PAPI Installation & Aiming Handbook
Working principle, 4-unit and 2-unit systems, aiming angles, threshold crossing height, siting, electrical supply, photometry, testing and troubleshooting. PAPI is a geometric system: each unit is aimed at its own elevation angle, so a small error in elevation, aiming, spacing or siting changes what the pilot sees.
Why PAPI Installation and Aiming Matter
PAPI looks simple from the runway: four light boxes producing red and white indications. Engineering-wise it is a controlled combination of siting, elevation, geometry, optics, aiming, electrical supply, photometry and obstacle clearance.
A small error in unit elevation, aiming, spacing or siting can change the visual indication presented to the pilot, even when every unit is lit and the electrical system is healthy.
What Is PAPI and How Does It Work?
Precision Approach Path Indicator (PAPI) is a visual approach slope guidance system. A conventional four-unit PAPI is a horizontal row of light units beside the runway. Each unit projects a sharply divided beam: aviation white in the upper sector, aviation red in the lower sector.
Because each unit is aimed at a different elevation angle, the pilot sees a different red/white combination depending on where the aircraft is relative to the approach path. Aircraft position changes → viewing angle changes → red/white indication changes. The indication is optical geometry, not a colour change commanded from a controller.
| Aircraft position | 4-unit PAPI | 2-unit APAPI |
|---|---|---|
| Too high | 4 White | 2 White |
| Slightly high | 3 White + 1 Red | — |
| On approach path | 2 White + 2 Red | 1 White + 1 Red |
| Slightly low | 1 White + 3 Red | — |
| Too low | 4 Red | 2 Red |
PAPI Quick Reference
PAPI 4-Unit vs APAPI 2-Unit Systems
| Feature | PAPI | APAPI |
|---|---|---|
| Typical units | 4 | 2 |
| FAA equipment type | L-880 | L-881 |
| On-path indication | 2 White + 2 Red | 1 White + 1 Red |
| Graded indications | 5 | 3 |
| Main use | Full visual approach slope indication | Abbreviated installation |
| Unit aiming | Four angles | Two angles |
Style A
Voltage-powered PAPI.
Style B
Current-powered PAPI on a series lighting circuit.
Typical assembly
Light units, optics, adjustable mounts, levelling and aiming mechanism, frangible mounting, power/control and monitoring equipment.
Light Unit, Optical System and Red/White Transition
Light unit
Generates output, forms the beam, separates red and white sectors and allows accurate horizontal and vertical aiming. LED sources must meet the applicable equipment standard.
Red/white transition
A sharp, narrow transition gives the pilot a clear indication. FAA AC 150/5345-28H limits the transition band at 1,000 ft (300 m) in minutes of arc, tighter at beam centre.
Colour and beam
Aviation white and aviation red. Intensity, distribution, coverage and colour stability all matter, so a unit that merely lights up is not proof of compliance. A PAPI can look acceptable in intensity and still have an optical or chromaticity problem.
PAPI Distance from Runway, TCH and Obstacle Clearance
Distance from threshold
There is no single universal distance. PAPI location is a calculated result of the approach angle, threshold crossing height, aircraft height group, runway profile and elevations, PAPI elevation, obstacles and the obstacle clearance surface, ILS glide path, the instrument approach procedure and local regulations.
Threshold Crossing Height (TCH)
TCH is the height of the lowest on-course signal above the intersection of runway centreline and threshold. Required TCH depends on the most demanding aircraft height group using the runway. Moving the PAPI changes the approach-path geometry at the threshold: PAPI elevation, PAPI-to-threshold distance and the selected approach angle together set it, but the final calculation must also account for runway profile and obstacle clearance.
With an ILS
Do not design PAPI in isolation. Visual and electronic glide paths must be reconciled during siting and commissioning, and FAA guidance has specific aiming provisions for certain height-group-4 aircraft on ILS runways. The aim is to avoid a conflicting visual and electronic indication.
Obstacle clearance
An obstacle clearance surface starts ahead of the PAPI and extends into the approach area, related to the lowest on-course signal. Survey terrain, buildings, trees, masts, poles, relevant vehicle positions and planned development, not just the area around the units.
PAPI Angle and Aiming Angle Explained
Side view (angles exaggerated): each unit switches from red to white at its own elevation angle
Two different things are often confused. The approach path angle is the selected visual glide path, normally around 3°. The unit aiming angle is the elevation each individual unit is set to around that path. The four units are not all aimed at 3°.
| Unit (outer → inner) | Offset (FAA L-880 standard) | Example for 3° path |
|---|---|---|
| Outer unit | 30′ below path | 2°30′ |
| Second unit | 10′ below path | 2°50′ |
| Third unit | 10′ above path | 3°10′ |
| Inner unit (nearest runway) | 30′ above path | 3°30′ |
PAPI Indication Simulator
Move the slider to change the aircraft's angle above the horizontal. Each unit shows white when the aircraft is above that unit's aiming angle and red when below it. This uses the standard 3° example (2°30′, 2°50′, 3°10′, 3°30′) and is for learning only, not design.
PAPI Installation Procedure
Site survey
Runway geometry, threshold, centreline, elevations, profile, aircraft group, obstacles, ILS relationship, access.
Siting calculation
Position, TCH, approach angle, unit aiming angles, obstacle clearance, lateral location.
Foundation and mounting
Elevation, stability, drainage, frangible requirements, hardware, maintenance access.
Unit positioning
Sequence, spacing, runway-edge distance, perpendicularity, physical elevation.
Levelling
Common horizontal reference plane. Tilt error changes the effective aiming angle.
Horizontal alignment
Azimuth, parallelism, unit-to-unit consistency against a reference.
Vertical aiming
Reference plane → inclinometer/aiming device → angle setting → locking → verification.
Electrical connection
AC supply and power control unit, or CCR series circuit with isolating transformer arrangement; control and monitoring.
Functional testing
All units operate, correct red/white presentation, intensity steps, monitoring and control.
Photometric testing
Intensity, colour, beam pattern, transition, angular performance.
Flight check
Operational verification where the authority requires it. Field alignment alone does not prove acceptability.
Documentation
Coordinates, elevations, spacing, aiming angles, TCH, survey, obstacle assessment, test and photometric results, as-builts.
PAPI Electrical Supply and Series Circuit
Voltage-powered
AC supply → Power control unit → PAPI light units
Series circuit
CCR → primary cable → primary connector → isolating transformer / approved PAPI interface → secondary connection → light unit
PAPI Photometry and Why It Differs from Alignment
Alignment testing asks
Is the unit pointing at the correct angle and direction?
Photometric testing asks
Does the unit produce the required optical performance?
Photometry verifies intensity, beam pattern, colour, red/white transition, horizontal alignment and vertical aiming. A PAPI can have correct intensity with wrong aiming, correct aiming with inadequate intensity, or both faults together, so commissioning must control both.
PAPI photometry should verify, in this order
PAPI Testing and Commissioning
| Test | Purpose |
|---|---|
| Visual inspection | Physical condition |
| Dimensional check | Position and spacing |
| Level check | Common reference plane |
| Azimuth check | Horizontal alignment |
| Vertical aiming | Individual unit angles |
| Electrical test | Supply and circuit |
| Functional test | Correct indications |
| Photometric test | Optical performance |
| Obstacle review | Clearance |
| Documentation review | Traceability |
Common PAPI Problems
Click a fault to see where to investigate. Work through the checks in order (observation position, levelling, horizontal alignment, vertical aiming, optics, electrical, photometry, retest, document) so healthy equipment is not replaced when the cause is alignment or siting. Click each item to expand.
All four white
Possible Cause
- Aircraft position
- Aiming configuration
- Incorrect unit settings
- Beam geometry
All four red
Possible Cause
- Aircraft position
- Vertical aiming
- Siting geometry
- Unit elevation
- Optical obstruction
Wrong 3W + 1R
Possible Cause
- Individual aiming
- Levelling
- Unit spacing
- Optical condition
- Mechanical movement
Wrong 2W + 2R
Possible Cause
- Selected angle
- Aiming angles
- Unit sequence
- Tch geometry
- Elevation
- Runway profile
Unequal brightness
Possible Cause
- Led/lamp degradation
- Contamination
- Damaged optics
- Electrical or power imbalance
- Ageing
Poor transition
Possible Cause
- Filter problem
- Optical damage
- Contamination
- Internal adjustment
- Defect
- Measurement issue
Aiming shifts after rain or wind
Possible Cause
- Mounting
- Foundation movement
- Fasteners
- Tilt mechanism
- Housing
Related Guides
Detailed fault diagnosis for PAPI and APAPI.
PAPI/APAPI failures Troubleshooting EnginePAPI Maintenance
Routine maintenance should protect both optical and mechanical performance.
Optical
- Lens cleanliness
- Filter condition
- LED / lamp condition
- Light output
- Beam condition
Mechanical
- Foundation
- Mounting legs
- Level
- Fasteners
- Frangible components
- Housing
Alignment
- Horizontal alignment
- Vertical aiming
- Unit-to-unit relationship
Electrical
- Power supply
- Connections
- Series circuit where applicable
- Monitoring
- Protective devices
Environmental
- Water ingress
- Moisture
- Dust
- Corrosion
- Condensation
- Frost / ice
PAPI Engineering Checklist
Click an item to mark it complete.
Design
Installation
Testing
Documentation
PAPI Standards and References
Visual approach slope indicator systems including PAPI and APAPI.
Aerodrome Design Manual, Visual Aids.
Design and Installation Details for Airport Visual Aids (J edition cited).
Applicable Engineering Briefs, including EB 67 criteria for alternative light sources.
Applicable flight inspection and operational requirements for PAPI acceptance.
PAPI equipment specification (H edition cited), with Engineering Brief 67 for alternative light sources.
8 Key PAPI Engineering Concepts
PAPI is a geometric system
The visible indication is produced through controlled angular relationships.
Four units are not aimed at the same angle
Each unit contributes a different portion of the visual guidance.
PAPI location affects TCH
Moving the PAPI changes the geometry of the approach path at the threshold.
Siting affects obstacle clearance
Location and aiming must protect the applicable obstacle clearance surface.
Alignment affects operational meaning
A mechanically misaligned unit can give the pilot an incorrect visual indication.
Photometry is more than intensity
Colour, beam pattern and angular transition are also important.
Electrical faults can look like optical faults
An abnormal electrical input may produce reduced or inconsistent light output.
Commissioning should end with evidence
Final aiming, electrical, photometric and siting results should be documented.
Quick Knowledge Check
Q1. Are all four PAPI units aimed at the same 3° angle?
Q2. What is the normal on-path indication of a four-unit PAPI?
Q3. How should PAPI distance from the threshold be determined?
Q4. A PAPI has correct intensity. Can it still give wrong guidance?
Q5. Which test answers "does the unit produce the required optical performance?"
Explore More AGL Engineering
AGL Fundamentals
Build your understanding of the complete Airfield Ground Lighting system.
Open AGL Fundamentals →PAPI Engineering Guide
Siting, geometry, foundations and electrical systems of PAPI.
Open PAPI Engineering Guide →PAPI Indication Reference
Red/white indications by aircraft position.
Open PAPI Indication Reference →PAPI Testing & Commissioning
Test and commission PAPI and APAPI.
Open PAPI Testing & Commissioning →AGL Photometry
Verify the optical performance of the installed system.
Open AGL Photometry →Approach Lighting Systems
The lighting PAPI works alongside on approach.
Open Approach Lighting Systems →ICAO Annex 14 AGL Requirements
The SARPs covering visual approach slope indicators.
Open ICAO Annex 14 AGL Requirements →FAA AC 150/5340 Series Explained
The FAA visual-aid design and installation documents behind PAPI siting.
Open FAA AC 150/5340 Series Explained →Aim Precisely. Measure Objectively.
From siting calculation to aiming angle, from photometry to commissioning records, 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 Frequently Asked Questions
Q1What does PAPI stand for?
Precision Approach Path Indicator.
Q2What is the purpose of PAPI?
It gives visual approach slope guidance so the pilot can judge the aircraft position relative to the desired approach path.
Q3How many lights are in a PAPI?
A conventional PAPI has four light units (FAA L-880). The abbreviated two-unit system, APAPI, is L-881.
Q4What does 4 white lights mean on PAPI?
The aircraft is significantly above the approach path.
Q5What does 3 white and 1 red mean?
The aircraft is slightly above the approach path.
Q6What does 2 white and 2 red mean?
The normal on-path indication for a four-unit PAPI.
Q7What does 1 white and 3 red mean?
The aircraft is slightly below the approach path.
Q8What does 4 red lights mean?
The aircraft is significantly below the approach path.
Q9What angle is PAPI set at?
The visual glide path is commonly around 3°, but the selected value comes from the approved design and siting requirements.
Q10Are all PAPI units aimed at 3 degrees?
No. Units are aimed at different elevations around the selected glide path, for example 2°30′, 2°50′, 3°10′ and 3°30′ for a 3° path. Exact settings follow the applicable standard and design.
Q11How far should PAPI be from the runway threshold?
There is no universal distance. Location is determined from TCH, approach angle, aircraft requirements, runway geometry, obstacle clearance and, where applicable, ILS geometry.
Q12What is PAPI TCH?
Threshold Crossing Height: the height of the lowest on-course PAPI signal above the runway threshold centreline.
Q13What colours does PAPI use?
Aviation white (upper sector) and aviation red (lower sector).
Q14Why is PAPI photometry required?
It verifies intensity, colour and beam pattern, and supports confirmation of optical performance.
Q15Can PAPI be installed on the right side of the runway?
It is normally on the left as seen from the approach, but alternative siting is allowed where the standard location is impracticable or specific operational requirements apply.
Q16What is the difference between PAPI and APAPI?
PAPI normally uses four units; APAPI is an abbreviated two-unit arrangement, so indications and aiming differ.
Q17What can cause a PAPI to fail photometry?
Optical contamination, damaged optics, light-source degradation, incorrect aiming or beam pattern, colour deviation, electrical problems, or measurement and setup errors.
Q18Is PAPI alignment the same as PAPI photometry?
No. Alignment verifies physical direction and aiming; photometry evaluates optical performance.
Engineering & Expert Notes
Verify the Setting Out Before Pouring
InstallationCheck positions with survey equipment before foundations are fixed.
Level Units Carefully
LevellingUse proper tools and record readings. Level errors affect aiming angles.
Aim Each Unit to Its Own Angle
AimingCheck that each unit's angle follows the design and is recorded individually.
Verify Electrical Connections
ElectricalCheck transformers, leads and connectors before energising.
Run a Functional Test
TestingConfirm indications from the approach side as well as instrument checks.
Document As-Built Data
RecordsCapture angles, positions and photographs for the handover file.
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.