PAPI

The Precision Approach Path Indicator (PAPI) is a visual guidance system that helps pilots maintain the correct glide slope during final approach. It consists of four light units arranged in a row, typically installed on the left side of the runway (or both sides for Category II/III). 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.

PAPI is the most widely used visual glide slope system worldwide, providing a simple, unambiguous signal that requires no special equipment on the aircraft. It is essential for both precision and non‑precision runways, enhancing safety and operational efficiency during all weather conditions.

PAPI's purpose is to let a pilot self-correct a descent angle to within a very tight margin using only a glance at the runway, without any onboard glide slope receiver or electronic aid — a capability that matters on every approach, but becomes essential in marginal weather, at night, or when an ILS glide path isn't available or isn't being used. The four-unit row translates a continuous descent-angle deviation into a simple, countable pattern of red and white lights, so a pilot can judge not just whether they're high or low but roughly by how much.

This combination of precision and universality — no aircraft equipment required, immediately readable by any pilot regardless of aircraft type — is why PAPI has become the default visual glide slope system at the overwhelming majority of the world's runways, serving as either the primary glide path reference or a critical backup to an electronic system.

PAPI is deployed on a wide range of runways and heliports:

• Precision approach runways (CAT I, II, III) – providing primary visual glide slope guidance.
• Non‑precision runways – enhancing approach safety for visual and circling approaches.
• Military airfields – supporting tactical and training operations.
• Heliports – providing glide path guidance for helicopter approaches (helicopter PAPI).
• Regional and general aviation airports – improving approach accuracy and safety.
• Runways with terrain or obstacle constraints – where a defined glide path is critical.

The wing bar of four light units is mounted on one side of the runway (normally the left, viewed from the approach), at a fixed lateral offset from the runway edge chosen to keep the units clear of the obstacle-limitation surface, wingtip clearance envelope, and any blast zone. The bar is set back from the threshold along the runway at a distance derived from the design tables so the visual glide path crosses the threshold at the standard eye-to-wheel height for the aircraft the runway serves, all tied into the runway centreline and threshold survey. Each unit sits on its own frangible mount at equal lateral spacing along the bar, and is individually aimed in elevation during commissioning so the four units together produce a sharp two-red/two-white split exactly on the design glide angle, with the transition band between units kept tight. Cabling runs underground from each unit back to the constant current regulator, and the units and mounting frame must stay within the frangibility requirements for the runway's code number.

PAPI operates on a simple optical principle that provides immediate visual feedback to the pilot:

• Optical Design: Each light unit contains a lamp (halogen or LED) and a precision optical system that projects a beam with a distinct red/white transition. The transition angle is set to the desired glide slope (typically 3°).
• Colour Indication: When the aircraft is on the correct glide path, the pilot sees two red lights (the units closest to the runway) and two white lights (the units farthest from the runway). Above the path, more white lights appear; below, more red lights appear. The system provides a four‑light display with the following meanings:
  • All white – aircraft is well above the glide path.
  • 3 white / 1 red – slightly above the glide path.
  • 2 white / 2 red – on the correct glide path.
  • 1 white / 3 red – slightly below the glide path.
  • All red – well below the glide path.
• Power and Control: PAPI units are powered from the constant current series circuit via isolating transformers, with intensity controlled by the CCR (typically 3‑step or 5‑step). Brightness can be adjusted to suit ambient light conditions (day, night, twilight).
• Integration: PAPI is often integrated with runway approach lighting systems and may be controlled remotely via the ALCMS. Some systems include automatic brightness adjustment based on ambient light sensors.

PAPI has no per-unit electronic addressing — the "control logic" that produces the glide path signal is set optically at commissioning, not by any processor. Each of the four light units is levelled and its internal lens/shutter assembly is precisely tilted so its red/white transition occurs at a specific elevation angle, with the units aimed a fraction of a degree apart from one another so that, taken together, they straddle the design glide path (typically 3°). Once set, that elevation is fixed hardware, not a software parameter. All four units are powered from the same constant current series circuit through their own isolating transformers, with the ALCMS or local CCR setting the shared brightness step for day, twilight, or night; that command changes how bright the whole array is, never which unit shows which colour. Where automatic brightness control is fitted, an ambient light sensor feeds the CCR/ALCMS to select the appropriate step. Loop monitoring works the same way as any series circuit — the CCR watches loop voltage against commanded current, and a rise flags an open-circuit lamp or failed transformer as a loop fault for maintenance to isolate, rather than the system detecting or correcting a misaimed unit itself. Optical alignment is verified periodically by ground or flight check, not by any feedback sensor in the fixture.

Each of the four PAPI units is a rectangular, box-section housing in die-cast aluminium alloy or louvred galvanised steel, weatherproofed and finished to resist UV degradation and de-icing fluid over years of outdoor service. Inside, the light source sits behind a two-part optical system: a lower red filter glass and an upper clear (white) glass, precisely separated so the beam transitions sharply between colours at the set glide-path angle, with a mechanical spirit-level and micrometer adjuster built into the housing so each unit can be levelled and pitched to within fractions of a degree during commissioning. The housing bolts to a frangible mounting mast set at a fixed, surveyed height above the runway shoulder, positioned so a shear point lets the mast break away cleanly if struck by an aircraft or vehicle. Primary cabling runs underground in conduit from each unit to its own isolating transformer, housed in a nearby L-867/L-868 base can, with sealed direct-burial connectors and a drainage provision to keep moisture away from the transformer and cable entries.