PVASI

The Pulsating Visual Approach Slope Indicator (PVASI) is a compact and economical visual glide slope guidance system that uses a single light unit with a pulsating (flashing) beam instead of the fixed red/white colour transitions used in PAPI or VASI. The system projects a beam that appears steady when the aircraft is on the correct glide path, and pulsates (flashes) at varying rates when the aircraft is above or below the path.

PVASI is one of the simplest and most cost‑effective visual glide slope systems available. It typically consists of a single elevated light unit with a precision optical system and a pulsating mechanism. The system is easy to install, requires minimal electrical power, and provides an intuitive indication that is easily understood by pilots. It is particularly well‑suited for smaller airfields, heliports, and military forward operating bases where a full PAPI or VASI is not justified.

PVASI exists to give smaller airfields, heliports, and austere operating locations a working glide slope reference without the cost, complexity, and power demands of a multi-unit PAPI or VASI array. Its single-unit, pulsating design condenses the same essential message — on path, above path, or below path — into one fixture that a pilot reads by whether the beam is steady or flashing, rather than by comparing the colour pattern across several separated lights.

This simplicity is the point: at locations where traffic volume or budget doesn't support a full system, PVASI still gives approaching pilots a real, trustworthy glide slope cue rather than leaving them with no visual glide guidance at all, which matters most during night operations or marginal visibility when judging descent angle by the runway alone becomes unreliable.

PVASI is deployed on a variety of smaller and specialised landing sites:

• General aviation airports – with low traffic volumes and limited budgets.
• Heliports – where a compact, lightweight system is needed.
• Remote and regional airstrips – with limited electrical infrastructure and space.
• Military forward operating bases – requiring rapid deployment and simplicity.
• Temporary or emergency runways – where a portable, lightweight system is advantageous.
• As a backup or supplement to ILS – on larger runways for additional visual guidance.

PVASI uses a single light unit mounted on one side of the runway, set back from the threshold at the distance needed to place the on-slope indication over the threshold at the correct height, and tied into the runway centreline and threshold survey. The unit sits on a frangible mount at a fixed lateral offset from the runway edge, clear of the obstacle-limitation surface, and is individually aimed and levelled during commissioning so the pulse-rate transitions occur at the correct angular deviations either side of the design glide path. Cabling runs underground back to its regulator and pulse-control unit, and the single-light footprint makes PVASI suited to sites where a full multi-unit bar system isn't warranted by traffic volume or budget.

PVASI operates on a simple but effective principle using pulsating light to convey glide path information:

• Optical Design: The system consists of a single light unit (or occasionally a small array) that projects a narrow beam along the desired glide path. The optical system is designed to produce a beam with a specific vertical angular coverage.
• Pulsating Indication: Unlike colour‑based systems (PAPI, VASI), PVASI uses the rate of pulsation (flashing) to indicate the aircraft's position:
  • Steady (no pulsation): The aircraft is on the correct glide path.
  • Slow pulsation (e.g., 1–2 flashes per second): The aircraft is slightly above or below the glide path (depending on system configuration).
  • Fast pulsation (e.g., 4–6 flashes per second): The aircraft is significantly above or below the glide path, indicating a need for immediate correction.
  The specific pulsing pattern (steady vs. pulsating, and the rate of pulsation) provides an intuitive indication that does not rely on colour recognition, making it suitable for colour‑blind pilots.
• Glide Path Angle: PVASI is typically set to a standard 3° glide path angle, but can be adjusted to suit specific approach requirements (2.5°–4°).
• Power and Control: PVASI units are powered from the constant current series circuit via isolating transformers, with intensity controlled by the CCR (typically 3‑step). The pulsating mechanism is typically controlled by an internal electronic driver or an external control unit.
• Integration: PVASI is typically deployed independently and can be easily integrated with the runway lighting system.

PVASI splits its architecture in two: a conventional power path and an electronic angle-sensing path that neither PAPI nor VASI needs. The light unit itself is powered from the constant current series circuit through an isolating transformer exactly like other visual approach aids, with the CCR setting overall lamp brightness for day, twilight, or night — that part is fixed-current, loop-level control, same as always. What's different is the pulsating behaviour: an internal electronic driver (or an external control unit, depending on installation) modulates the beam on and off at a rate that depends on how far the aircraft's line of sight sits from the design glide path, rather than relying on a fixed optical colour split. Steady illumination means on-path; a slow flash means a small deviation; a fast flash means a large one. Because that logic lives in the driver electronics rather than in fixed lens geometry, PVASI's angular thresholds can be tuned without physically re-aiming the unit, but it also means the driver itself is an additional active component to monitor — its failure mode (stuck steady, stuck flashing, or dark) needs separate consideration from the loop-level lamp/transformer fault that the CCR already reports.

The PVASI unit is a single box-section housing in die-cast aluminium alloy or louvred galvanised steel, weatherproofed against UV exposure and de-icing fluid. Unlike a split red/white indicator, the optical assembly projects a single white beam whose apparent behaviour changes with the aircraft's position on the glide path, achieved through an internal pulsing circuit and lens/reflector geometry that produces a steady white signal on-path and a pulsating signal above or below it. A built-in levelling and pitch-adjustment mechanism is used to aim the unit to the set glide-path angle during commissioning. The housing mounts on a frangible mast at a fixed, surveyed height beside the runway, engineered to shear away cleanly if struck. Cabling runs underground in conduit to an isolating transformer housed in a nearby base can, with sealed, direct-burial connectors and a drainage provision to keep the transformer and cable entries free of standing water.