PAPI Calibration, Aiming & Alignment Procedure
How PAPI is calibrated and aimed: vertical angle, horizontal alignment, unit-to-unit geometry, elevation, installation position, calibration equipment, photometry and recalibration. A PAPI can be electrically healthy and still need recalibration.
PAPI Calibration, Aiming and Alignment
PAPI calibration and aiming ensure the system gives the intended visual approach-slope indication. Performance depends on more than the units being operational: each must be at the correct location and elevation, aligned to the runway and set to the right vertical aiming angle.
A healthy PAPI can still need recalibration
Foundation movement, accidental impact, mechanical adjustment, runway or pavement works, threshold changes, relocation, optical-component or equipment replacement, and deterioration or movement of mounting hardware.
Calibration vs maintenance
Maintenance covers cleaning, physical, connector, transformer and electrical checks and general serviceability, and may reveal the need for recalibration. Calibration measures angles, sets vertical aiming and horizontal alignment, verifies elevation and unit-to-unit geometry, and checks optical performance.
Ground Calibration vs Flight Inspection
Ground calibration / aiming verification
Done on the airfield with surveying and aiming equipment. It establishes the physical and optical settings.
Flight inspection / flight calibration
Airborne verification by the competent authority or an approved flight-inspection organisation, where required.
AAI's operational circular on PAPI calibration (as cited) states that ground calibration is carried out before flight calibration, that flight inspection of a new PAPI installation covers items such as range, setting angles, brilliancy control and compatibility with the ILS glide path, and that calibration is required after major maintenance or developments affecting the system. AAI also operates a Flight Inspection Unit that can inspect visual glide slope indicators. Confirm against the current circular and your authority's procedure.
PAPI Aiming Angle and the 3° Geometry
The aiming angle is the elevation each light unit is set to. The four units are not set identically; different angles create the sequence 4 White → 3W/1R → 2W/2R → 1W/3R → 4 Red.
| Unit | Typical setting for a 3° slope | Step to next |
|---|---|---|
| Nearest runway | 3°30′ | 20′ |
| Second unit | 3°10′ | 20′ |
| Third unit | 2°50′ | 20′ |
| Farthest from runway | 2°30′ | — |
On-course sector
The pilot's on-course indication is the angular relationship between the relevant unit beams, conventionally about 20′ of arc. ICAO-derived guidance allows widening to 30′ where harmonisation with an electronic glide path requires it. That is a design decision, not a routine maintenance change.
PAPI angle is not always 3°
3° is common but runway-specific. Indian AAI eAIP entries publish approach angle and MEHT per runway. Records should state the approved approach angle and the measured unit angles, not just "PAPI = 3°".
PAPI Alignment Measurement Sheet
Enter your measured angles. The sheet shows the difference in minutes and, if you enter a tolerance, pass or fail.
Vertical Angle, Equipment and Levelling
Clinometer / inclinometer
Measures the vertical aiming angle. AAI documents call for calibration settings to be checked with clinometers and spirit levels, and FAA maintenance guidance also checks vertical aiming with a clinometer. Use an instrument suited to the required accuracy.
Sighting stick
Used for routine angle verification where the maintenance procedure specifies it, with the device correct for the PAPI model.
Spirit level and levelling
Check the mechanical reference before measuring: spirit level, manufacturer levelling device, mounting reference, tilt indication. Aiming without levelling gives unreliable readings.
Other equipment
Survey equipment (coordinates, elevation, distance, runway relationship, foundation levels), optical or photometric equipment (intensity, distribution, colour, alignment), and the manufacturer-specific aiming tool where one is specified.
Electronic vs manual
Electronic inclinometers give digital readings, high resolution, data logging and easier documentation. Sighting sticks, mechanical clinometers and spirit levels remain useful for field verification. Choose by tolerance and manufacturer procedure.
Horizontal Alignment, Spacing and Position
Vertical aiming alone does not complete alignment. The units must also have the correct horizontal relationship with the runway centreline, the PAPI row reference and the intended approach direction. Errors come from foundation movement, impact, loose hardware, maintenance, civil works or incorrect installation.
| Parameter | ICAO-derived design guidance (as cited) | FAA AC 150/5340-30 (see Field Guide) |
|---|---|---|
| Unit spacing | About 9 m between units | 20 to 30 ft (6 to 9 m), ±1 ft |
| Inner edge from runway edge | About 15 m ±1 m | At least 50 ft (15 m); 30 ft for small non-jet runways |
Verify at each visit
Unit spacing unchanged, units not moved, foundations stable, PAPI row undisturbed. Aiming cannot compensate for a wrongly positioned PAPI, so check distance from the threshold, lateral offset, elevation, runway relationship and approved coordinates.
Unit-to-unit relationship
Calibrate the four units as a system. The question is not only "is Unit 2 at its angle?" but "is Unit 2 correctly related to Units 1, 3 and 4?". Never calibrate only the unit that seems faulty.
Elevation, Foundations, TCH and MEHT
Verify foundation elevation, unit elevation, relative elevation between units, threshold elevation and the runway reference. An angle is only meaningful when the mechanical reference is right. If the foundation settles, the unit can move vertically while an instrument zeroed to a local reference still reads correctly, changing the PAPI-to-threshold relationship.
PAPI Alignment Procedure
Review approved design
Location, unit identification, approach angle, unit settings, elevation, spacing, tolerance.
Establish survey reference
Runway centreline, threshold, runway edge, PAPI reference, elevation datum.
Inspect foundations
Stability, level, damage, movement, anchor hardware.
Check horizontal alignment
The PAPI row and each unit's orientation.
Check unit spacing
Measure each adjacent pair.
Level the unit
Use the approved mechanical reference.
Measure vertical angle
Clinometer, inclinometer or sighting equipment.
Adjust
Small controlled adjustments to the approved setting.
Lock the adjustment
As the manufacturer specifies.
Repeat for all four units
Never fix only the unit that looks wrong.
Verify unit-to-unit relationship
Confirm the complete angular sequence.
Photometry
Verify optical output and beam characteristics.
Functional test
Confirm the complete visual indication.
Record results
Create the final calibration record.
Photometric and Colour Verification
Aiming is not complete until optical performance is verified where required. A correctly aimed unit can still have an optical defect, and a photometrically healthy unit can still be wrongly aimed, so consider aiming + photometry together. Photometry may check intensity, colour, chromaticity, distribution, transition, coverage and alignment.
Beam transition
Establish that the white/red transition falls within the required angular region. Problems include a transition too high, too low, irregular or inconsistent between units, any of which should trigger optical and aiming investigation.
Colour
Visual inspection, chromaticity measurement and unit-to-unit comparison, depending on equipment and standard. AAI field-photometry specifications include colour and chromaticity in AGL and PAPI testing.
The strongest verification workflow
Mechanical calibration + survey verification + photometric verification + functional verification. Aiming alone is not enough, photometry alone is not enough, and electrical testing alone is not enough. Together they give a much stronger indication of PAPI condition.
Related Guides
AGL Photometric Testing Guide AGL photometry Visual inspection vs photometric testing Photometric test data analyzerCommon PAPI Alignment Errors
One unit at the wrong angle
Can distort the white/red indication.
Row not parallel to the runway reference
Creates horizontal alignment error.
Foundation settlement
Changes the elevation relationship.
Incorrect unit spacing
Changes the intended geometry.
Incorrect survey reference
Can leave all four units set wrongly.
Wrong unit label
A documentation error even if the physical units are correct.
Adjusting without rechecking all units
Leaves an inconsistent angular relationship.
Aiming without levelling
Gives unreliable angle measurements.
Unverified instrument
Makes the result questionable.
Adjustment not locked
The unit can move after calibration.
Impact, Civil Works, Threshold Change, ILS Harmonisation
| Cause | Why it matters |
|---|---|
| Vehicle or maintenance impact | May not damage a unit visibly yet cause slight angular movement, distorted mounting, foundation damage or optical displacement while it still operates. Recheck after any known impact |
| Civil works (pavement, resurfacing, trenching, drainage, taxiway construction, grading) | Review the PAPI reference geometry afterwards |
| Runway threshold change | Changes distance, approach geometry, TCH, MEHT and obstacle protection, so a full PAPI engineering review may be needed |
| PAPI relocation (extension, threshold relocation, obstacles, geometry change) | Treat as a new installation: survey, installation verification, aiming, photometry, functional testing, documentation and, where required, flight inspection |
| ILS or electronic glide path | Harmonise the visual and electronic paths. ICAO-derived guidance allows defined unit-setting adjustments for this, as an engineering design activity, not an informal field fix |
| New obstacles (structures, cranes, vegetation, temporary works) | Report physical obstructions found during inspection, since PAPI must keep its relationship to the obstacle protection surface |
When to Recalibrate and the Recalibration Workflow
Recalibration may be needed after major maintenance, unit or optical-component replacement, foundation work, physical impact, runway rehabilitation, threshold or PAPI relocation, any change affecting geometry, or abnormal photometric results. A small routine correction is not necessarily a full recalibration: the level of verification depends on what changed, why, its effect on geometry, airport procedure and authority requirement. A major change can need formal ground and flight verification.
PAPI Calibration and Commissioning Checklist
Click an item to mark it complete.
Design
Physical and Aiming
Electrical and Optical
Functional and Documentation
Calibration Report and Maintenance Schedules
Report contents
Project (airport, runway, PAPI ID, date, conditions); reference data (threshold, position, elevation, approved angle, unit settings); survey (coordinates, elevations, spacing, horizontal alignment); aiming (required, measured, difference, adjustment); photometry; functional test; corrective work (defect, action, retest); final status (accepted, accepted with observations, or corrective work outstanding). Do not pre-fill a universal angle into forms.
AAI schedule examples
One cited AAI schedule has routine serviceability and visual checks for damage or displacement, weekly angle checks with a sighting stick, half-yearly calibration-setting checks with clinometer and spirit level, and earthing, cable-insulation and isolating-transformer connection checks. Another includes recurring angle calibration and checks of wiring, earthing and remote control. Use these as examples only.
PAPI Calibration Support
Precision AGL Solutions supports PAPI calibration as a specialist engineering service. This covers ground calibration work only; formal airborne flight inspection is performed by the competent authority or an approved organisation.
Quick Knowledge Check
Q1. Which statement about PAPI calibration is correct?
Q2. One PAPI unit looks wrong. What should you do?
Q3. Why record "approved approach angle" and "measured unit angles" separately?
Q4. A foundation settles. What can happen?
Q5. Which should set the tolerance for an Indian airport PAPI?
Explore More AGL Engineering
PAPI Engineering Guide
Siting, geometry and electrical systems.
Open PAPI Engineering Guide →PAPI Installation & Aiming Handbook
Installation procedure and aiming angles.
Open PAPI Installation & Aiming Handbook →PAPI Alignment, Testing & Maintenance
Records, checks and maintenance.
Open PAPI Alignment, Testing & Maintenance →PAPI Alignment Calibration Tool
The aiming and calibration tool.
Open PAPI Alignment Calibration Tool →PAPI Testing & Commissioning
Test and commission PAPI and APAPI.
Open PAPI Testing & Commissioning →AGL Photometric Testing Guide
Photometric testing procedure, measurement, evaluation and reporting.
Open AGL Photometric Testing Guide →Aim Precisely. Verify Objectively.
Explore the full PAPI cluster, or talk to our engineers about PAPI calibration and commissioning support.
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Standards & Compliance
Runway, Taxiway & Approach Lighting
Surface Movement & Safety
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Electrical Engineering & Power
Photometry & Testing
Maintenance, Inspection & Safety
Resources, Documentation & Careers
PAPI Calibration FAQ
Q1How is PAPI calibrated?
Calibration verifies the approved position, elevation, horizontal alignment, unit spacing and individual vertical aiming angles, followed by photometric and functional verification. Formal flight inspection, where required, is separate.
Q2What equipment is used to calibrate PAPI?
Typically a calibrated clinometer or inclinometer, spirit level, sighting stick, survey equipment and suitable photometric equipment, plus the manufacturer's aiming equipment where specified.
Q3What is the PAPI angle?
There is no single universal angle. 3° is common, with a conventional ICAO-derived four-unit arrangement of 3°30′, 3°10′, 2°50′ and 2°30′. Project settings come from the approved design.
Q4Are all four PAPI units set at the same angle?
No. Their vertical aiming angles are deliberately different to create the white/red sequence.
Q5What is the standard angle for a 3° PAPI?
The conventional ICAO-derived settings are 3°30′, 3°10′, 2°50′ and 2°30′, from the unit nearest the runway to the farthest.
Q6Can the PAPI angle be changed during maintenance?
Only under the approved maintenance and design procedure, never just to make the lights look aligned.
Q7What is PAPI horizontal alignment?
The positioning and orientation of the PAPI row and individual units relative to the runway reference.
Q8What happens if one PAPI unit is misaligned?
Its white/red transition occurs at the wrong elevation, changing the indication presented to the pilot.
Q9Does PAPI calibration include photometry?
A complete verification should include photometry where required, but they measure different things: aiming establishes geometry and photometry verifies optical performance.
Q10When should PAPI be recalibrated?
After major maintenance, relocation, runway or threshold developments, physical impact, foundation changes or other work affecting geometry. AAI's circular calls for calibration after major maintenance or developments affecting PAPI.
Q11Is PAPI ground calibration the same as flight inspection?
No. Ground calibration sets the physical and optical settings. Flight inspection is airborne verification where the authority or procedure requires it.
Q12Does PAPI need calibration after runway resurfacing?
It may. Any change affecting runway elevation, threshold geometry or the PAPI-to-runway relationship should trigger an engineering assessment.
Q13What is the difference between PAPI aiming and alignment?
Aiming is mainly the vertical optical angle of each unit. Alignment is broader: horizontal orientation, position, spacing and the relationship between the four units.
Engineering & Expert Notes
Use Calibrated Instruments
EquipmentRecord instrument identity and calibration date with the measurement.
Confirm Reference Points
ReferenceEstablish and document the reference used for angle measurement so it can be repeated.
Ground Calibration Is Not a Flight Check
VerificationUnderstand what each activity verifies and what it does not.
Follow a Written Procedure
ProcedureA defined sequence reduces errors and makes results comparable.
Check Photometry After Alignment
PhotometryAlignment does not guarantee intensity or colour. Verify both.
Issue a Complete Calibration Report
ReportInclude measurements, conditions and corrective actions.
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