PAPI Light Unit

The PAPI Light Unit is a precision optical fixture that serves as the fundamental building block of a PAPI system. Each unit is a self-contained assembly comprising a light source (halogen lamp or LED module), a precision optical system (reflector and prism/lens), a housing with integral frangible mounting, and an electrical interface (isolating transformer connection).

The unit projects a tightly collimated beam with a sharp red-to-white colour transition at a precisely defined angle. When installed as part of a four‑unit PAPI array, the individual units are set to slightly different elevation angles so that the pilot sees a combination of red and white lights that indicates their position relative to the 3° glide slope. Each unit is designed for long service life, low maintenance, and reliable performance in extreme environmental conditions.

The PAPI light unit's purpose is to give a pilot instant, unambiguous feedback on whether they are above, on, or below the correct glide slope, encoded entirely in colour rather than requiring any instrument interpretation — a red light means too low, white means too high, and the specific mix of red and white across the four-unit array tells the pilot exactly how far off the 3° path they are. This design exists because glide slope deviation is one of the most consequential and time-critical pieces of information a pilot needs on approach, and a system that can be read correctly in a fraction of a second, without conscious calculation, removes a significant source of workload and error at exactly the phase of flight where both are most dangerous.

A PAPI light unit consists of several precision-engineered sub-assemblies working together to produce the characteristic red/white beam pattern:

PAPI light units are installed in arrays at airfields worldwide, on a variety of runway types and operational environments:

• Precision approach runways (CAT I, II, III) – as part of the primary visual glide slope system, typically on the left side of the runway.
• Non‑precision runways – providing essential visual guidance for circling and visual approaches.
• Military airfields – supporting tactical and training operations where accurate glide slope control is critical.
• Heliports – in a specialised configuration (Helicopter PAPI) with steeper beam angles (4°–6°).
• Temporary or portable deployments – using battery‑powered or solar‑powered units for rapid setup.
• Runways with obstacle constraints – where a defined glide path is essential for safe obstacle clearance.

Each of the four units is mounted on its own base at a precisely surveyed position and elevation, since the array's glide slope indication depends on the units being co-planar and set at the correct height relative to the runway threshold and touchdown zone. The units sit far enough back from the runway edge to remain clear of wingtip and blast clearances while still falling within the obstacle-limitation surface requirements for the runway category, with a frangible mounting used given the array's position close to the operational runway strip. Each unit's elevation angle is set during commissioning using a digital level or similar precision instrument, calibrated against the runway's specific glide path angle (commonly 3°) and re-verified periodically, since even a small tilt error shifts the visual glide slope the array presents to an approaching pilot. Power and control cabling runs from the array back to its own dedicated regulator circuit, with cable routing and pit layout planned to keep the four units' wiring runs matched so brightness steps switch in unison across the full array.

The PAPI light unit operates on a precise optical principle to provide unambiguous glide slope information to the pilot:

• Beam Projection: The light source emits a broad beam that is collected and collimated by a parabolic reflector. The collimated beam then passes through a precision prism or lens that creates a sharp transition between white light (above the transition angle) and red light (below the transition angle).
• Colour Transition: The transition angle is set to the desired glide slope (typically 3°). The white sector of the beam is directed above the transition angle, while the red sector is directed below. The transition is sharp (typically less than 3 arc‑minutes) to provide an unambiguous indication.
• Intensity Control: The unit receives power from the isolating transformer on a constant current series circuit. The intensity is controlled by the CCR, typically offering 3 or 5 brightness steps (day, twilight, night). Some LED units include automatic intensity adjustment based on ambient light.
• Array Configuration: In a standard four‑unit PAPI, the units are set to slightly different elevation angles so that the pilot sees:
  • All white – well above the glide path.
  • 3 white / 1 red – slightly above.
  • 2 white / 2 red – on the correct glide path.
  • 1 white / 3 red – slightly below.
  • All red – well below.
• Frangible Protection: The mounting includes a frangible coupling that breaks on impact, allowing the unit to shear away from the pole without causing significant damage to an aircraft that strikes it.

A PAPI light unit's electrical control is identical in principle to the runway edge light's — the ALCMS or control unit selects a brightness step, and the CCR delivers the corresponding current through the unit's isolating transformer — but the four units in the array are never controlled to show different things from each other electrically. What produces the red/white transition pattern the pilot reads is a fixed optical setting, not a control signal: each unit's elevation angle is staggered at installation, so the same shared current produces a white or red indication depending purely on where the pilot's eye falls relative to that unit's own transition angle.

This means the array's glide-slope information comes from four units acting in electrical lock-step but optical independence — the control system only ever adjusts overall brightness together, while it's the mounting-base alignment of each unit that determines whether the pilot sees 2 white/2 red on the correct path or something else.

The sub-assemblies come together inside a single die-cast aluminium or composite body, machined internally to hold the reflector and light source at a fixed, precisely toleranced optical alignment so the beam's angle and sharp red/white transition are set by the housing geometry itself rather than by field adjustment of the internals. The prism or lens sits at the front behind the optical window, both gasket-sealed to the housing with O-rings to keep the internal optics free of dust and moisture, while a pressure-equalisation valve or moisture vent lets the sealed cavity breathe with temperature changes without drawing in liquid water. At the rear, the electrical interface — the secondary connector and any surge protection or monitoring contacts — passes through its own sealed gland into the housing, and the frangible mounting foot bolts to the underside, positioned so the coupling's designed shear point sits below the optical assembly and protects it from a lateral impact. Fasteners throughout are corrosion-resistant to hold the assembly rigid, since any flex between housing and optics would shift the beam angle away from its calibrated setting.