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Safety-critical technical reference — read before use. Independent educational guidance for qualified AGL and aerodrome personnel. It is not an approved procedure, design, specification or regulatory instruction and does not replace your aerodrome’s permit-to-work, isolation/LOTO procedures, the approved design or the manufacturer’s instructions. Series-circuit AGL equipment can carry lethal voltages. Read the full disclaimer ↓
THE AGL ENGINEERING CENTRE by Precision AGL Academy · PAPI Explained

How Does PAPI Work?

Why PAPI uses white and red lights, what 2 white and 2 red means, and the optical engineering behind the four-light indication: approach slope, threshold crossing height, aiming, photometry and maintenance.

Start Learning PAPI Engineering Guide
Overview

How Does PAPI Work?

A Precision Approach Path Indicator (PAPI) gives pilots visual information about the aircraft's position relative to the desired approach slope. A conventional PAPI is four light units in a horizontal row beside the runway. Each unit produces a precisely controlled transition between white and red light, and as the aircraft moves above or below the designed path, the combination of white and red changes.

Think of it as four light units + four optical settings + one approach reference. For the pilot it is simple. For the engineer, the units need the correct location, elevation, spacing, horizontal alignment, vertical aiming and optical performance for the indication to represent the intended path.

IndicationMeaning
4 WhiteAircraft significantly above the indicated approach path
3 White + 1 RedSlightly above the indicated approach path
2 White + 2 RedApproximately on the indicated approach path
1 White + 3 RedSlightly below the indicated approach path
4 RedSignificantly below the indicated approach path
Exact response to any indication is governed by the approved approach procedure and aircraft operating practice.

The Precision Approach Path Indicator: four light units beside the runway that show the pilot, by colour alone, whether the aircraft is on, above or below the approach slope.

HOW A PAPI WORKS: POWER IN, COLOUR SIGNAL OUT CCR CONSTANT-CURRENT REGULATOR SERIES CIRCUIT PRIMARY CABLE LOOP ISOLATION TRANSFORMER ONE PER UNIT PAPI UNIT LAMP · LENS · COLOUR FILTER PILOT READS THE COLOURS × 4 UNITS IN ONE WING BAR WHAT THE PILOT SEES (UNITS A TO D, A NEAREST THE RUNWAY) ABOVE THE SLOPE A B C D TOO HIGH · ALL WHITE ON THE SLOPE A B C D 2 RED · 2 WHITE BELOW THE SLOPE A B C D TOO LOW · ALL RED SCHEMATIC · NOT TO SCALE · PATTERN SHOWN FOR A WING BAR OF FOUR UNITS
Figure 1. The Precision Approach Path Indicator: four light units beside the runway that show the pilot, by colour alone, whether the aircraft is on, above or below the approach slope.
AGL Visual Encyclopedia: A PAPI PAPI unit arrangement
PAPI light boxes beside a runway at Washington Dulles, showing a red and white pattern
Photo 2. A PAPI installation at Washington Dulles, from the FAA. The red and white pattern the pilot sees depends on siting and aim.Credit: Federal Aviation Administration. US Government work, public domain. Source page
Interactive

PAPI Indication Simulator

Move the slider to change the aircraft's angle above the horizontal. A unit shows white when the aircraft is above its aiming angle and red when below.

3.00° · On approach path
INNER
2
3
OUTER
2 White + 2 Red
Standard 3° example with unit angles 2°30′, 2°50′, 3°10′, 3°30′. A learning aid only, not design data.
Working Principle

Why Red and White? The Optical Transition

Why does PAPI show red and white?

Each unit is designed so that, viewed from the approach, one angular region appears white, another appears red, and the change between them happens at a controlled elevation angle. What matters is not that a unit can make two colours, but that the transition sits at a defined vertical viewing angle. The four units are then aimed at progressively different elevations, which turns a continuous change in aircraft height into an intuitive pattern of colours.

Side view (angles exaggerated): the aircraft crosses each unit's transition line as it moves up or down

PAPI units4 White3 White + 1 Red2 White + 2 Red1 White + 3 Red4 Red

Does every unit point at the same angle?

No. The four units have different vertical aiming settings, carefully related so the transitions occur at the required angular positions. Installation is therefore not "install four units in a row" but survey, elevation, spacing, alignment, individual aiming and optical verification.

Why one moved unit matters

If Unit 3 moves, it may still turn on and show white and red, yet its transition now happens at the wrong elevation, changing the indication an approaching aircraft sees. PAPI aiming is a safety-related engineering parameter, not just a mechanical adjustment.

What PAPI Is Not

What PAPI Does and Does Not Tell the Pilot

Angular reference

PAPI shows the angular relationship between the aircraft line of sight and the four beams. 2 White + 2 Red means approximately on the indicated path.

Not an altimeter

It does not give altitude, airspeed, distance or attitude. The same indication occurs at different altitudes depending on distance from the runway.

Colour logic

White and red are used for vertical approach-path indication only. Runway and taxiway lighting use a wider colour set for their own functions.

Saying "PAPI can always be seen from X kilometres" is too simplistic. Range depends on intensity, atmospheric visibility, background, aircraft and pilot eye position, runway environment, design and operational requirements.

The descent slope the PAPI marks for the pilot. Two red and two white means on the path; more white means high, more red means low.

THRESHOLD PAPI WING BAR AIMING POINT ≈ TOUCHDOWN ZONE ≈ 300 m CROSSING HEIGHT ABOVE PATHHIGH · 3–4 WHITE ON GLIDE PATH2 RED · 2 WHITE BELOW PATHLOW · 3–4 RED 3° GLIDE PATH NOT TO SCALE · SLOPE EXAGGERATED APPROACH · SIDE VIEW
Figure 2. The descent slope the PAPI marks for the pilot. Two red and two white means on the path; more white means high, more red means low.
AGL Visual Encyclopedia: The PAPI glide path PAPI beam
Geometry

Approach Slope, TCH and MEHT

Threshold Crossing Height (TCH) relates the visual approach path to aircraft height over the threshold. Minimum Eye Height over Threshold (MEHT) is the associated eye-height characteristic of the approved PAPI and runway geometry. Both belong to the larger approach geometry, so a PAPI cannot be designed by just choosing a four-light unit.

Is PAPI always 3°?

No. 3.00° is common, but the correct value is the one set by the approved runway design and current aeronautical information. Indian AAI eAIP entries publish the PAPI side, approach angle and MEHT per runway direction, with examples at 3.00° and at 3.25°. Check the current eAIP for any runway.

Why location matters

Location depends on threshold position, runway elevation, approach slope, TCH/MEHT, obstacles, aircraft considerations and runway geometry. Moving a PAPI without recalculating changes its relationship to the intended path. Resurfacing, extension, re-thresholding or threshold relocation should trigger a PAPI engineering review.

Related Guides

PAPI Engineering Guide Glide path geometry tool ICAO PAPI Requirements

The vertical angle at which each PAPI unit’s beam changes from white to red. Four units, four angles, spread around the approach slope.

3° SLOPE UNIT A · 3°30′ UNIT B · 3°10′ UNIT C · 2°50′ UNIT D · 2°30′ ELEVATION ANGLE PAPI UNIT INCLINOMETER ON REFERENCE SURFACE ABOVE A UNIT'S ANGLE · THAT UNIT SHOWS WHITE BELOW A UNIT'S ANGLE · THAT UNIT SHOWS RED NOT TO SCALE · ANGLES EXAGGERATED · FOUR UNITS SHOWN FROM THE SIDE AS ONE POINT APPROACH · SIDE VIEW
Figure 3. The vertical angle at which each PAPI unit’s beam changes from white to red. Four units, four angles, spread around the approach slope.
AGL Visual Encyclopedia: The PAPI elevation angle
Comparison

PAPI vs APAPI, ILS and Approach Lighting

SystemWhat it isRelationship to PAPI
APAPIAbbreviated configuration using fewer units, giving a different indication arrangementChoice depends on aerodrome design and operational requirements
ILSRadio-navigation precision approach systemDifferent system. They can be used together, and PAPI gives an independent visual reference. PAPI does not replace ILS
Approach lightingVisual reference extending before the thresholdDifferent information; both work together in the visual approach environment
Runway lightingEdge, threshold, end and centreline lightsThe pilot sees the whole visual environment, not PAPI in isolation
PAPI can provide visual approach-slope guidance on runways without ILS, subject to the applicable design and operational requirements. It is useful at night and may contribute in reduced visibility where the approved procedure permits, alongside approach lighting, runway lighting, markings, weather/RVR and airport procedures.

Related Guides

PAPI vs APAPI PAPI vs ILS Approach Lighting Systems Traditional vs LED PAPI

Four units against two. The same white-above, red-below code, but the PAPI shows the error in finer steps.

HIGHLOWAIRCRAFT HEIGHT RELATIVE TO THE APPROACH SLOPEPAPI4 UNITS · 5 STEPSALL WHITEHIGH3 WHITE · 1 REDSLIGHTLY HIGH2 WHITE · 2 REDON SLOPE1 WHITE · 3 REDSLIGHTLY LOWALL REDLOWAPAPI2 UNITS · 3 STEPSBOTH WHITEHIGH1 WHITE · 1 REDON SLOPEBOTH REDLOWPILOT’S VIEW OF THE WING BAR · SCHEMATIC
Figure 4. Four units against two. The same white-above, red-below code, but the PAPI shows the error in finer steps.
AGL Visual Encyclopedia: PAPI versus APAPI: what is the difference VASI versus PAPI ILS glide slope
Medium-intensity approach lighting system with runway alignment indicator lights, shown on an FAA page
Photo 1. A medium-intensity approach lighting system (MALSR) as published by the FAA: white light bars and flashers on the extended centreline, with a green threshold line.Credit: Federal Aviation Administration. US Government work, public domain. Source page
Photometry

PAPI Photometry: Why "ON" Is Not Enough

Visual inspection tells you a unit is on. Photometry tells you how much light it produces and where it goes. A PAPI can be electrically functional but photometrically defective because of a dirty lens, optical deterioration, incorrect aiming, a damaged optical component, an LED/driver problem or mechanical displacement.

Intensity

A low result can come from contamination, optical degradation, electrical problems, LED/driver degradation or wrong test geometry. Verify the measurement before replacing equipment.

Colour and chromaticity

Colour depends on optical components, filters, lens condition, LED performance, contamination and correct replacements. AAI field-photometry specifications include colour and chromaticity for AGL and PAPI testing.

Beam alignment

Aiming errors come from foundation movement, impact, loose hardware, construction, incorrect adjustment or mechanical degradation, so verify aiming periodically.

01

Review design data

Runway, PAPI location, approach angle, unit arrangement, criteria.

02

Inspect units

Lenses, housings, foundations, surroundings.

03

Verify alignment

Horizontal and vertical settings.

04

Energize

Operate under the defined test condition.

05

Measure

Record the relevant photometric parameters.

06

Evaluate

Compare against the applicable criteria.

07

Correct

Clean, align, repair or replace.

08

Retest

Confirm the result.

Related Guides

AGL photometry Visual inspection vs photometric testing Photometric test data analyzer

Making sure each of the four units still shows its white-to-red change at the right angle and points parallel to the runway, and re-setting it when it has moved.

PAPI ALIGNMENT PLAN VIEW: AZIMUTH SIDE VIEW: ELEVATION RUNWAY CENTRELINE UNIT 4 UNIT 3 UNIT 2 · DRIFTED UNIT 1 EACH UNIT AIMED PARALLEL TO THE CENTRELINE aircraft approach from the left HORIZONTAL WHITE CUT-OFF RED θ INCLINOMETER ON REFERENCE SURFACE θ SET TO THE UNIT SETTING ANGLE cut-off is the white-to-red change WHEN TO RE-ALIGN After workon lens, lamp or housing After impactor settlement of the unit When tests showdrift in angle SCHEMATIC · ANGLES EXAGGERATED · SETTINGS FROM THE STANDARD AND MANUFACTURER · NOT TO SCALE
Figure 5. Making sure each of the four units still shows its white-to-red change at the right angle and points parallel to the runway, and re-setting it when it has moved.
AGL Visual Encyclopedia: A PAPI re-aligned
Electrical

Electrical Chain and Isolating Transformers

CCR
Primary cable
Isolating transformer
Connector
PAPI unit
PAPI is primarily optical, but a failed transformer, connector, cable, control or circuit supply can make a healthy optical unit look defective. Troubleshoot the whole chain. Architecture depends on the approved installation.

Related Guides

Isolating transformers CCR deep dive Surge protection

The isolating transformer that feeds each PAPI unit — one per unit, its primary in the series circuit, placed in the base can, the unit or a nearby pit, and joined to the lamps by a short secondary lead.

ONE TRANSFORMER PER UNIT primaries in series, secondary lead to each unit WHERE THE TRANSFORMER SITS set by the PAPI type and the design SERIES PRIMARY · SAME CURRENT THROUGH EVERY PRIMARY TX4 U4 TX3 U3 TX2 U2 TX1 U1 NEAREST RUNWAY TX IN BASE CAN under the unit TX IN UNIT HOUSING self-contained TX IN NEARBY PIT short secondary lead SIX ARRANGEMENT CHECKS · IN ORDER 1 MATCH rating suits the unit's lamp load and the design 2 ONE PER UNIT each unit has its own transformer on the series path 3 PLACE base can, unit or pit, as the type and design require 4 CONNECT primary in series, short secondary lead to the lamps 5 BOND case and can bonded to the earthing system 6 IDENTIFY unit and TX labelled, then recorded as-built
Figure 6. The isolating transformer that feeds each PAPI unit — one per unit, its primary in the series circuit, placed in the base can, the unit or a nearby pit, and joined to the lamps by a short secondary lead.
AGL Visual Encyclopedia: A PAPI transformer arranged PAPI power supply
Maintenance

PAPI Maintenance and Testing After Work

CategoryCheck
OpticalLens, colour, intensity, beam coverage, beam alignment
MechanicalFoundation, mounting, horizontal alignment, vertical aiming, unit relationship
ElectricalSupply, cable, connector, transformer, insulation, earthing
FunctionalAll units on, colour indication, control, brightness, final indication
FinalRetest and documentation

After maintenance follow this order: alignment check → electrical check → photometry → functional test → retest → documentation, so there is evidence that the work restored intended performance. AAI maintenance specifications give examples of this integrated approach, including physical inspection, lens cleaning, PAPI angle checks, calibration, earthing, insulation and transformer and connector checks.

Flight inspection

Ground maintenance and photometry are not formal airborne verification. Where the approved procedure requires it, PAPI is evaluated from the air. An AAI document describes ground calibration before flight calibration, with checks such as PAPI setting angles, brilliancy control and compatibility with the ILS glide path. The current airport and authority procedure decides when it is required.

Related Guides

AGL maintenance India PAPI/APAPI maintenance PAPI Alignment, Testing & Maintenance Field Guide

Five checks that prove each unit is sound, set at the right angle, aimed down the runway and changing colour exactly where it should.

PAPI WING BAR · SIDE VIEW OF THE FOUR TRANSITION ANGLES 2°30′2°50′3°10′3°30′ 3°00′ ABOVE SLOPE · WHITE BELOW SLOPE · RED RUNWAY 21345 1Electrical checks2Elevation angle: inclinometer3Azimuth aiming along runway4Colour + sharp transition5Flight inspection SCHEMATIC · ANGLES EXAGGERATED, NOT TO SCALE · HIGHEST SETTING IS THE UNIT NEAREST THE RUNWAY
Figure 7. Five checks that prove each unit is sound, set at the right angle, aimed down the runway and changing colour exactly where it should.
AGL Visual Encyclopedia: A PAPI tested PAPI inspection
India

PAPI in India

PAPI is widely deployed across Indian aerodromes. AAI eAIP material gives runway-specific data (PAPI side, approach angle and MEHT), so installed values differ between aerodromes and runway directions. For Indian projects the basis should include current DGCA requirements, AAI or project specifications, approved aerodrome information and manufacturer requirements, and the work chain runs survey, design review, civil foundation, installation, electrical integration, aiming, photometry, testing, commissioning and maintenance.

Related Guides

Airport runway lighting India AGL installation India AGL standards compliance India Airport lighting standards India
Standards

PAPI and ICAO

ICAO Annex 14 Volume I is the principal international aerodrome standard framework, and ICAO's Aerodrome Design Manual Part 4 (Visual Aids) gives engineering guidance on visual approach slope indicators. The ICAO Store lists Annex 14 Vol. I as the 9th Edition (July 2022) as amended, including Amendment 18. Corrigenda have followed, so confirm the latest corrigendum or edition and your State's implementation before relying on any requirement.

Related Guides

ICAO PAPI Requirements & Compliance Guide ICAO Annex 14 AGL Requirements

How the four units of a wing bar stand beside the runway: one straight line, equally spaced, in a fixed order, so the pilot reads them as a single bar of lights.

THRESHOLD RUNWAY APPROACH UNIT A · NEAREST RUNWAY UNIT B UNIT C UNIT D · FARTHEST WING BAR · 4 UNITS ≈ 300 m OFFSET FROM RUNWAY EDGE EQUAL SPACING OPTIONAL SECOND WING BAR · RIGHT SIDE NOT TO SCALE · LINE OF UNITS IS AT RIGHT ANGLES TO THE RUNWAY CENTRELINE APPROACH · PLAN VIEW
Figure 8. How the four units of a wing bar stand beside the runway: one straight line, equally spaced, in a fixed order, so the pilot reads them as a single bar of lights.
AGL Visual Encyclopedia: The PAPI unit arrangement
Troubleshooting

Common PAPI Problems

Click a fault to see where to investigate.

All units off

Investigate

  • Agl circuit
  • Ccr
  • Transformer circuit
  • Control
  • Power

One unit off

Investigate

  • Transformer
  • Connector
  • Secondary cable
  • Internal unit

One unit dim

Investigate

  • Lens
  • Optical system
  • Led/driver
  • Electrical supply
  • Transformer

Wrong colour pattern

Investigate

  • Aiming
  • Optical condition
  • Colour performance
  • Unit-to-unit relationship

PAPI misaligned

Investigate

  • Foundation
  • Mounting
  • Horizontal alignment
  • Vertical aiming

Photometry failure

Investigate

  • Measurement setup
  • Operating condition
  • Optical condition
  • Electrical condition
  • Aiming
If PAPI is not working, the operational consequence depends on runway category, approach procedure, other visual aids, airport rules, current aeronautical information and authority requirements. Follow the approved procedure, not an informal interpretation.

Related Guides

PAPI/APAPI failures Troubleshooting Engine

The symptom shows whether a unit, the circuit, the aim or the optics failed, and a short set of visual, electrical and inclinometer checks confirms it.

PAPI TROUBLESHOOTING · SYMPTOM, CAUSE, CHECKRUNWAY EDGEWING BAR · 4 UNITS · ABOUT 300 M BEYOND THRESHOLDUNIT 12°30′UNIT 22°50′UNIT 33°10′UNIT 43°30′EACH UNIT FED BYITS OWN ISOLATINGTRANSFORMERSETTING ANGLES SHOWN FOR A 3° SLOPE · HIGHEST NEAREST THE RUNWAY1SYMPTOMONE UNIT DARK/DIMCause: lamp or LED, lead,connector or secondaryCheck: look, then secondary2SYMPTOMALL UNITS OUTCause: CCR off or openloop: primary, joint, cableCheck: CCR, loop continuity3SYMPTOMWRONG SLOPECause: unit moved by aknock or settlementCheck: inclinometer, re-aim4SYMPTOMUNSHARP / UNEQUALCause: dirty or cracked lens,water, debris in frontCheck: lens, housing, view5SYMPTOMCCR EARTH FAULTCause: water in housing,connector or cableCheck: insulation to earthISOLATE AT THE CCR · LOCK OFF · PROVE DEAD · ATC CLEARANCE BEFORE WORK
Figure 9. The symptom shows whether a unit, the circuit, the aim or the optics failed, and a short set of visual, electrical and inclinometer checks confirms it.
AGL Visual Encyclopedia: How do you troubleshoot a PAPI
Systems

Digital Integration and LED PAPI

Monitoring

Depending on the installation, PAPI can interface with AGL control, ALCMS, remote monitoring, maintenance systems and operational procedures, so status is monitored as part of the airfield system.

LED PAPI

Opportunities include lower energy use, longer component life, fewer lamp replacements and better diagnostics. Weigh these together with optical performance, reliability, maintainability and operational requirement.

Related Guides

ALCMS Traditional vs LED PAPI

The PAPI reports its state over a link to the control room, so a failed unit is found at once. The reading is a warning; the signal itself is still proved on site.

PAPI REMOTE MONITORING FROM THE WING BAR TO THE CONTROL ROOM WHAT CAN BE SEEN STATUS TRAVELS FROM THE UNITS TO THE CONTROL ROOM PAPI UNITSU1 – U4 CONTROLUNIT LINKBY DESIGN ALCMSOPERATOR HOW EACH STATE IS SHOWN HEALTHYas expected WARNINGdrift or low FAULTunit out UNKNOWNno data NO DATA IS SHOWN AS UNKNOWN, NEVER AS HEALTHY STATE on or off, and the intensity step UNITS each unit's lamp or LED, by identity SUPPLY CCR or supply state, standby in use AIM drift warning, on some types only DOES NOT PROVE THE SIGNAL colour change and aim are verified on site WHERE EACH PART HAPPENS At the sitesense and gather status On the linkcarry it, securely In control roomshow, alarm, log SCHEMATIC · VARIES BY PAPI TYPE AND MAKER · LINK TYPE COMES FROM THE DESIGN · NOT TO SCALE
Figure 10. The PAPI reports its state over a link to the control room, so a failed unit is found at once. The reading is a warning; the signal itself is still proved on site.
AGL Visual Encyclopedia: A PAPI monitored remotely Smart PAPI ALCMS to PAPI
In One Line

PAPI converts the aircraft's vertical position relative to a designed approach slope into a simple white-and-red visual indication.

Behind four lights sit location, elevation, alignment, aiming, optics, electrics and maintenance.

Test Yourself

Quick Knowledge Check

Q1. What does 2 White + 2 Red mean?

A. Aircraft far too high
B. Approximately on the indicated approach path
C. Aircraft too low
D. A unit has failed

Q2. Why does moving one PAPI unit change the indication?

A. It changes the colour of the LED
B. Its white/red transition angle moves relative to the other units
C. It changes the CCR current
D. It does not

Q3. Is every PAPI set to exactly 3°?

A. Yes
B. No, the angle comes from the approved runway design and aeronautical information
C. Only in India
D. Only APAPI differs

Q4. Can a PAPI be switched on but still defective?

A. No
B. Yes, for example with wrong aiming, low intensity or poor colour
C. Only if all four are dark
D. Only in rain

Q5. What does PAPI give the pilot?

A. Exact altitude
B. An angular visual reference to the approach path
C. Airspeed
D. Distance to touchdown
Explore More

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Open ICAO PAPI Requirements →
COMPARISON

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Open PAPI vs APAPI →
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Open PAPI vs ILS →
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Open AGL Photometric Testing Procedure →
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Intensity, coverage, colour and testing.

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Airport photometers and reporting.

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INDIA

AGL Maintenance India

Maintenance practice at Indian airports.

Open AGL Maintenance India →
SYSTEMS

Approach Lighting Systems

The lighting PAPI works alongside.

Open Approach Lighting Systems →
CONTROL

ALCMS

Airfield lighting control and monitoring.

Open ALCMS →

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FAQ

How PAPI Works: FAQ

Q1How does PAPI work?

PAPI uses four optical units with different vertical aiming settings. Each has a controlled white/red transition, and the combination the pilot sees shows whether the aircraft is above, approximately on, or below the intended approach path.

Q2What does PAPI stand for?

Precision Approach Path Indicator.

Q3How many lights are in a PAPI?

A conventional PAPI uses four light units.

Q4Why does PAPI use red and white?

The two colours give a simple visual indication of the aircraft's relationship to the intended approach slope.

Q5What does 2 white and 2 red mean?

The aircraft is approximately on the intended PAPI approach path.

Q6What does 3 white and 1 red mean?

The aircraft is slightly above the indicated approach path.

Q7What does 1 white and 3 red mean?

The aircraft is slightly below the indicated approach path.

Q8What does 4 red mean on PAPI?

The aircraft is significantly below the indicated visual approach path.

Q9Does 4 white always mean the same altitude?

No. PAPI gives an angular visual reference, not a fixed altitude. Actual altitude depends on distance and position relative to the runway.

Q10Is every PAPI set to 3 degrees?

No. 3.00° is common, but designs can differ. Indian AAI publications show examples at both 3.00° and 3.25°.

Q11What is PAPI TCH?

Threshold Crossing Height: the relationship between the approach path and aircraft height over the threshold.

Q12What is PAPI MEHT?

Minimum Eye Height over Threshold, associated with the approved PAPI and runway approach geometry and published per runway direction in Indian AAI aeronautical information.

Q13Is PAPI the same as ILS?

No. PAPI is a visual approach-slope indicator and ILS is a radio-navigation system. They can be used together.

Q14Is PAPI the same as approach lighting?

No. They provide different visual information and work together in the approach environment.

Q15Does PAPI need photometric testing?

It may, during commissioning, inspection, maintenance or acceptance per the applicable programme. Measurements can include intensity, colour, chromaticity and beam characteristics.

Q16Can PAPI work without ILS?

Yes. It can give visual approach-slope guidance on appropriately equipped runways, subject to the applicable design and operational requirements.

Q17Can PAPI be on but still defective?

Yes. It can operate electrically while having incorrect aiming, low intensity, damaged optics or poor colour.

Q18Why is PAPI aiming important?

The white/red transition is angular, so changing a unit's physical aiming changes where the transition is seen.

Q19What happens if one PAPI unit moves?

Its transition moves relative to the other three units, potentially changing the indication presented to the pilot.

Q20Does PAPI need maintenance?

Yes. Maintenance should cover optical, mechanical, electrical, aiming, photometric and functional performance.

Field Insight

Engineering & Expert Notes

RS
By Rahul Sharma
Airfield Ground Lighting Specialist
Indications Depend on the Pilot's Position
Principle

The red and white pattern changes with the angle of approach relative to the set slope.

Setting Angles and Siting Work Together
Geometry

Correct angles are only part of the story. Position relative to the threshold is equally important.

A Light That Is On May Still Be Wrong
Photometry

Check intensity, colour and the sharpness of the transition, not just illumination.

PAPI Does Not Replace Other Aids
Limits

It provides visual slope guidance, and pilots use it together with other information.

Re-Check After Impacts and Civil Works
Maintenance

Movement of units or changes in surroundings can alter the indications.

Keep the Calibration Record
Records

Document angles and measurements so changes can be recognised.

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