Visual Glide Slope PAPI / VASI

Visual Approach Slope Indicator (VASI) and Precision Approach Path Indicator (PAPI) are light systems that provide pilots with visible glide-slope guidance during the final approach segment. They are located beside the runway threshold and project a pattern of red and white light, enabling the pilot to determine whether the aircraft is above, on, or below the optimal descent path.

PAPI has largely replaced VASI on modern airfields because it offers more precise guidance with a single row of lights (typically four units) that all show the same colour for a given angle, whereas VASI uses a two-bar system with separate upwind and downwind bars. Both systems serve the same purpose: preventing approaches that are too high (risk of overshoot) or too low (risk of obstacle collision).

The purpose of PAPI and VASI systems is to solve a problem that runway and approach lighting alone can't: telling a pilot not just where the runway is, but whether they are actually descending at the correct angle to reach it safely. An approach that looks visually fine to the eye can still be dangerously high (risking a long landing or a go-around) or dangerously low (risking obstacle or terrain collision short of the runway), and neither error is easy to judge from the runway's appearance alone, especially at night or in marginal visibility. By encoding glide-slope deviation directly into a simple red/white colour pattern that requires no calculation or instrument cross-check, PAPI (and its predecessor VASI) gives the pilot instantaneous, continuous feedback throughout the approach — which is why PAPI's single-row, four-unit design has displaced VASI's older two-bar system on most modern airfields, since a single glance at one row of lights is faster and less ambiguous to interpret than comparing two separate bars.

Visual glide slope systems are installed on the approach side of runways that serve instrument or visual approaches, and are particularly important for precision approaches:

• PAPI: Typically a row of four light units, each projecting a red or white beam. Installed on the left side of the runway (or both sides for some configurations) at a defined distance from the threshold.
• VASI: A two-bar system (near and far) showing red/red when below the glide path, white (near)/red (far) when on the correct path, and white/white when above it. Now less common, but still in use on some smaller or older airfields.
• APAPI: Abbreviated PAPI with only two light units, used on non-precision or visual-only runways.
• Tri-colour or PVASI: Other visual slope indicators, though PAPI is the international standard.

The system is sized and sited according to the approach category of the runway (e.g., CAT I, II, III) and the specific geometry of the approach obstacle-limitation surface.

The four PAPI units are installed in a straight row perpendicular to the runway centreline, on the left side of the runway abreast of the touchdown zone at a fixed distance from the threshold, with even spacing between units set by the applicable design standard so the row reads as one continuous bar rather than separate lights. Each unit's foundation and frangible mounting are surveyed and levelled precisely, since PAPI's entire function depends on every unit being set to the exact same glide-slope elevation angle — the units are not simply bolted down and left, but individually adjusted and checked against the design glide path using a theodolite or the manufacturer's alignment tool before being accepted into service. Cabling to each unit runs in duct or direct-buried conduit back to its own isolating transformer and onward to the circuit's CCR, following the same series-circuit wiring practice as other precision approach lighting so all four units can be intensity-stepped together. Where VASI's older two-bar layout is retained on a legacy installation, the near and far bars are similarly surveyed to their respective design angles, with the greater physical separation between the two bars making precise siting even more critical to the system reading correctly from the cockpit.

Each PAPI unit contains a lamp, a red filter, and a lens or reflector that projects a beam with a sharp cut-off angle. The beam is aimed so that below the glide path the pilot sees red, and above the glide path white. With four units, the pilot sees a combination of red and white lights that indicates the deviation from the nominal 3° glide slope.

Key functional aspects include:

• Colour Coding: On a four-unit PAPI, the number of white vs. red lights indicates the deviation:
  • 4 White: Too high (steep approach)
  • 3 White / 1 Red: Slightly high
  • 2 White / 2 Red: On glide slope (correct)
  • 1 White / 3 Red: Slightly low
  • 4 Red: Too low (shallow approach)
• Light Source: Traditionally halogen lamps, but increasingly LED modules for longer life and lower maintenance.
• Power Supply: Fed from the AGL series circuit (like other runway lighting) via isolating transformers, with intensity steps controlled from the CCR.
• Adjustment: Each unit can be mechanically tilted to set the exact glide-slope angle (typically 3°).
• Frangibility: PAPI units are mounted on frangible supports so they break away on impact, reducing the risk to aircraft.

The PAPI or VASI light bar is controlled the same way a runway edge light circuit is: at the loop level, not per fixture. All units in the bar — typically four for PAPI — are wired into the same constant current series circuit and fed through their own isolating transformers, so a single ALCMS-commanded brightness step changes every unit's intensity simultaneously. This is not incidental — a pilot reading the four-light pattern is comparing red and white across units in the same glance, so if one unit ran at a different intensity to the others (rather than simply switching between red and white internally) the comparison itself would be unreliable. The glide-path angle each unit indicates is set mechanically during aiming and calibration, not by the ALCMS; the control system's only real-time influence is the shared brightness step and on/off state for the whole bar, with loop-level fault detection identifying an open circuit or failed transformer the same way it would on any other series circuit.

Each PAPI or VASI unit is a weatherproof, elongated housing, typically die-cast aluminium, containing two or three separate light sources (halogen or LED) each viewed through its own precisely positioned optical system. A split-colour lens or a combination of a red filter over the lower portion of each lamp's beam and clear glass over the upper portion, positioned to a fraction of a degree of accuracy, creates the sharp red/white transition that defines the glide path angle for that unit. The housing mounts on a frangible support structure similar to elevated runway edge lights, engineered to break away cleanly under aircraft impact, with the unit's angular tilt precisely set and locked using shims or an integrated adjustment mechanism during commissioning to establish the correct glide path. The optical compartment is gasket-sealed against the housing to protect the lamps and precision optics from moisture and dust, and the units in a wing bar are installed on a common baseline with tightly controlled spacing and levelling so their combined light pattern reads consistently from the approach.