T‑VASIS

The T‑Visual Approach Slope Indicator System (T‑VASIS) is a visual glide slope guidance system built from 20 light units, all of which emit white light in normal operation — unlike PAPI and VASI, T‑VASIS does not rely on a red/white colour split. The units are arranged as a central "wing bar" (also called the crossbar) of four lights running across the approach direction (the "top" of the T), together with two "fly lines" (also called stems) of lights running down the approach path on either side of it that light up progressively to show "fly-up" or "fly-down" corrections. The T‑shaped configuration provides a more precise and intuitive graduated indication of the aircraft's position relative to the correct glide path, particularly for larger aircraft with higher cockpit eye heights.

T‑VASIS was originally developed in Australia to give pilots of larger aircraft a more precise, graduated glide-path read than a simple two-colour bar, by encoding how far off path an aircraft is directly in the number of fly-up or fly-down lights illuminated alongside the wing bar. It is normally set to the same standard 3° glide path as PAPI and VASI; like any VGSI, the angle can be adjusted (typically up to around 4°) at sites with specific obstacle clearance or noise abatement requirements. It was historically used at military airfields and some commercial airports, though ICAO Annex 14 has recommended discontinuing T‑VASIS and AT‑VASIS as standard systems since 1 January 2020, so it is increasingly found only at legacy or military installations.

T-VASIS exists to give pilots a more precise and immediately readable glide slope indication than a simple two-colour bar system, using the number of illuminated fly-up or fly-down lights as the signal: how many lights show alongside the wing bar, and on which side, tells the pilot at a glance not just whether they're on path but roughly how far off and in which direction. This matters most for larger aircraft with higher cockpit eye heights, where the viewing geometry to a standard PAPI or VASI bar can be harder to judge precisely.

Where a steeper angle is needed for obstacle clearance or noise abatement, T‑VASIS — like any VGSI — can be set up to around 4° instead of the standard 3°, giving airport planners a way to steepen the visual glide path over a specific approach without redesigning the whole runway environment or necessarily changing the electronic glide path if one exists.

T‑VASIS is deployed on runways where a steeper glide path or enhanced precision is required:

• Military airfields – supporting tactical approaches and training for large transport aircraft.
• Commercial airports with obstacle constraints – where terrain or buildings require a steeper approach angle.
• Airports with noise abatement procedures – where steeper approaches reduce noise footprint.
• Runways serving large transport aircraft – such as C‑17, C‑130, or commercial wide‑bodies.
• Airfields with high approach speeds – where a more precise glide path reference is beneficial.
• As a complement to ILS – providing visual backup to instrument guidance.

The T-VASIS array is mounted on one side of the runway (normally the left), set back from the threshold at the distance needed to place the wing bar's on-slope indication over the threshold at the correct height, and tied into the runway centreline and threshold survey. The wing bar and the two fly-up/fly-down lines of lights are set out together as a single surveyed group, symmetrically about the runway centreline, so the T shape reads correctly from the cockpit at the design glide angle, with each unit on its own frangible mount at a fixed lateral offset from the runway edge, clear of the obstacle-limitation surface. Every unit is individually aimed and levelled during commissioning so the fly-up and fly-down lights illuminate in the correct sequence as the aircraft deviates above or below the glide path. Cabling runs underground from each unit back to its constant current regulator, and the larger footprint compared to PAPI or VASI — T-VASIS installations typically extend around 500 m along the runway from just beyond the threshold — means clearance and frangibility requirements are checked across the full array, not just a single bar.

T‑VASIS operates on a fundamentally different principle from PAPI and VASI: instead of a colour transition, it signals deviation through which and how many white lights are illuminated, with red reserved only as a severe-deviation warning. The system consists of:

• Configuration: Twenty light units arranged symmetrically about the runway centreline, in the form of two "wing bars" of four lights each (one wing bar on each side, together forming the horizontal top of the T) and two bisecting longitudinal "fly lines" of six lights each (forming the vertical stem of the T on each side). All units project white light in normal operation.
• Colour Indication: The pilot reads the number and position of illuminated lights, not a colour split:
  • On the correct glide path: Only the wing bar is visible, steady white. No fly-up or fly-down lights are lit.
  • Above the glide path: The wing bar plus one, two, or three "fly-down" lights are visible — the more fly-down lights lit, the further above the slope.
  • Below the glide path: The wing bar plus one, two, or three "fly-up" lights are visible — the more fly-up lights lit, the further below the slope.
  • Well below the glide path: The wing bar and fly-up lights change from white to red as a distinct, higher-severity warning that the aircraft is significantly low.
• Configurable Approach Angle: Like PAPI and VASI, T‑VASIS is normally set to the standard 3° glide path; it can be set up to around 4° at sites with obstacle clearance or noise-abatement needs.
• Power and Control: T‑VASIS units are powered from the constant current series circuit via isolating transformers, with intensity controlled by the CCR (typically 3‑step or 5‑step).
• Integration: T‑VASIS is typically deployed independently or integrated with the runway lighting system and ALCMS.

T‑VASIS encodes its signal entirely through fixed optical geometry across two arrays that share one control layer. The wing-bar units are aimed to appear steady white only near the correct glide-path angle. Each fly-line unit's lens/reflector is tilted at commissioning so it switches on at a specific elevation deviation from the design glide path, with units spaced along the fly line so that, together, they show progressively more fly-down lights above the design glide path (typically 3°, or up to around 4° where a steeper angle has been set for site-specific reasons) and progressively more fly-up lights below it — the fly-up units also switch from white to red once the deviation passes a severe-low threshold. That geometry is set once, mechanically, and re-verified by ground or flight check — no processor decides which unit lights up. Both the wing bar and the fly lines draw from the same constant current series circuit through their own isolating transformers, and the CCR (typically 3‑step or 5‑step, sometimes via ALCMS) sets one shared brightness level for the entire T — wing bar and fly lines together — for day, twilight, or night. Loop-level fault monitoring is standard series-circuit practice: a voltage rise at constant current flags an open lamp or failed transformer as a loop fault, without distinguishing which wing-bar or fly-line unit is affected until sectionalised testing is done.

Each unit in the T-VASIS array is an individual box-section housing in die-cast aluminium alloy or louvred galvanised steel, projecting a single white beam that switches on or off as a whole (with the fly-up units able to switch to red for the severe-deviation warning) rather than a split red/white beam like PAPI or VASI; the fly-line units run in line along the approach path and the wing-bar units run perpendicular to it, together forming the ground-visible "T" pattern. Optics are shaped as a lens/reflector assembly tuned to the fixture's role in the array, with a built-in levelling and pitch-adjustment mechanism used to aim each unit precisely to the set glide-path angle during commissioning. Every unit mounts on its own frangible mast at a fixed, surveyed height, engineered to shear away cleanly if struck, and units are spaced and wired individually so the control system can switch fly-up or fly-down lights independently as the aircraft departs the glide path. 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 transformers and cable entries free of standing water.