PAPI Alignment Telescope / Aiming Device for Airfield Ground Lighting

What It Is

PAPI light units must be accurately positioned and aimed to establish the intended visual approach path. Precision AGL Solutions supplies PAPI alignment telescopes and aiming devices for installation, commissioning, maintenance and periodic verification.

Depending on configuration, the equipment provides optical line-of-sight alignment, angular reference measurement, or both.

For FAA systems, AC 150/5345-28H requires an aiming tool that measures the vertical angle of the light-beam centre from 2° to 8°, in 10-minute-of-arc increments, with repeatable accuracy of ±3 arc minutes.

Establishes the Relationship Between

• PAPI light-unit optical axis
• Runway centreline
• Approved approach path
• PAPI installation reference
• Required vertical aiming angle
• Horizontal alignment reference
• Survey or engineering reference points

Configurations may include a telescope, angular scale, sighting arrangement, bubble level, digital angular measurement, adjustable mount, reference reticle, targeting system or PAPI-specific fixture. Select according to the PAPI manufacturer, installation procedure and airport authority requirements.

A Defined Visual Glide Path

FAA installation guidance requires the PAPI to be sited and aimed so the approach path provides the required obstacle clearance and threshold crossing height. Where a runway has a published instrument approach glide path or vertical descent angle, the PAPI path should be coordinated with it where practicable.

Optical Reference

Each light unit produces a red/white transition. Its centre is the light-beam centre used for vertical aiming. AC 150/5345-28H defines it as the transition band between red and white and requires built-in vertical adjustment across the specified range.

Reference Transfer

Transfer a known survey or installation reference to the PAPI light unit.

Installation Verification

Confirm the installed equipment corresponds to the approved design and adjustment settings.

Optical Characteristics to Review

• Magnification
• Field of view
• Reticle design
• Optical clarity
• Focusing range
• Target visibility
• Angular reference
• Sight-line stability

Higher magnification helps identify distant targets but narrows the field of view and increases vibration sensitivity. The appropriate specification depends on target distance and required accuracy.

Reticle and Angular Reference

The reticle may provide a central crosshair, angular reference, target alignment marks, horizontal and vertical references or calibration markings.

Angular measurement may use a mechanical scale, optical graduated scale, vernier, digital inclinometer, digital encoder or integrated display. For FAA PAPI aiming, meet the system's performance requirements rather than relying on the nominal resolution of a generic instrument.

Vertical Aiming

The units in a row are aimed at different elevation angles around the desired glide path to give the required red/white indications. FAA Order JO 6850.2C describes the siting and aiming criteria.

For a four-unit PAPI the setting angles are normally the approach angle θ −30′, θ −10′, θ +10′ and θ +30′. A 3° path therefore gives 2°30′, 2°50′, 3°10′ and 3°30′, which is why a 10-minute graduation is the minimum useful resolution. Confirm the settings, including those for a two-unit system, against the applicable Annex 14 or FAA edition.

The installation angle is set by approved approach geometry, obstacle clearance, threshold crossing height and design criteria. Use the device to reproduce the approved engineering value, not to select an angle independently.

Horizontal / Azimuth Alignment

The row of PAPI units is laid out perpendicular to the runway centreline, and each unit is aimed in azimuth parallel to the runway centreline (the vertical angle is a separate setting). A telescope can establish the line of sight related to the centreline, particularly where:

• The centreline is not visually obvious
• The PAPI is offset
• Terrain affects the installation
• Obstacle clearance needs a modified arrangement
• The installation is being relocated
• Existing alignment is under investigation

A Typical Inspection Package

• Alignment Telescope / Aiming Device — primary optical or angular alignment
• PAPI Check Stick — mechanical / reference verification
• Clinometer / Digital Inclinometer — supporting angular measurement
• Survey Equipment — runway and installation references
• PAPI Mounting Stand — stable positioning

Integrated Aiming Device

An integrated device may combine a telescope, angular scale, precision level, mounting bracket, PAPI interface, adjustable sighting axis, reference target, carrying case and calibration / verification documentation, reducing dependence on multiple independent references.

Still verify the specification against the applicable PAPI equipment standard.

Mounting Interface

• Stable attachment
• Minimal mechanical play
• Repeatable positioning
• Controlled adjustment
• Compatibility with PAPI light-unit geometry
• Secure locking after adjustment

Loose hardware can introduce apparent alignment changes not caused by the PAPI itself.

Environmental Conditions

• Temperature variation
• Wind and dust
• Moisture
• Solar exposure
• Mechanical vibration
• Transport impacts

For use on an operational airfield, confirm environmental suitability from the manufacturer's specification.

Verification May Include

• Optical reference check
• Angular scale verification
• Zero and level checks
• Reference target check
• Repeatability test
• Comparison against a known standard

FAA Testing

FAA PAPI requirements include testing the aiming device to demonstrate that light-unit movement is correctly indicated, with the measuring device checked at one-degree intervals from 2° to 8°. The device is controlled measurement equipment, not merely a viewing telescope.

Verify

• Instrument identification and condition
• Reference target condition
• Optical cleanliness
• Mounting stability
• Angular scale / measurement condition
• Zero / reference setting
• Applicable PAPI model
• Current installation procedure

Why It Matters

A damaged or incorrectly referenced aiming device can produce a consistent but incorrect alignment. After realignment or unit replacement, the authority may require a flight inspection or a notified unserviceability before the PAPI returns to service. Follow the airport procedure.

Typical Triggers

• Scheduled inspection
• Suspected alignment change
• Light-unit replacement
• Mounting adjustment
• Pavement rehabilitation or runway work
• Vehicle impact
• Severe weather
• Reported abnormal PAPI indication

Post-Impact

Units can be disturbed by maintenance vehicles, grass-cutting, snow-removal and service equipment, or construction. Even with an intact enclosure, a small alignment change can affect the optical indication. The device determines whether the unit has moved relative to its established reference. FAA-qualified PAPI units also include an excessive-tilt cut-out that switches the system off when a unit moves out of its aiming angle by a fraction of a degree, so a unit that is still operating can nevertheless be slightly out of alignment.

Mechanical

• Mounting interface
• Adjustment range
• Locking mechanism
• Reference surfaces
• Stand / tripod compatibility

Documentation

• Product datasheet and drawing
• Operating instructions
• Calibration / verification procedure
• Calibration certificate where required
• Compatibility documentation

FAA References

• AC 150/5345-28H — active PAPI equipment circular; requires an aiming tool meeting the specified range, increments and repeatability
• AC 150/5340-30J — visual-aid installation guidance
• Order JO 6850.2C — PAPI siting, approach-path coordination and aiming
• AC 150/5340-26C — Maintenance of Airport Visual Aid Facilities
• Engineering Brief #95 — Additional siting and survey considerations for PAPI and other visual glide slope indicators

Product Selection Should Distinguish

• PAPI-compliant aiming tool — meets the applicable PAPI equipment specification
• Alignment telescope — line-of-sight / reference alignment
• General survey instrument — broader site and positional references

One product may combine functions; confirm from its technical documentation.

Design and Verification

Design and verify the PAPI against the applicable edition of ICAO Annex 14, Volume I (9th Edition, July 2022, with Amendment 18 at the time of writing), the ICAO Aerodrome Design Manual (Doc 9157) Part 4, Visual Aids, and the aerodrome's national implementation requirements, such as DGCA CAR Section 4 in India.

The Specification May Define

• PAPI approach angle and threshold crossing height
• Light-unit spacing
• Survey tolerances
• Alignment method
• Inspection frequency
• Acceptance testing and documentation

Equipment

• Primary: PAPI Alignment Telescope / Aiming Device
• Supporting: PAPI Check Stick; Clinometer / Digital Inclinometer; Survey Equipment
• Installation: PAPI Mounting Stand

Documentation

• PAPI installation drawing
• Manufacturer maintenance manual
• Alignment procedure
• Survey data
• Inspection records
• Equipment calibration / verification records