APAPI

The Abbreviated Precision Approach Path Indicator (APAPI) is a visual glide slope guidance system consisting of two light units arranged in a row, typically installed on the left side of the runway. Each unit projects a red or white light depending on the pilot's viewing angle. The combination of red and white lights indicates whether the aircraft is above, below, or on the correct glide path, but with fewer lights than the standard four‑unit PAPI.

APAPI provides a simplified but effective glide path reference, making it ideal for smaller airports, general aviation fields, heliports, and temporary installations. It is also used as a backup system on larger runways. The two‑unit configuration reduces cost and footprint while still delivering clear guidance for visual approaches.

APAPI exists to bring the same red/white, above/below/on-path logic that pilots already trust on standard PAPI to airfields where a full four-unit installation isn't warranted — smaller general aviation strips, heliports, or temporary sites. Using two light units instead of four means the pilot reads a coarser indication of glide slope deviation, but the fundamental colour-transition principle stays identical, so no new interpretation skill is required.

Because it mirrors standard PAPI so closely, APAPI also works well as a backup or supplementary indicator on larger runways, giving controllers and pilots an additional visual reference at reduced cost and physical footprint without introducing a system pilots would need to learn to read differently.

APAPI is deployed on a variety of airfields and landing sites:

• General aviation airports – providing cost‑effective glide slope guidance.
• Heliports – where a compact system is needed for helicopter approaches.
• Regional and remote airstrips – with limited electrical capacity or space.
• Military forward operating bases – requiring rapid deployment.
• As a backup to PAPI – on precision runways for redundancy.
• Temporary or emergency runways – where a full PAPI is not justified.

The wing bar consists of two light units mounted on a single frangible mast, positioned on one side of the runway (normally the left, viewed from the approach) at a fixed lateral offset from the runway edge, clear of the obstacle-limitation surface and any propeller or jet-blast zone. The bar is set back from the threshold along the runway a distance chosen from the design tables to place the visual glide path over the threshold at the standard eye-to-wheel height, tied into the runway centreline and threshold survey so the resulting glide angle matches the promulgated approach path. Each unit sits on a frangible coupling at a set height and is individually aimed and levelled during commissioning to give a sharp red/white transition at the design angle. Cabling runs underground from the bar back to its constant current regulator, and units are set out with enough separation from the runway strip edge that a frangible failure does not intrude on the graded area.

APAPI operates on the same optical principle as PAPI but with two light units:

• Optical Design: Each light unit contains a lamp and a precision optical system that projects a beam with a sharp red/white transition. The transition angle is set to the desired glide slope (typically 3°).
• Colour Indication: With only two units, the indication is coarser but still intuitive:
  • Both white – aircraft is well above the glide path.
  • Near unit red / far unit white – on the correct glide path. The unit nearest the runway is set to a higher transition angle than the far unit, so on the design glide path the near unit still reads red and the far unit has already gone white. This is the same near-red/far-white convention used by the standard four‑unit PAPI, just with only one pair of units instead of two.
  • Both red – aircraft is well below the glide path.
• Power and Control: APAPI units are powered from the constant current series circuit via isolating transformers, with intensity controlled by the CCR (typically 3‑step). Brightness can be adjusted for day, twilight, and night operations.
• Integration: APAPI is often deployed independently or integrated with the runway lighting system. It may be controlled remotely via the ALCMS if connected.

APAPI carries the same control architecture as full PAPI, simply with fewer units to power and aim. Each of the two light units is levelled and its lens/shutter tilted so its red/white transition sits at a specific elevation, with the near unit set to a fractionally higher transition angle than the far one so the near unit reads red and the far unit reads white on the design glide path; that split is fixed optical hardware set at commissioning, not a runtime signal. Both units draw from the same constant current series circuit through their own isolating transformers, so the CCR (or ALCMS, if the site connects one) commands a single shared brightness step for the whole two-light array — it has no way to change which unit shows red versus white, only how bright both burn. Loop monitoring follows the standard series-circuit pattern: the CCR watches loop voltage against commanded current, and a rise reads back as a lamp or transformer fault on that loop, not a pinpointed unit or a misalignment — those are only caught by periodic ground or flight check of the fixed optical aim.

Each of the two APAPI units shares the same rectangular, box-section housing used on full PAPI installations, in die-cast aluminium alloy or louvred galvanised steel, weatherproofed against UV exposure and de-icing fluid. The light source sits behind a lower red filter glass and an upper clear (white) glass, split at a precise internal line so the beam changes sharply from red to white at the set glide-path angle, with a built-in spirit level and micrometer pitch adjuster used to set each unit during commissioning. The housing mounts on a frangible mast at a fixed, surveyed height, engineered to shear away cleanly on impact, and is typically installed on runways too narrow or too lightly trafficked to justify a full four-unit wing bar. Cabling runs underground in conduit to a dedicated isolating transformer per unit, housed in a nearby base can with sealed, direct-burial connectors and a drainage provision to protect the transformer and cable entries from moisture.