lighting, threshold lighting, edge/centreline lighting and markings to provide complementary visual cues.
PAPI & Obstructions
Assessed against terrain, buildings, structures, vegetation, vehicles and temporary construction.
Interfering Lights
Apron, road, building, vehicle and construction lighting can interfere with the red/white presentation and should be controlled.
PAPI Lighting System: Working Principle, Angle Setting & Calibration
How a PAPI gives the pilot a glide path signal, and how it is set and checked.
PAPI lighting system
A PAPI lighting system is a row of four light units beside the runway that shows the pilot whether the aircraft is on, above or below the correct approach slope. It is sited, aimed and powered to the design for the runway it serves. See the earlier sections of this guide for siting and approach geometry.
PAPI lights working principle
Each PAPI unit projects a beam that is white above a set angle and red below it. The four units are aimed at slightly different angles, so the pilot sees a different mix of red and white lights depending on the aircraft's height on the approach. When the pilot sees an even mix, the aircraft is on the correct slope.
PAPI light angle setting
PAPI light angle setting is the vertical aiming angle of each unit, which fixes where its red-white transition falls. The angles are derived from the approach slope and the standard being followed, so none are given here. They are set with a calibrated instrument and recorded against each unit.
PAPI light calibration
PAPI light calibration checks that each unit's red-white transition is at the correct angle and that the beam is stable and level. It is done at installation, after any impact or unit change, and at intervals set by your maintenance plan. The results go into the PAPI test record. See the alignment and photometry sections of this guide.
PAPI System Installation, VASI Lighting System & LED PAPI Lights
Installing a PAPI, how it relates to the older VASI, and the move to LED.
PAPI system installation
PAPI system installation covers siting the units to the design, building the foundations, installing the units level and aligned, connecting power and control, and then aiming, calibrating and testing. Siting and alignment errors are hard to correct later, so each step should be checked before the next. See the installation checklists in this guide.
VASI lighting system
A VASI lighting system is an older visual approach slope indicator that uses bars of red and white lights to show the correct glide path. PAPI has replaced VASI at many aerodromes because it gives a clearer and more precise signal. Where VASI remains, it is maintained to the same standard of alignment and serviceability.
LED PAPI lights
LED PAPI lights use LED light sources in the PAPI units. They offer low power use and long life, but they must still meet the required photometric and colour performance and the aiming accuracy of the standard. Check compatibility with the existing circuit and CCR before replacing halogen units. See PAPI Maintenance, Heavy Weather & LED Systems above.
Design Review, Installation & Commissioning Checklists
The installation should not be considered complete simply because all four units illuminate — the final installation must satisfy the approved engineering requirements. Click an item to mark it complete.
Design Review — Geometry
Design Review — Mechanical & Electrical
Installation
Commissioning
Common PAPI Engineering Mistakes
Treating PAPI as four independent lights
PAPI is an integrated approach guidance system.
Ignoring foundation elevation
Small physical movement can affect optical geometry.
Setting alignment by visual judgement alone
Engineering references and measurement should be used.
Checking only electrical performance
A PAPI can be electrically healthy but optically incorrect.
Ignoring nearby lights
Visual interference can affect pilot interpretation.
Selecting cable without voltage-drop consideration
Electrical supply quality affects equipment performance.
Ignoring connector condition
A connector fault can produce intermittent or complete failure.
Failing to record final alignment
Without records, future maintenance becomes difficult.
Replacing a unit without rechecking geometry
Replacement or adjustment can change optical alignment.
Treating commissioning as an ON/OFF test
Commissioning must verify the complete engineering system.
PAPI Engineering Workflow
PAPI Performance Chain
Correct Location → Correct Elevation → Correct Geometry → Correct Alignment → Correct Electrical Supply → Correct Optical Output → Correct Photometric Performance → Correct Pilot Visual Indication.
If one link is wrong, the final guidance can be affected.
PAPI Standards & References
For engineering work, always verify the current applicable edition rather than relying on an old copy.
Precision Approach Path Indicator (PAPI) Systems — the current active FAA PAPI equipment specification
Design and Installation Details for Airport Visual Aids
Maintenance of Airport Visual Aid Facilities
Isolation Transformers for Airport Lighting Systems
L-823 Plug and Receptacle, Cable Connectors
Applicable ICAO requirements
Applicable national / DGCA requirements
Airport-specific technical specifications
Explore PAPI Engineering
PAPI Testing & Commissioning
From electrical testing to final operational verification.
Open handbook →PAPI Troubleshooting
Diagnosing abnormal indications, failures and alignment problems.
Diagnose faults →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
Frequently Asked Questions
Q1What is PAPI?
Precision Approach Path Indicator (PAPI) is a visual approach slope guidance system consisting of a horizontal array of light units positioned beside the runway. The units produce combinations of white and red light depending on the aircraft's vertical position relative to the designed approach path, so it is both a lighting system and an approach guidance system.
Q2What does a pilot see from a PAPI?
More white means the aircraft is above the desired approach path, more red means below, and a balanced transition means near the path. On a 4-unit PAPI the on-slope indication is an even split of 2 red and 2 white. The exact interpretation must follow the applicable aviation standard and approved procedure.
Q3Is a PAPI just four light boxes?
No. The objective is a reliable visual reference, combining correct location, elevation, aiming, optical performance, reliable power and a stable structure.
Q4Can a PAPI work electrically and still give wrong guidance?
Yes. A PAPI can be electrically functional and still provide incorrect approach guidance if its geometry or optical alignment is incorrect, and it can appear visually normal while incorrectly aimed.
Q5How should PAPI location be chosen?
Never simply because there is convenient space beside the runway. Location must be established against the runway centreline, threshold, intended approach path, threshold crossing height, approach slope, runway elevation, obstacle environment, runway safety areas, aircraft operating characteristics and applicable airport design standards, then checked against the approved design.
Q6Why is PAPI engineering fundamentally a geometry problem?
Because a small change in elevation, foundation settlement or optical aiming can affect the perceived approach guidance. PAPI location, elevation, aiming angle and threshold crossing height are all defined relative to the same approach geometry.
Q7How is PAPI alignment done?
Alignment should be treated as an engineering measurement rather than a visual guess: reference, measurement, adjustment, verification and recording. Controls include vertical aiming, horizontal alignment, unit-to-unit alignment, mechanical level, reference elevation, optical cleanliness and beam integrity.
Q8When should PAPI alignment be re-checked?
After initial installation, foundation work, mechanical adjustment, light-unit replacement and major maintenance, among other events.
Q9What components make up a PAPI system?
Light units, an optical system, mounting structure and foundation, power supply, a Power and Control Unit where applicable, and a CCR, isolation transformers and connectors in series-circuit installations. Control, monitoring, cabling, frangible mounting and photometry equipment complete the picture. The exact configuration depends on the design, manufacturer, airport and standards.
Q10What is the PAPI engineering chain?
It runs through runway requirement, approach geometry, PAPI location and elevation, light-unit arrangement, foundation and mounting, optical aiming, power and control, electrical installation, commissioning, photometric verification, operational monitoring and maintenance. A problem at any stage can affect the final guidance the pilot receives.
Q11How does PAPI work?
Each PAPI unit projects a beam that is white above and red below, and the units are aimed at slightly different elevation angles. The number of red and white units the pilot sees shows whether the aircraft is above, on or below the approach slope.
Q12Why does PAPI show red and white?
Red means the aircraft is below the unit's aiming angle and white means it is above. On a 4-unit PAPI an even 2 red, 2 white split indicates the aircraft is on the slope.
Q13What is the difference between PAPI and VASI?
VASI uses bars of light units and is largely superseded. PAPI uses a single wing bar of four units (or two in APAPI) with a sharper red-white transition, so the indication is easier to read and more precise.
Q14What is PAPI photometric testing?
It measures the intensity, colour and red-white transition of each PAPI unit with a calibrated photometer and compares the results with the applicable photometric requirements.
Q15How is PAPI calibrated?
Each unit's elevation angle is set against the surveyed reference with a calibrated inclinometer, locked, verified photometrically and recorded, with flight inspection where required.
Q16What causes PAPI failure?
Common causes are lamp or LED failure, transformer or connector faults, dirty or flooded optics, foundation movement that spoils alignment, and surge or lightning damage.
Q17What is the difference between a PAPI 4 unit system and a PAPI 2 unit system?
A PAPI 4 unit system gives five graded indications from all white to all red. A PAPI 2 unit system (APAPI) gives three: both white, one red and one white, or both red.
Q18What should a PAPI acceptance test include?
Visual, electrical, alignment, photometric and functional checks, then a PAPI test report recording measured against required values.
Q19How to set PAPI angle?
Set each PAPI unit's vertical angle from the design values for the approach slope and the standard being followed. Use a calibrated instrument to aim each unit so its red-white transition falls at its required angle, check that the housing is level, then lock the setting and record it against the unit. Re-check after any impact, unit change or pavement work, and confirm the result during the PAPI acceptance and calibration tests. The angle values come from your design and standard, so none are given here.
Engineering & Expert Notes
Siting and Angles Are Design Decisions
DesignThey derive from approach geometry and requirements. Document the basis.
Installation Tolerance Affects Indications
InstallationSmall errors in level or position change how the pilot sees the transition.
Check the Electrical Chain
ElectricalSupply, transformers and connectors affect reliability. Verify each element.
Measure, Do Not Assume
PhotometryVerify colour and intensity as part of commissioning.
Frangible Mounting Requires Care
MountingConfirm that mounting meets frangibility requirements and is maintained.
Plan Re-Verification
MaintenanceSet checks after impacts and at regular intervals.
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.
- Enquiries and services. The email and call buttons on this page open your own email or telephone application; nothing is sent to us until you send it. Descriptions of consultancy, support, documentation or other services are not an offer, quotation or tender, and contacting us or our replying creates no engagement, duty of care or contract; an engagement exists only under a written agreement. We are not a regulator or certification body, and “support” does not mean certification or approval. Please do not send confidential, classified or security-sensitive information.
- 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.