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Aero-Tips 05.23.06

Over the years a number of visual guidance systems have been developed to help pilots avoid obstacles on final approach. Probably the most common design is the two-bar visual approach slope indicator -- commonly called the VASI.

VASI is a system of lights arranged to provide visual descent guidance during the approach to a runway. VASI lights are visible from three to five miles during the day and up to 20 miles or more at night. The visual glide path provides obstruction clearance within +10 degrees of the extended runway centerline, and to four nautical miles from the runway threshold.

Lateral course guidance is provided by the runway or runway lights. Don't descend on the VASI glide path until you're visually aligned with the runway. VASI is particularly useful at unfamiliar airports or when landing at night, to keep you safely above unknown or unseen obstacles on the approach path. Instrument pilots find following the VASI critical when "breaking out" before the Missed Approach Point, especially on nonprecision approaches.

The basic principle of the VASI is color differentiation between red and white. Each light unit projects a beam of light having a white segment in the upper part of the beam and red segment in the lower part of the beam. The light units are arranged so that the pilot using the VASIs during an approach will see the combination of lights shown in the figure below.

Two-bar VASI

Most VASI installations consist of two bars, near and far, and may consist of two, four, or 12 light units. Light units are located on one side of the runway, usually the left. Where the installation consists of 12 light units, the units are located on both sides of the runway.

Two-bar VASI installations provide one visual glide path which is normally set at three degrees. Angles at some locations may be as high as 4.5 degrees to give proper obstacle clearance. VASI angles in excess of 3.5 degrees may cause an increase in runway length required for landing and rollout.

Using VASI

• Once you're visually aligned with the runway, fly level until the VASI shows "red over white". Then initiate a normal descent to maintain this visual indication until the flare.
• If you are high on glidepath descent at a slightly higher-than-normal descent rate until you see "red over white", then transition to a normal descent rate. NOTE: You'll often see the upper bars start to "turn pink" when you're almost on glidepath. Use this as your cue to begin moderating your descent rate.
• If you're low on glidepath and see "red over red", level off or begin a slight climb until you reacquire "red of white", then resume a normal descent.
• In many locations you'll need to manually activate the VASI. See the Airport/Facility Directory.

The VASI glidepath usually aims you at a point about 1000 feet from the runway threshold. If the runway is particularly short or you're flying an airplane requiring a long landing roll, it's especially critical to use the VASI to help you clear obstacles and land safely.

Aero-tip of the day: Understand and use the two-bar VASI if it's available at your destination airport.

FMI: Aero-Tips