Visual glideslope indicators provide a visual reference to aid a pilot in determining the aircraft's position relative to the proper glide path.
To achieve this, visual glideslope indicators use a system of lights or other elements that change colors to indicate the proper glide path to the pilot during both day and night conditions.
Visual Approach Slope Indicator(s) (VASI) are the most common visual glideslope indicators and consist of an installation of lights arranged to provide visual descent guidance information during an approach to a runway.
Each light unit in a VASI installation projects a beam of light with an upper and lower segment. The upper segment is colored white, and the lower segment is colored red. The basic principle of the VASI system is the differentiation between this red and white light.
Different variations of VASIs exist and may consist of 2, 4, 6, 12, or 16 light units arranged into 2 or 3 rows, referred to as bars, of lights. Each bar in a VASI installation is then referred to as a near, middle, or far bar.
All VASIs are visible from 3-5 miles during the day, and up to 20 miles or more at night. The visual glide path provided by the VASI provides safe obstruction clearance within plus or minus 10 degrees of the extended runway centerline up to 4 nautical miles from the runway threshold.
Pilots are advised to not initiate a descent until visually aligned with the runway. VASIs do not provide lateral guidance. Therefore, pilots must use the runway or runway lights for lateral guidance during an approach.
The most popular type of VASI installation is the two-bar VASI. The two-bar VASI consists of a near and far bar and may have 2, 4, or 12 light units making up the entire installation.
The two bar VASI provides one visual glide path normally set at 3 degrees. However, angles at some airports may be as high as 4.5 degrees to provide proper obstacle clearance.
The Three-Bar VASI consists of 3 bars (near, middle, and far) and may consist of either 6 or 16 light units.
The additional middle bar allows the Three-Bar VASI to provide two visual glide paths (an upper and lower glide path) instead of just the one glide path provided by the Two-Bar VASI.
The lower glide path is provided by the near and middle bars and is normally set at three degrees. However, if determined necessary a higher glide path as high as 4.5 degrees may be used.
The upper glide path is provided by the near and middle bars and is normally set a quarter of a degree higher than the lower glide path.
The lower glide path is used by most aircraft during an approach. The upper glide path is intended to be used only by aircraft with a cockpit located high up on the aircraft. This is to allow larger aircraft sufficient threshold crossing height during an approach.
A Precision Approach Path Indicator (PAPI) use similar light units to a VASI. However, a PAPI consists of a single row of either two or four light units normally installed on the left side of the runway, rather than 2 or 3 rows found in a VASI that may be installed on the left or both sides of the runway.
PAPIs have similar visual ranges to a VASI, with an effective visual range of 5 miles during the day, and up to 20 miles at night.
Tri-Color Systems, otherwise known as Tri-color visual approach slope indicators, consist of a single light unit that projects a three-color visual glide path into the final approach area of the landing runway.
Depending on visibility conditions, tri-color systems have an effective visual range of approximately one-half to one mile during the day, and up to 5 miles at night.
Pulsating systems are also known as Pulsating Visual Approach Slope Indicator(s) (PVASI), consist of a single light unit projecting a two-color visual approach path into the approach area of the landing runway.
PVASIs have an effective visual range of about 4 miles during the day and up to ten miles at night.
Alignment of Elements Systems is a low-cost system installed on some small general aviation airports.
The Alignment of Elements System consists of painted plywood panels, normally painted black and white, or black and fluorescent orange. The Alignment of Elements System is generally a daytime use-only system. However, some systems are lighted to permit night use.
Alignment of Elements Systems have an effective visual range of about three-quarter miles during both day and nighttime conditions.