Alignment Jig

An Alignment Jig is a specialised tool designed to check and adjust the angular orientation of airfield ground lighting (AGL) fixtures. Proper alignment is critical because each fixture has a defined beam pattern that must be aimed precisely to provide the intended visual guidance to pilots. Even a small angular error (e.g., 0.5°) can shift the beam centre, reducing light intensity at the critical viewing angle and potentially compromising safety.

Alignment jigs come in various forms, depending on the fixture type and the required accuracy:

• Mechanical Level Jigs: Simple tools with spirit levels and reference surfaces that attach to the fixture to check its physical level and tilt. Used for runway edge lights, threshold lights, and other fixtures where the beam is referenced to the horizontal plane.
• Optical Sighting Jigs: Feature a telescope or laser pointer that projects a reference beam. The jig is mounted on the fixture, and the operator checks that the reference beam aligns with a target at a known distance and height. This is common for approach lights and PAPI systems where precise elevation angles (e.g., 3°) must be verified.
• Digital Inclinometer Jigs: Incorporate an electronic inclinometer that displays the fixture's tilt angle digitally. These offer high accuracy (typically ±0.1°) and are used for fixtures that require precise angular settings, such as runway centreline lights or PAPI units.
• Fixture-Specific Jigs: Custom‑made tools designed for a particular fixture model. They engage with the fixture's optical housing or mounting points to provide a repeatable reference for alignment checks.

The alignment jig's purpose is to guarantee that a fixture's certified beam pattern is actually delivered where the airfield design intends it — because a fixture that is optically perfect but mounted a fraction of a degree out of true will put its peak intensity at the wrong angle, potentially leaving pilots with a dim, off-axis, or invisible light exactly where they need reliable guidance. By giving a technician a precise, repeatable reference for azimuth and elevation, the jig turns alignment from a matter of judgement into a verifiable, documented setting, whether the fixture is being installed for the first time, checked after ground settlement, or reassembled following a lens or LED module replacement. Its function is therefore less about the fixture itself and more about closing the gap between a design specification on paper and the actual light a pilot sees in the air.

Alignment jigs are deployed at various stages of the AGL lifecycle to ensure correct beam orientation:

• Installation and Commissioning: During new AGL system installation or major upgrades, alignment jigs are used to set the correct azimuth and elevation for every fixture according to the approved lighting layout plan. This ensures that the system provides the required visual guidance from the first day of operation.
• Routine Maintenance Inspections: As part of periodic maintenance (e.g., annual checks), alignment jigs are used to verify that fixtures have not shifted due to pavement settlement, ground movement, or mechanical damage. Any misalignment can be corrected before it affects pilot visibility.
• Post-Repair Verification: After a fixture has been repaired or replaced (e.g., lens replacement, LED module change), the alignment jig is used to confirm that the new components have not altered the beam direction. This is particularly important for fixtures with replaceable optical assemblies.
• Troubleshooting: When pilots report that a specific light appears dim or off‑centre, an alignment jig can quickly determine whether the fixture is misaligned or whether the issue is due to other factors (e.g., low intensity, contamination).
• Calibration of PAPI Systems: PAPI (Precision Approach Path Indicator) units require very precise elevation angle settings (typically 3° ± 0.1°). Specialised alignment jigs with spirit levels and sighting telescopes are used to set and verify the PAPI's glide slope angle during commissioning and periodic checks.

As a portable field tool, the alignment jig has no permanent airfield location — it is kept in the maintenance vehicle or tool store and carried out to each fixture as required. In use, it is temporarily clamped or seated onto the fixture's mounting flange or lens housing so its reference surfaces, spirit level, or sighting optics line up with the fixture's own optical axis, and it is removed once the check or adjustment is complete. For PAPI and other precision-angle work, the jig is set up alongside a fixed target or sighting point at a known distance and height so the elevation angle can be verified against that external reference. Between uses, the jig itself is periodically checked against a known-good reference to confirm it has not lost calibration from handling or vehicle transport.

The use of an alignment jig typically involves the following steps:

• Preparation: The jig is cleaned and checked for calibration (if it has built‑in instruments). The fixture is powered and allowed to stabilise, though alignment checks are often performed with the fixture off or at a low intensity to avoid glare.
• Mounting: The jig is attached to the fixture using the fixture's existing mounting points or a dedicated reference surface. For elevated fixtures, the jig may be clamped to the pole or the fixture housing. For inset fixtures, the jig is placed over the fixture or inserted into a recess.
• Alignment Reference: The jig provides a reference line or plane that corresponds to the fixture's optical axis. For mechanical level jigs, the operator adjusts the fixture until the spirit level shows the required tilt. For optical jigs, the operator sights through the telescope or aligns the laser spot with a target set at the specified distance and height.
• Adjustment: The fixture is adjusted (using its mounting screws or shims) until the jig indicates the correct orientation. For fixtures with two‑axis adjustment (azimuth and elevation), both axes are checked and set.
• Verification: The adjustment is checked again with the jig to ensure that the fixture has not moved during tightening. A final check is often performed with the fixture powered at the required intensity to confirm that the beam pattern appears correct.

The specific procedure depends on the fixture type and the jig design. For example, runway edge lights are typically aligned to be level (0° tilt) and aimed perpendicular to the runway centreline. Runway centreline lights are aligned to have their beam peak at a specific elevation angle (e.g., 4° above the horizontal) and no azimuth offset. PAPI units are set to a specific glide slope angle (e.g., 3°) using an inclinometer and sighting telescope.

The alignment jig itself carries no active electronics or control loop — it is a mechanical reference instrument, not a monitored subsystem. Its "architecture" is really the calibration chain that keeps its output trustworthy: a certified survey control point (or GPS-derived coordinate) establishes the geodetic reference, a theodolite, total station or laser level is set up and checked against that control point, and the jig is then mounted to the fixture base and read against the same reference. Because the jig has no self-monitoring, its accuracy depends entirely on periodic recalibration against a certified reference standard and on correct field procedure — there is no automated fault detection, so operator technique and calibration currency are the only safeguards against a systematically misaligned fixture.

The jig is machined from lightweight aluminium alloy or corrosion-resistant stainless steel, with precisely toleranced locating pins, slots, or a fixture-specific cradle that mates directly to the AGL fixture's mounting flange or base can to guarantee repeatable positioning. Depending on type, the jig integrates a bubble-level vial, an optical sighting scope, or a digital inclinometer module rigidly bonded to the reference frame so that any tilt reading reflects the true orientation of the fixture rather than jig flex. Adjustment screws or vernier scales allow fine-tuning of tilt and azimuth within the jig's rated angular range, with locking collars or clamps to hold the set position once achieved. The complete jig is supplied in a foam-lined carrying case sized to protect its calibrated accuracy and prevent damage to sighting optics or digital sensors during transport between fixture locations on the airfield.