Visual approach slope indicator (VASI) lights are a system used to provide slope guidance during the visual stage of the approach. They will assist pilots to maintain a stable descent path down to the runway surface for the flare and landing.
A typical two-bar VASI system has two wing bar single lights, one above the other, which are positioned at the side of the runway, usually at 300 m from the approach threshold. A duplicated set is positioned on the opposite side of the runway. This is sometimes known as the red-white system because the colors seen by the pilot tell him or her if he or she is right on slope, too high, or too low as follows:
• All bars white = high on the slope
• Near bars white and the far bars red = right on slope
• All bars red = low on the slope
This has an extra wing bar single light further down the runway to assist pilots of larger aircraft, such as the B747. This is so because the guidance given by VASIs depends on the position of the pilot’s eyes. Because the wheels of a large aircraft are much further below the pilot’s eyes, it is essential that his or her eyes follow a parallel but higher slope to ensure adequate clearance over the runway threshold. Pilots of large aircraft should use the second and third wing bar lights and ignore the first, whereas pilots of smaller aircraft should use the two nearer wing bar lights and ignore the third furthest. The red-white guidance system is the same as the two-bar VASI system.