PAPI Elevation Adjustment

The PAPI Elevation Adjustment is the mechanical system that allows the vertical (up/down) angle of a PAPI light unit to be set with extreme precision. This adjustment determines the angle of the red/white transition beam relative to the horizontal plane. For a standard fixed‑wing PAPI, the transition is set to 3° above the horizontal—the nominal glide slope. However, this angle can be adjusted to suit specific runway requirements, obstacle clearance needs, or aircraft types.

The elevation adjustment is typically a fine‑threaded mechanism integrated into the mounting base or a dedicated elevation sub‑assembly. It allows the entire PAPI unit to be tilted up or down in controlled increments, with a typical range of 2.5° to 4.5° and a resolution of 0.02° or better.

In a standard four‑unit PAPI system, each unit is set to a slightly different elevation angle to produce the characteristic red/white pattern:

• Unit 1 (far from threshold): lowest angle (e.g., 2°30′)
• Unit 2: 2°50′
• Unit 3: 3°10′
• Unit 4 (nearest threshold): 3°30′

This staggered arrangement creates the 2‑white/2‑red indication when the aircraft is on the correct 3° glide path.

The elevation adjustment is what actually encodes the glide slope into the PAPI array — without it, four identical light units would just be four identical light units, not an instrument. It is the fractions of a degree separating each unit's angle that create the boundaries a pilot reads as "too high," "on path," or "too low," so the mechanism's precision directly determines how precisely the visual indication matches the intended approach angle. A coarse or imprecise adjustment would blur those boundaries, widening the transition zone between red and white until the indication became ambiguous exactly where it needs to be most decisive — right around the correct glide path. Because the whole system's credibility rests on this handful of arc-minutes being set correctly on every one of the four units, the elevation adjustment is arguably the single mechanism responsible for whether a PAPI tells the truth about an aircraft's position on approach.

The elevation adjustment mechanism is a precision assembly designed for repeatable, accurate angle setting:

The elevation adjustment is a standard feature on all PAPI systems, with variations for different operational requirements:

• Fixed‑wing runways – typically set to 3° (2.5°–4° range), with each unit staggered for the 4‑light pattern.
• Helicopter PAPI – set to steeper angles, typically 4°–6°, to accommodate helicopter approach profiles.
• Runways with obstacle constraints – adjusted to steeper angles (e.g., 3.5°–4°) to provide obstacle clearance.
• Runways with noise abatement – steeper approaches can reduce noise footprint, requiring non‑standard elevation settings.
• Portable PAPI – with quick‑adjust mechanisms for rapid reconfiguration at temporary airfields.
• Calibration and maintenance – used whenever the glide slope angle needs to be verified or changed.

The elevation mechanism is built into the fixture head where it pivots on the mounting base, positioned so a technician can access the adjustment screw or graduated scale without dismounting the unit from its foundation. It's set during initial commissioning, after the mounting base and wing bar are already fixed in their final surveyed position, since the elevation angle only makes sense relative to a base that's already correctly sited and levelled. Each unit on the bar is adjusted individually to its own staggered angle (each unit typically offset by a fraction of a degree from its neighbours to produce the four-light pattern), and the mechanism is locked off once set so vibration or weather doesn't drift the angle between calibration checks. On portable PAPI, the same mechanism is designed for quicker access, since the whole system may be re-levelled and re-aimed each time it's redeployed.

The elevation adjustment process is a precise operation that requires care and the right tools:

• Initial Setup: The PAPI unit is mounted on its base with the elevation adjustment screws or shims in their neutral positions. The unit is loosely secured to allow tilting.
• Reference Measurement: A theodolite or electronic inclinometer is set up to measure the current elevation angle of the unit’s optical axis. The reference is typically the horizontal plane or a known survey point.
• Calculating the Required Angle: The required elevation angle is determined from the airfield’s AGL documentation. For a standard 3° glide path, the unit’s transition angle must be set to 3° (or a staggered value for each unit in the array).
• Adjustment: The adjustment screw is turned to raise or lower the front of the unit. Each full turn changes the angle by a known amount (e.g., 0.03° per turn). The adjustment is made incrementally, with the angle re‑measured after each turn.
• Locking: Once the correct angle is achieved, the locking bolts or nuts are tightened securely. The angle is verified one final time to ensure it did not shift during locking.
• Verification: For a complete PAPI system, the elevation of all four units is set to the staggered values. The system is then tested photometrically to ensure the red/white transition pattern is correct when viewed from the approach.

The adjustment is typically performed using a precision inclinometer or theodolite with an accuracy of ±0.01° or better, as even a 0.05° error can shift the glide path by several feet at the runway threshold.

Elevation adjustment, like azimuth, has no electrical control logic — it's a mechanical, technician-closed loop rather than an ALCMS-driven one. With the pivot hinge loosened, a theodolite or inclinometer measures the current elevation angle against the required glide-path value, and the adjustment screw is turned incrementally, with the angle re-measured after every turn, until the correct staggered value for that unit's position in the four-unit array is achieved and locked.

Because the four PAPI units in an array are each set to a slightly different elevation angle to produce the red/white transition pattern, this loop is repeated separately for every unit, and the final photometric check across all four is what actually confirms the array's electrical intensity control and the mechanical elevation settings are working together correctly.

Like its azimuth counterpart, the elevation mechanism is a fine-threaded assembly integrated into the mounting base or a dedicated sub-assembly beneath the PAPI unit, rather than a separate visible fitting. A precision-ground threaded rod or worm-gear arrangement tilts the unit up or down about a horizontal pivot axis as it is turned, with the fine thread pitch giving the sub-arc-minute resolution the glide slope setting requires. A calibrated scale or vernier alongside the adjustment point lets the technician read the current angle directly, or in combination with an alignment tool such as an inclinometer for the final precise setting. Once set, a locking collar, clamping plate, or lock nut is tightened against the adjustment thread to fix the angle permanently against wind loading and vibration, and all load-bearing parts are machined from corrosion-resistant metal so the mechanism continues to move smoothly and hold its setting through years of outdoor service.

For a standard 4‑unit PAPI system with a 3° nominal glide slope, the individual units are set to staggered elevation angles. The table below shows typical values:

The stagger creates a 1° spread across the four units, with 20 arcminutes (0.33°) between adjacent units. When the aircraft is on the 3° glide path, Units 2 and 3 appear white (above their transition angles), while Units 1 and 4 appear red (below their transition angles), giving the pilot the 2‑white/2‑red indication. The exact stagger values may vary depending on the manufacturer and specific installation requirements.