AT‑VASIS

The Abbreviated T‑Visual Approach Slope Indicator System (AT‑VASIS) is a compact visual glide slope guidance system built from ten light units, all of which emit white light in normal operation — like full T‑VASIS, it does not rely on a red/white colour split. Where full T‑VASIS mirrors its wing bar and fly line on both sides of the runway (20 units total), AT‑VASIS installs a single wing bar of four lights plus one bisecting fly line of six lights on one side only, halving the unit count while keeping the same graduated, number-of-lights signal.

AT‑VASIS reads the same way as full T‑VASIS: the wing bar alone means on path, and progressively more fly-down or fly-up lights show how far above or below the aircraft is. Like T‑VASIS, it is normally set to the standard 3° glide path used by VASI and PAPI, and — like any VGSI — can be adjusted up to around 4° at airfields with obstacle clearance constraints or noise abatement requirements.

AT-VASIS exists to make the graduated, T-shaped glide-path guidance of T-VASIS available to airfields that can't justify the cost or space of the full installation. By trimming the number of light units while keeping the distinctive T-shaped visual cue, it preserves the core benefit of the design — a precise, intuitive read on descent angle — at the standard 3° path, or steepened up to around 4° at sites facing obstacle clearance limits or noise abatement requirements.

Because pilots read the T-shape as a single visual pattern rather than tallying individual light colours, the abbreviated version still delivers a fast, unambiguous sense of high, low, or on-path even with fewer units, which is what makes it a workable lower-cost alternative rather than a compromised one.

AT‑VASIS is deployed on a variety of airfields where a steeper glide path and a compact system are required:

• Military forward operating bases – where rapid deployment and compact footprint are essential.
• Smaller commercial airfields – with obstacle clearance or noise abatement requirements.
• Heliports – where a steeper approach angle is beneficial for helicopter operations.
• Regional airstrips – with limited electrical infrastructure and space.
• As a backup or supplement to ILS – on larger runways for additional visual guidance.
• Airfields serving turboprop and regional jet aircraft – that benefit from steeper approach profiles.

AT-VASIS uses a reduced light-bar arrangement mounted on one side of the runway, set back from the threshold at the distance needed to place the glide path over the threshold at the correct height, and tied into the runway centreline and threshold survey. Bars sit on frangible mounts at a fixed lateral offset from the runway edge, clear of the obstacle-limitation surface, with each bar individually levelled and aimed during commissioning to reproduce the standard T-VASIS visual cues from a smaller unit count. Cabling is run underground from each bar back to its constant current regulator, and the reduced footprint compared to full T-VASIS allows installation on narrower strips or sites with tighter clearance margins, while still requiring the same setting-out discipline against the runway survey.

AT‑VASIS operates on a fundamentally different principle from PAPI and VASI, the same as full T‑VASIS: 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 — just with half the unit count of the full system. The system consists of:

• Configuration: Ten light units on one side of the runway: a single "wing bar" of four lights running across the approach direction, and one bisecting longitudinal "fly line" of six lights running down the approach path. All units project white light in normal operation.
• Colour Indication: The pilot reads the number 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 T‑VASIS, AT‑VASIS is normally set to the standard 3° glide path and can be set up to around 4° where obstacle clearance requires it.
• Power and Control: AT‑VASIS units are powered from the constant current series circuit via isolating transformers, with intensity controlled by the CCR (typically 3‑step).
• Integration: AT‑VASIS is typically deployed independently or integrated with the runway lighting system.

AT‑VASIS carries the same fixed-optics control logic as full T‑VASIS, just with one wing bar and one fly line instead of two of each. 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 the six units spaced along the line so that, together, they show progressively more fly-down lights above the design glide path 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, exactly as on the full system. That optical aim is fixed hardware, re-verified by ground or flight check — no processor decides which unit lights up. Both the wing bar and the fly line draw from the same constant current series circuit through their own isolating transformers, and the CCR (typically a basic 3‑step unit) sets one shared brightness level for the whole array. 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 pinpointing which unit is affected until sectionalised testing is done.

Each unit is an individual box-section housing in die-cast aluminium alloy or louvred galvanised steel, built the same way as a full T-VASIS fixture and installed as the same wing-bar-plus-fly-line group, but on one side of the runway only rather than mirrored on both — the abbreviation for lower-traffic runways that don't warrant the complete, symmetric system. 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 is wired individually so the control system can switch fly-up or fly-down units independently as needed. 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.