Understanding VASI Lights – A Guide for Pilots

What Are VAST Lights? – Overview of the System

The Visual Approach Slope Indicator (VAST) is a lighting system that gives pilots visual cues to maintain a proper descent angle for landing.

One major advantage of the VAST system is its impressive long-range visibility—typically 3–5 miles during the day and up to 20 miles at night—allowing pilots to acquire and stabilize on the glide path well before the runway threshold.

Per FAA standards, VAST units are installed alongside the runway, typically on the left. Configurations vary, ranging from 2-bar systems with two, four, or six light units to larger 12 or 16-unit arrays that can flank both sides of the runway. A standard two-bar VAST provides a single glide path, usually set at a 3-degree angle.

More than just guidance, the VAST system is a critical safety feature. Its glide path is precisely calibrated to ensure clearance over all obstacles within a wide corridor—extending 10 degrees from the runway centerline and reaching 4 nautical miles from the threshold.

How VAST Lights Work – Glide Path Guidance

Each VAST light unit projects an ingenious split beam of light—white in its upper portion and red in its lower. A pilot’s vertical position on the approach dictates which color they see from each light bar, giving them an immediate, intuitive cue about their altitude relative to the ideal glide path.

For a standard two-bar VAST, the combination of colors from the near and far bars translates into the following simple guidance:

• All White: Too high.

• Red over White: On the correct glide path.

• All Red: Too low.

Pilots often rely on simple mnemonics to remember these indications: “White over white, you’re high as a kite.” “Red over white, you’re alright.” “Red over red, you’re dead.”

The transition between colors isn’t instantaneous. As an aircraft drifts to the edge of the glide path, the lights pass through a narrow pink zone. This subtle shift serves as a crucial early warning, prompting pilots to make small, smooth corrections and re-intercept the optimal descent angle before a major deviation occurs.

Types of VAST Systems – Variants Explained

While the standard two-bar VAST is the most common system you’ll encounter, it’s far from the only one. Airports around the world have unique operational needs, from accommodating jumbo jets to fitting guidance systems into tight spaces. To meet these demands, several VAST variants have been developed, each with a distinct way of communicating glide path information.

Three-Bar VAST

The three-bar VAST is the three-bar VAST, designed for airports handling aircraft with widely varying cockpit-to-wheel heights. It effectively provides two distinct glide paths: pilots of smaller aircraft use the lower two bars as a standard VAST, while pilots in large, wide-body jets reference the top two. This dual-path system ensures that even as the pilot follows the correct visual glide path, the aircraft’s landing gear safely clears the runway threshold.

Pulsating VAST (PVA SI)

The Pulsating VAST (PVA SI) offers a single-unit solution, using a combination of color and flashing lights to convey guidance:

• Pulsating White: Too high.

• Steady White: On glide path.

• Steady Red: Slightly low.

• Pulsating Red: Dangerously low.

The pulsating signal is very noticeable, making it an effective warning system, especially at night.

Tricolor VAST

The Tricolor VAST is another single-light system that projects a three-color beam:

• Amber: Too high.

• Green: On glide path.

• Red: Too low.

The system’s utility is limited, however. Its narrow “on-path” green corridor can make a stable approach difficult, and its color scheme may prove confusing for pilots with color vision deficiencies.

TEXAS and ADIVASI

The T-Visual Approach Slope Indicator (TEXAS) uses an intuitive arrangement of white lights in a ‘T’ shape to signal deviations from the glide path:

• Horizontal Line: On glide path.

• Upright ‘T’: Too high.

• Inverted ‘T’: Too low where the number of lights in the stem indicates how far below the path you are.

The Abbreviated TEXAS (ADIVASI) is a simplified version of this system.

VAST vs PAPI – Key Differences and Similarities

While VAST and the Precision Approach Path Indicator (PAPI) serve the same fundamental purpose—providing vertical guidance on final approach—they are distinct systems with important differences. PAPI is generally considered the more modern and precise successor, but understanding both is crucial for any pilot.

The most obvious distinction lies in their light configuration. A standard VAST system uses light units arranged in two separate rows, or bars. In contrast, a PAPI system features a single horizontal row of four lights. This distinct visual layout allows a pilot to immediately identify which system is in use and how to interpret its signals.

This layout difference allows PAPI to provide more granular guidance than the VAST’s simpler three-part indication, offering five distinct steps of guidance:

• Four White: Well above glide path.

• Three White, One Red: Slightly high.

• Two White, Two Red: On glide path.

• One White, Three Red: Slightly low.

• Four Red: Well below glide path.

Although PAPI’s more detailed feedback allows for finer corrections, its obstacle clearance surface is slightly shorter (3.4 NM) compared to VAST’s (4 NM). VAST remains a common sight, largely due to its lower installation cost, while PAPI’s precision is typically reserved for larger airports with stricter requirements.

Future of VAST Lights – Trends and Innovations

Although VAST has been a reliable tool for decades, the future of visual approach guidance is moving toward more advanced systems.

A major development is the widespread adoption of LED technology to replace traditional incandescent lamps. LEDs are not only more energy-efficient but also offer a longer operational lifespan and greater reliability, which translates directly to less maintenance downtime.

In the longer term, the evolution of glide path indicators will may include integration with digital cockpit technologies. Future systems could incorporate enhanced lighting or even augmented reality overlays on a heads-up display, providing pilots with an exceptionally clear and intuitive view of the approach. These advancements are aimed at reducing pilot workload and improving situational awareness during the most critical phase of flight.

Still, VAST systems are far from obsolete. Their simplicity and cost-effectiveness mean they will remain a fixture at smaller airports and general aviation fields where the high precision of a PAPI system is not a necessity.