PAPI Alignment/Calibration Tool

The PAPI Alignment/Calibration Tool is a specialised instrument used by airfield maintenance teams to set the elevation (vertical angle) and azimuth (horizontal orientation) of individual PAPI light units. The tool ensures that the red/white transition occurs at the exact glide slope angle required—typically 3° for fixed‑wing runways or 4°–6° for helicopter PAPI—and that the beam is aligned with the runway centreline.

Several types of tools are used, depending on the accuracy required and the installation environment:

Modern airfields often use a combination of these tools, with the electronic inclinometer becoming the preferred choice for its speed, accuracy, and data‑logging capabilities.

The calibration tool exists to bridge the gap between the elevation and azimuth adjustment mechanisms being physically capable of fine positioning, and a technician actually achieving the exact angle those mechanisms were built for. A tolerance measured in arc-minutes cannot be judged by eye or by feel, no matter how experienced the technician — without a precision reference instrument, the adjustment mechanism's fine threads and locking hardware are simply unusable at the accuracy PAPI requires. The tool's real function is repeatability as much as accuracy: at commissioning, after a pole is disturbed by a storm or ground settlement, or during periodic re-verification, it lets the same exact angle be re-established with confidence, and — for the electronic inclinometer especially — with a logged record proving it was done. In effect, the calibration tool is what turns a mechanically adjustable fixture into one that reliably meets its designed glide slope angle in practice, not just on paper.

The alignment/calibration tool is an essential part of every PAPI installation and maintenance program:

• Initial installation – setting the beam angles of all four PAPI units during system commissioning.
• Periodic recalibration – verifying and adjusting angles as part of routine maintenance (typically every 6–12 months).
• After maintenance or replacement – re‑aligning a unit after lamp/LED replacement, optical assembly servicing, or frangible coupling replacement.
• After impact or storm damage – checking alignment after any event that could have shifted the unit.
• Runway surveys – used in conjunction with surveying equipment to establish reference points for PAPI placement.
• Quality assurance – verifying that the PAPI system meets ICAO/FAA photometric requirements.

Between calibration sessions, the tool is returned to its padded case with the jig, theodolite optics, or inclinometer sensor protected from knocks, since the sub-degree accuracy this tool provides depends on internal reference surfaces and sensors that a hard bump can throw out of true. Mechanical jigs are checked for flatness and squareness against a reference surface before each use, and any jig showing wear at its mounting or reference points is set aside for recalibration or replacement rather than used on the assumption it's still accurate. The electronic inclinometer's zero calibration is verified against a known flat reference periodically, and its battery is checked before a commissioning or recalibration session where a mid-sequence power loss would mean re-measuring units already completed. All tool types are kept dry and within their rated storage temperature range, and any stored calibration data on the electronic inclinometer is backed up or transferred before the unit is reused for a different runway's PAPI array, so records from one site aren't inadvertently mixed with another's.

The alignment process varies depending on the tool type, but the fundamental steps are similar:

• Setup: The tool is mounted or positioned relative to the PAPI unit. For mechanical jigs, the jig is clamped onto the unit’s optical window or housing. For theodolites, the instrument is set up on a tripod at a known distance from the PAPI. Electronic inclinometers are placed on a flat reference surface on the unit.
• Reference Setting: A reference point or angle is established. For example, the theodolite may be aimed at a survey marker on the runway threshold to set azimuth. The inclinometer may be zeroed on a known horizontal surface.
• Angle Measurement: The tool reads the current elevation (vertical angle) and azimuth of the PAPI unit. The measured angle is compared with the required glide slope angle (e.g., 3°). The difference is noted.
• Adjustment: The PAPI unit’s elevation and azimuth adjustments (on the mounting base) are loosened, and the unit is rotated or tilted until the tool indicates the correct angles. This is done incrementally, with re‑checking after each adjustment.
• Locking & Verification: Once the correct angles are achieved, the adjustment mechanisms are locked. The tool is used again to verify that the angles have not shifted during locking.
• Documentation: The final measured angles are recorded (often automatically by electronic tools) for maintenance logs and future reference.

For a complete PAPI system, each of the four units is aligned individually, with the elevation angles set to produce the characteristic red/white transition pattern (e.g., 2°30′, 2°50′, 3°10′, 3°30′ for a 3° glide path).

The calibration tool sits outside the airfield's electrical control system entirely — it has no place in the ALCMS/CCR command chain that runs the PAPI's lamps day to day. Its "architecture" is instead a manual measure-adjust-verify loop, with the technician acting as the controller and the tool providing the reference measurement at each step: read the current angle, compare it against the required glide-path value, adjust, and re-measure, repeating until the reading holds within tolerance before locking the adjustment down.

This is a fundamentally different kind of control loop from the ones elsewhere in the AGL system — there is no continuous automatic feedback, no ALCMS visibility into it, and no fault reporting; the loop closes only for as long as the technician is actively cycling through it, and its result (the locked mechanical angle) is what every subsequent electrical control function in the PAPI unit then depends on.

Build varies by tool type but each is designed to mount directly on, or reference against, the PAPI unit being set. A mechanical jig is a rigid metal template or fixture, machined to seat squarely against the PAPI unit's mounting face, with a flat reference surface and an integral bubble level or protractor scale that the technician reads directly. An optical theodolite is a precision instrument on its own levelling tripod, with a rotating telescope on a graduated horizontal and vertical circle, allowing the operator to sight a surveyed reference point and read off the angle to fine resolution independent of contact with the PAPI unit itself. An electronic inclinometer is a compact, sealed housing containing a solid-state angle sensor, digital display or Bluetooth output, and battery, designed to sit flat against a machined reference face on the PAPI unit and provide a direct digital angle reading; some versions log or transmit the reading for calibration records. All three are built for repeated field use and are stored and transported in a protective case to preserve their calibration accuracy between uses.