PAPI INDIA
Precision Approach Path Indicator systems, installation, testing and maintenance — PAPI supply, aiming, photometry, commissioning and troubleshooting for runway visual approach guidance at Indian airports.
A PAPI is not simply four lights: it is a precision optical, mechanical and electrical system that depends on correct siting, elevation, spacing, aiming, optical performance, electrical supply and maintenance.
PAPI India at a Glance
Precision Approach Path Indicator (PAPI) is one of the most important visual approach guidance systems used at airports. It gives the pilot a visual indication of the aircraft's position relative to the selected approach slope during final approach to a runway.
A PAPI installation normally consists of a horizontal array of light units beside the runway approach end. Each unit produces a precisely controlled transition between white and red light over defined elevation angles, and the pilot interprets the combination of white and red indications to judge whether the aircraft is high, low or approximately on the intended path.
PAPI is therefore not simply a set of four lights. It is a precision optical, mechanical and electrical system that depends on correct siting, elevation, spacing, aiming, optical performance, electrical supply and maintenance.
PAPI is widely represented in India's aeronautical information publications. Current AAI eAIP entries show PAPI installations at Indian aerodromes with published approach slopes and MEHT values, including examples of 3.00° PAPI systems.[1]
For airport projects, the PAPI system should be designed, installed, tested and maintained according to the applicable ICAO provisions, DGCA requirements, AAI / project specifications, aerodrome procedures and the manufacturer's approved requirements.
What Is PAPI and How Does It Work?
PAPI is a visual approach slope indicator system that provides vertical guidance to pilots. A conventional installation uses four optical light units in a row, each with a precisely established elevation setting so the white-to-red transition occurs at a controlled angular position.
4 White
Aircraft significantly high
3 White / 1 Red
Above the intended path
2 White / 2 Red
Approximately on the intended path
1 White / 3 Red
Below the intended path
4 Red
Aircraft significantly low
The fundamental principle is an optical boundary between white and red light. The four units are aimed at progressively different elevation settings — they do not simply emit four identical lights. Their optical elevation settings are deliberately offset to create the required approach-path indication. The exact operational interpretation should always follow the approved aerodrome procedure and applicable documentation.
PAPI System Components
A complete PAPI installation can include the following.
PAPI light units
- Light source and optical system
- Reflector / lens arrangement
- Colour filters or optical elements
- Housing
- Mechanical adjustment mechanism
- Mounting assembly and electrical connection
Support / foundation
Units require a mechanically stable installation.
- Concrete foundation
- Mounting plate and base
- Adjustable support
- Protective arrangement
- Drainage provisions
Isolating transformers
In series-circuit installations, individual PAPI units can be supplied through airfield isolating transformers appropriate to the system design.
Primary AGL circuit
CCR → Primary Cable → Isolating Transformer → Secondary Connection → PAPI Unit. The actual architecture depends on the approved PAPI system and airport electrical design.
Control and monitoring
Depending on the installation, PAPI may be integrated with the airfield lighting control and monitoring system.
Aiming and adjustment equipment
- Elevation measurement
- Horizontal alignment
- Unit aiming
- Reference verification
PAPI India Applications
PAPI can be installed on a wide range of aerodromes and project types.
Current Indian AAI eAIP entries demonstrate that PAPI is used across a range of Indian aerodromes. Published examples include PAPI on both runway directions and configurations associated with CAT I and other approach-lighting arrangements.[2]
PAPI Installation in India: Siting, Slope, Aiming, Spacing, MEHT & TCH
PAPI installation requires more than placing four light units beside the runway. It should start with an engineering review, and final installation parameters should come from the approved design and governing requirements.
Siting
Siting runs Runway Threshold → PAPI Reference Position → Aircraft Approach Path. An incorrect location can create a mismatch between the guidance given and the intended runway approach geometry — so PAPI is an aerodrome engineering activity, not an ordinary lighting installation.
- Threshold location
- Approach slope and PAPI elevation
- Horizontal distance
- Threshold crossing height
- Obstacle clearance requirements
- Runway longitudinal profile
- Aircraft operational assumptions
Approach slope
PAPI establishes a visual indication corresponding to a selected approach slope. Indian AAI eAIP entries frequently publish 3.00°, but the applicable slope is an aerodrome-specific design/operational parameter and should not be assumed identical for every runway.[2] A small aiming error can change the effective visual guidance given to the pilot.
Spacing
Spacing depends on the approved design and applicable standard, and should be set from controlled survey references rather than estimated during installation.
- Centre-to-centre spacing
- Perpendicularity / reference line
- Distance from runway centreline
- Distance from threshold
- Unit elevation
- Alignment with approved PAPI geometry
Horizontal alignment
Elevation aiming is only part of alignment. A unit that has moved horizontally may still appear operational but introduce an alignment error. Causes include:
- Foundation movement
- Accidental impact
- Loose mounting hardware
- Pavement works
- Maintenance activity
- Mechanical deterioration
MEHT
Minimum Eye Height Over Threshold relates the PAPI indication to pilot eye height over the runway threshold under the defined approach geometry. AAI eAIP entries publish PAPI data together with MEHT for individual runways, for example 3.00° with a published MEHT.[4] MEHT is not simply an equipment specification: it is linked to overall approach geometry and should be calculated and verified in the approved aerodrome design.
TCH
Threshold Crossing Height is part of approach guidance design. The objective is a coherent relationship between PAPI, approach slope, threshold and runway touchdown geometry, so PAPI should be evaluated with the overall approach system, not as an isolated fitting.
PAPI vs APAPI: a conventional PAPI normally uses four light units; an Abbreviated PAPI (APAPI) uses fewer units and therefore provides a different visual indication arrangement. The choice depends on the applicable aerodrome design and operational requirements.
PAPI Elevation and Aiming
Correct aiming is one of the most critical parts of installation. Each unit must be set to its prescribed elevation angle relative to the appropriate reference.
Establish survey reference
Check foundation elevation
Determine each unit's elevation setting
Mechanically align unit
Check horizontal orientation
Lock adjustment
Independently verify
Functional & photometric tests
AAI maintenance documentation itself calls for periodic checks of PAPI angle, calibration and light-unit alignment, showing that aiming is a continuing maintenance requirement rather than a one-time installation activity.[3]
PAPI Optical System and Colour
The optical system generates the white/red transition.
Optical system
Performance depends on light source, reflector or lens, filter, optical alignment, housing condition, cleanliness and internal configuration. A PAPI lamp that is illuminated does not automatically prove the optical system is performing correctly.
- High-intensity directional light
- Defined horizontal coverage
- Defined vertical distribution
- White light above the transition
- Red light below the transition
- A controlled transition zone
Colour
The white/red indication is fundamental. Incorrect colour can result from the following. Visual inspection can detect obvious problems, but where acceptance or compliance depends on measured optical performance, use appropriate test procedures.
- Damaged filters
- Degraded optical components
- Incorrect replacement parts
- Lens contamination
- Internal optical faults
- Wrong equipment configuration
PAPI Photometric Testing
PAPI should be subjected to suitable photometric testing during commissioning and, where required, periodic inspection and maintenance.
AAI technical specifications for in-field AGL photometry explicitly include PAPI units among the fittings to be tested, and require evaluation of parameters such as colour, coverage, configuration and chromaticity.[5]
PAPI functional test
Does the system operate?
PAPI photometric test
Does the system produce the required optical performance?
Document review
Review the approved PAPI design, runway data and applicable criteria.
Physical inspection
Check units for damage, contamination, displacement and mechanical condition.
Electrical inspection
Verify circuit, transformers, connectors and supply arrangements.
Aiming verification
Measure elevation and horizontal alignment of each unit.
Photometric measurement
Measure the relevant optical characteristics with calibrated equipment.
Data analysis
Compare results with the applicable requirements.
Defect identification
Record any unit or parameter requiring correction.
Corrective work
Clean, repair, realign or replace components as required.
Retesting
Confirm final performance after corrective action.
Final report
Document measured values, equipment information, defects and final status.
PAPI Installation Testing and Commissioning
PAPI commissioning should be systematic. A useful sequence has eight stages.
Stage 1 — Documentation
- Approved drawings
- Equipment manuals
- PAPI layout
- Runway survey information
- Electrical drawings
- Applicable standards / specifications
Stage 2 — Civil verification
- Foundation
- Elevation
- Drainage
- Mechanical stability
- Access
- Finished-ground relationship
Stage 3 — Mechanical installation
- PAPI units
- Mounting assemblies
- Spacing
- Horizontal alignment
- Protective arrangements
Stage 4 — Electrical installation
- Primary circuit
- Isolating transformers
- Secondary connections
- Earthing
- Connectors
- Control interfaces
Stage 5 — Aiming
- Unit-by-unit elevation setting
- Horizontal alignment
- Reference verification
- Locking and sealing
Stage 6 — Functional test
- ON / OFF
- Brightness / control operation where applicable
- White/red indication
- Unit operation
- Circuit response
Stage 7 — Photometric test
- Intensity
- Colour
- Optical distribution
- Beam transition
- Alignment
Stage 8 — Final acceptance
- Defect correction
- Retesting
- Documentation
- As-built records
- Maintenance baseline
PAPI Electrical System, Transformers, Cables & Connectors
Although PAPI is primarily a visual guidance system, its electrical design is equally important. A typical AGL architecture is shown below; the actual architecture varies by equipment and project.
Airport Power Supply
Constant Current Regulator
Primary AGL Cable
Isolating Transformer
Secondary Connector
PAPI Light Unit
Electrical design should address
- Circuit loading and isolation
- Transformer selection
- Cable condition, connectors and insulation
- Earthing
- Surge protection as specified
- Control interface and monitoring
- Secondary power arrangements
Isolating transformers
Where PAPI is fed through an AGL series circuit, isolating transformers isolate the primary circuit from the individual light units. Selection must match the approved equipment and circuit design — not just physical dimensions; electrical compatibility is essential.
- Rated primary current
- Secondary voltage
- Power rating
- Insulation
- Environmental protection
- Connector arrangement and installation method
Cables and connectors
AAI maintenance schedules for PAPI include checks of plug/socket connections, transformer housings and moisture inside fittings.[3]
- Primary airfield cable
- Transformer leads
- Secondary cable
- Connectors and plug/socket assemblies
- Junction arrangements
- Common problems: moisture ingress, damaged connectors, poor termination, insulation degradation, mechanical damage
PAPI Maintenance in India
PAPI maintenance should be proactive.
Daily / operational checks
- Confirm system serviceability
- Switch on and check operation
- Visually inspect units
- Check obvious damage and displacement
Periodic inspection
- Clean lenses
- Inspect optical surfaces
- Remove vegetation
- Check aiming
- Inspect mechanical components and alignment
Electrical maintenance
- Check transformers
- Inspect connectors
- Examine moisture ingress
- Verify earthing
- Inspect cables and terminations
Photometric maintenance
- Photometric verification as required
- Investigate low or inconsistent output
- Verify colour
- Check beam characteristics
- Retest after corrective action
AAI tender documentation provides a practical example of such a structure, including daily serviceability checks, weekly angle checks, monthly calibration/alignment checks and half-yearly or pre- and post-monsoon inspections. These are contract-specific AAI requirements and should not automatically be treated as a universal frequency for every airport.[3]
Lens contamination
Dust and environmental deposits can affect optical output.
Vegetation obstruction
Vegetation can interfere with the pilot's view of the units.
Unit misalignment
Mechanical movement can change the intended aiming angle.
Moisture ingress
Water inside the fitting can damage optical or electrical components.
Damaged optical components
Reflectors, filters and lenses can deteriorate or become damaged.
Electrical connection failure
A poor secondary connection can cause intermittent or complete unit failure.
Foundation movement
A small physical displacement can create an optical aiming error.
Incorrect replacement
Replacement parts or units must be compatible with the approved PAPI system.
PAPI Troubleshooting
Troubleshooting should combine electrical, optical, mechanical and survey information rather than replacing components based only on the visual symptom.
| Symptom | Possible Investigation |
|---|---|
| One unit OFF | Supply, transformer, connector, internal electronics |
| All units OFF | CCR / circuit / control / power issue |
| One unit appears different | Aiming, optical condition, electrical performance |
| Unexpected colour indication | Optical / filter issue or aiming |
| Weak light output | Optical degradation, contamination or electrical issue |
| PAPI indication inconsistent | Unit aiming or optical performance |
| Unit displaced | Mechanical mounting / foundation |
| Moisture visible | Housing / sealing / drainage investigation |
| Repeated transformer failures | Electrical loading, environment or equipment compatibility |
| Photometry failure | Optical, aiming, electrical or installation investigation |
PAPI in Low-Visibility Operations and LED PAPI
Low-visibility airport operations
PAPI is a visual aid and part of the aerodrome's broader visual guidance infrastructure. Where visibility conditions are demanding, continued availability and correct performance of visual aids is especially important. PAPI, approach lighting, runway lighting, marking and instrument approach procedures must be considered as an integrated system; the specific requirements depend on the runway and approved operating category.
LED technology
Modern PAPI equipment can use LED sources. LED does not mean maintenance-free: optical inspection, cleaning, alignment verification, mechanical inspection, electrical testing, photometric verification and corrective maintenance are still required.
- Long service life
- Lower routine lamp replacement
- Stable optical performance
- Reduced maintenance burden
- Lower energy consumption
- Improved monitoring possibilities
PAPI for New Airports, Runway Extension, Rehabilitation and Relocation
New airport projects
Consider PAPI from the design stage: Survey → Design → Civil Foundation → PAPI Supply → Electrical Installation → Aiming → Commissioning → Photometry → Approval/Handover. It should not be treated as an isolated equipment package.
- Runway geometry
- Obstacle limitation considerations
- Approach lighting and threshold markings
- Runway centreline and edge lights
- Instrument approach and navigation procedures
- Maintenance access and airfield control systems
Runway extension and rehabilitation
Changing runway geometry changes the relationship between PAPI position, threshold and approach path, so leaving the existing PAPI in its old position may not be appropriate. An engineering review should be completed whenever runway geometry or threshold position changes. Triggers include a runway being:
- Extended or shortened
- Shifted or resurfaced
- Reprofiled
- Re-thresholded
- Geometrically modified
PAPI relocation
After relocation the system should be treated as an engineering installation requiring complete verification, not merely reconnecting the old equipment.
- Survey
- New foundation
- Revised coordinates and elevations
- Electrical modification
- New cable / transformer arrangements
- Mechanical installation and aiming
- Photometry and documentation
- Operational coordination
PAPI Manufacturer, Contractor and Consultant in India
Manufacturer and supplier
A supplier for an Indian airport project should demonstrate technical compatibility with the required application. AAI technical and maintenance procurement documents also stress manuals, subsystem descriptions, maintenance procedures, testing/adjustment procedures and circuit/layout information.[6]
- Optical performance
- Environmental protection and mechanical construction
- Electrical compatibility and adjustability
- Maintainability and spare-parts availability
- Technical documentation
- Calibration / aiming provisions
- Warranty and after-sales support
- Installation and commissioning capability
Contractor
For larger airport projects, PAPI work may be included within a broader AGL SITC package.
- Site survey and civil coordination
- Foundation and equipment installation
- Electrical cabling, transformers and connectors
- Aiming, testing and photometry
- Commissioning and as-built documentation
- Operator training and maintenance
Consultant
Particularly useful where PAPI is part of a larger AGL modernisation or airport development programme.
- Runway visual-aid design and PAPI siting
- Approach slope calculations and MEHT/TCH considerations
- Obstacle assessment coordination
- Technical specifications, BOQ and tender documents
- Technical evaluation and equipment approval
- Installation inspection and commissioning supervision
- Photometric testing and acceptance documentation
PAPI Standards in India and Current Indian Airport Evidence
PAPI design and installation should be evaluated against the applicable standards and project documentation rather than a single generic PAPI specification.
The international reference framework includes ICAO Annex 14, Volume I, Aerodromes — Design and Operations, while detailed visual-aid guidance is also addressed in ICAO's Aerodrome Design Manual Part 4, *Visual Aids*. The ICAO Store listing shows Annex 14 Volume I as the 9th Edition (July 2022), with Amendment 18 dated 4 August 2025 and 2026 corrigenda.[7]
For Indian airports, the applicable DGCA CAR requirements, AAI technical specifications, aerodrome operating procedures and approved project documents must also be incorporated. FAA requirements should not automatically be presented as Indian or ICAO requirements; where a contract specifically invokes FAA criteria, those can be evaluated separately.
PAPI is not merely a theoretical or legacy system in India. Current AAI aeronautical publications identify PAPI at numerous Indian airports, with different runway directions, PAPI side/location and published MEHT values.[2] AAI procurement documentation also includes PAPI within maintenance programmes, in-field photometric testing, airport AGL maintenance and runway visual-aid systems. For example, AAI's Rajkot International Airport photometry specification includes PAPI units in the field-testing scope, and AAI maintenance documentation details PAPI inspection and maintenance activities.[5]
PAPI Selection and Quality-Control Checklists
Use these checklists before selecting a PAPI system and before handover.
Optical
- Required approach slope
- White/red transition
- Intensity and optical coverage
- Beam characteristics
- Colour / chromaticity
Mechanical
- Housing and mounting
- Adjustment mechanism
- Foundation interface
- Environmental protection
- Frangibility / structural requirements where applicable
Electrical
- Input / circuit compatibility
- Transformer compatibility
- Connectors and earthing
- Control interface
- Monitoring
Operational
- Required runway configuration
- Visibility / operational category
- Aircraft considerations
- Threshold geometry
- MEHT / TCH requirements
Maintenance
- Spare parts
- Calibration / aiming tools
- Optical components
- Service manuals
- Local technical support
- Repair procedure
Documentation
- Technical datasheet
- Installation and maintenance manuals
- Wiring diagrams
- Optical data
- Test procedures
- Calibration information
Survey
- Coordinates verified
- Threshold reference verified
- PAPI position verified
- Elevations checked
Civil
- Foundations complete
- Drainage satisfactory
- Mounting stable
- No obstruction to the visual system
Mechanical
- Spacing checked
- Horizontal alignment checked
- Elevation adjustment checked
- Locking arrangements completed
Electrical
- Transformer checked
- Connectors checked
- Circuit tested
- Insulation condition verified
- Earthing verified
Optical
- White/red indication checked
- Lens condition checked
- Aiming verified
- Photometric performance measured
Documentation
- Test results and calibration records
- As-built drawings
- Equipment serial numbers
- Maintenance manuals
- Corrective-action records
- Final acceptance documentation
PAPI India with Precision AGL Solutions
Precision AGL Solutions treats PAPI as an engineering system, not just an equipment product, and supports the complete PAPI lifecycle.
Design Support
Supply Coordination
Installation
Aiming
Electrical Integration
Photometry
Testing
Commissioning
Maintenance & Troubleshooting
A reliable PAPI installation depends on correct location, elevation, spacing, aiming, optical performance, electrical integration, testing and maintenance. The Indian aeronautical information system shows PAPI as an established runway visual aid across many aerodromes, while AAI technical documents demonstrate the importance of dedicated PAPI inspection, aiming, maintenance and photometric verification.[2]
Frequently Asked Questions
What does PAPI stand for?
What is the purpose of PAPI?
How many lights are used in a PAPI?
What do 2 white and 2 red lights mean?
Is every PAPI set to 3 degrees?
What is MEHT in PAPI?
Does PAPI require photometric testing?
Can PAPI be installed on both runway directions?
Can an existing PAPI be relocated?
Is LED PAPI maintenance-free?
Who can handle PAPI installation in India?
Looking for PAPI Support in India?
PAPI design support, installation, aiming, photometry, commissioning, maintenance and troubleshooting — share your runway data, BOQ, technical specification or existing PAPI configuration.
Sources & References
- AIM India eAIP v2-01-2026 — AD 2.1 VIND
- AIM India eAIP archive 16-04-2026 — AD 2.1 VAPR
- Airports Authority of India — tender document (PAPI maintenance schedules)
- AIM India eAIP v2-01-2026 — AD 2.1 VAJL
- Airports Authority of India — in-field AGL photometry specification
- Airports Authority of India — technical and maintenance tender documentation
- ICAO Store — Annex 14, Aerodromes
External sources were referenced when this page was prepared; tender listings, AIP data and company websites change over time. Always confirm requirements against the controlled documents for your project.
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