VASI

The Visual Approach Slope Indicator (VASI) is a visual guidance system that helps pilots maintain the correct glide path during final approach. It consists of a series of light units arranged in bars (typically two or three bars) on the left side of the runway. Each bar projects a red or white light depending on the pilot's viewing angle. The combination of red and white bars indicates whether the aircraft is above, below, or on the correct glide slope.

VASI is one of the oldest and most widely used visual glide slope systems. It provides a clear, unambiguous signal that requires no special equipment on the aircraft. The system is simple, reliable, and cost‑effective, making it suitable for a wide range of airports, from small general aviation fields to larger commercial airports.

VASI exists to answer one question for an approaching pilot — am I on the correct glide path, or am I too high or too low — using nothing more than red and white light and the aircraft's own eyes, without requiring any onboard equipment or electronic glide slope receiver. This makes it usable by any aircraft regardless of avionics fit, which is part of why it became one of the most widely adopted visual glide slope systems at airports of every size.

The multi-bar arrangement gives the pilot a graduated read: the relationship between the near and far bars shifts as the aircraft moves above or below the ideal path, letting a pilot correct smoothly and early rather than discovering a deviation only once it has become significant, which is especially valuable during the final stages of approach when altitude for correction is limited.

VASI is deployed on a wide range of runways and landing sites:

• Non‑precision runways – providing essential visual glide slope guidance.
• General aviation airports – where cost and simplicity are key.
• Military airfields – supporting training and operational approaches.
• Regional and remote airstrips – with limited infrastructure.
• As a backup to PAPI – on precision runways for redundancy.
• Heliports – where a simple glide path reference is needed.

VASI uses two light bars — a near (downwind) bar and a far (upwind) bar — mounted on one side of the runway, normally the left, at a fixed lateral offset from the runway edge clear of the obstacle-limitation surface and blast zones. The bars are set back from the threshold at distances derived from the design tables, spaced apart along the runway so that on the correct glide path the near bar shows white and the far bar shows red, with the vertical separation between the two producing the visual "on-slope" cue. Setting-out is tied to the runway centreline and threshold survey so both bars sit on the same design glide path, and each unit within a bar is mounted on its own frangible coupling and individually aimed and levelled during commissioning. Cabling from each bar runs underground back to its constant current regulator, and on wide-body runways a second pair of bars may be installed on the opposite side to give a full box configuration.

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

• Configuration: VASI typically consists of two or three light bars (e.g., 2‑bar VASI or 3‑bar VASI). Each bar contains multiple light units that project a beam with a distinct red/white transition. The bars are positioned at different distances from the runway threshold and set to slightly different angles.
• Colour Indication: The pilot sees a combination of red and white bars. For a standard 2‑bar VASI (one near bar and one far bar):
  • Both white – aircraft is well above the glide path.
  • Near bar white / Far bar red – on the correct glide path.
  • Both red – aircraft is well below the glide path.
  For a 3‑bar VASI, the indication is more nuanced, with the middle bar providing a reference for specific aircraft types (e.g., high‑wing vs. low‑wing).
• Power and Control: VASI 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 for day, twilight, and night operations.
• Integration: VASI is typically deployed independently or integrated with the runway lighting system. It may be controlled remotely via the ALCMS if connected.

VASI's glide path signal is generated entirely by fixed optics, with the circuit doing nothing more than powering the lamps. Every light unit's lens/shutter assembly is levelled and tilted at commissioning so its red/white transition falls at a specific elevation, and the bars — near and far in a 2-bar system, or near/middle/far in a 3-bar system — are aimed at slightly different angles from each other so that, together, they straddle the design glide path. That geometry is permanent hardware alignment, checked periodically by ground or flight inspection, not something any control system adjusts in real time. All units across all bars are powered from the same constant current series circuit through their own isolating transformers, and the CCR (or ALCMS where connected) sets one shared brightness step for day, twilight, or night — a command that changes overall brightness only, never which bar or unit shows red or white. Loop-level fault detection works as on any series circuit: the CCR watches loop voltage against commanded current, and a rise reports an open lamp or failed transformer as a loop fault, sectionalised by further testing rather than pinpointed automatically.

Each VASI unit in a wing bar is a wide, box-section housing in die-cast aluminium alloy or louvred galvanised steel, containing several lamp/optic pairs side by side so the bar presents as a single elongated light source rather than a row of separate fixtures. The optics are split into an upper clear (white) glass and a lower red filter glass, precisely positioned so the beam transitions from white to red at the set glide-path angle, with a built-in levelling and pitch-adjustment mechanism used to aim each bar during commissioning. Downwind and upwind bars are mounted on separate frangible masts set at surveyed heights either side of the touchdown zone, engineered to shear away cleanly if struck. Cabling runs underground in conduit to isolating transformers housed in nearby base cans, with sealed, direct-burial connectors and a drainage provision to keep the transformer and cable entries free of standing water.