Visual approach slope indicator

Visual Approach Slope Indicator (VASI) is a system of lights on the side of an airport runway threshold that provides visual descent guidance information during the approach to a runway. These lights are designed to aid pilots in maintaining the correct approach path for a safe landing, especially during conditions of reduced visibility. The VASI is an important component in aviation safety, ensuring that aircraft are aligned with the runway and at the correct altitude for landing.

Overview

VASI systems consist of light units arranged in bars. These lights are visible from several miles during the day and up to 20 miles or more at night. The most common configurations are two-bar VASIs and three-bar VASIs. Each bar consists of a series of lights that project a visual glide path which is safe from obstacles and correctly aligned with the runway. The lights change color based on the angle at which they are viewed. Typically, if the pilot sees red lights, they are below the glide path; if they see white lights, they are above the glide path; and if they see a combination of red and white, they are on the correct glide path.

Operation

The basic principle behind the VASI is simple yet effective. It employs color-coded lights to indicate the correct path of descent to the pilot. The lights are arranged such that, from the air, pilots can easily discern their position relative to the desired glide path. If the aircraft is too high, the lights will appear white, and if it is too low, the lights will appear red. The goal is to maintain an equal number of red and white lights, indicating that the aircraft is on the correct approach path.

Types of VASI

There are several types of VASI systems, each designed to meet different operational requirements:

• Standard VASI: This is the most common type, consisting of two bars of lights. It is suitable for most aircraft, providing a visual glide path that ensures safe landing.

• Pulsating VASI: Unlike the standard VASI, which uses steady lights, the pulsating VASI uses a single light source that pulsates to indicate whether the pilot is above, on, or below the glide path.

• Tri-color VASI: This system uses a single light unit that changes color from green to red, through amber, depending on the aircraft's altitude relative to the glide slope.

• T-VASI: Arranged in a T-shape, this variant is designed for use at airports where space constraints prevent the installation of standard VASI systems.

Installation and Maintenance

VASI systems are installed on the side of the runway, typically in line with the runway threshold. The installation and maintenance of VASI systems are critical to ensure their reliability and effectiveness. Regular checks are conducted to ensure that the lights are functioning correctly and that their alignment is accurate.

Safety and Regulations

The use of VASI is governed by aviation authorities, such as the Federal Aviation Administration (FAA) in the United States. These authorities set the standards and regulations for the installation, operation, and maintenance of VASI systems. Compliance with these regulations is mandatory for airports to ensure the safety of aircraft operations.

Conclusion

The Visual Approach Slope Indicator is a crucial tool for pilots, especially during the final approach phase of flight. It provides a simple yet effective means of ensuring that aircraft maintain the correct glide path for a safe landing. As technology advances, VASI systems continue to evolve, but their fundamental role in aviation safety remains unchanged.