The 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 may be visible from up to eight kilometers (five miles) during the day and up to 32 kilometers (20 miles) or more at night.
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Basic visual approach slope indicators consist of one set of lights set up some seven meters (twenty feet) from the start of the runway. Each light is designed so that the light appears as either white or red, depending on the angle at which the lights are viewed. When the pilot is approaching the lights at the proper angle, meaning the pilot is on the glide slope, the first set of lights appears white and the second set appears red. When both sets appear white, the pilot is flying too high, and when both appear red he or she is flying too low. This is the most common type of visual approach slope indicator system.
Precision Approach Path Indicator (PAPI) consist of four sets of lights in a line perpendicular to the runway, usually mounted to the left side of the runway. These have a similar purpose to basic visual approach slope indicators, but the additional lights serve to show the pilot how far off the glide slope the aircraft is.
When the lights show White-White-Red-Red the aircraft is on the correct glide slope for landing, usually 3.0°. Three red lights (white–red–red–red) indicate that the aircraft is slightly below glide slope (2.8°), while four red lights (Red-Red-Red-Red) indicate that the aircraft is significantly below glide slope (<2.5°).
Conversely, three white lights (white–white–white–red) indicate that the aircraft is slightly above glide slope (3.2°), and four white lights (White-White-White-White) indicated that the aircraft is significantly above glideslope (>3.5°).
Most large airports utilize this system. Although most airports use a PAPI based on a 3.0° glide slope, some airports may use a glide slope as great as 5.0° in order to have proper obstruction clearance.
Pulsating visual approach slope indicator (PVASI) is a single box found at non FAA Part 139 airports, heliports or airparks. The signal format is solid white when established on the proper descent profile, and solid red when below the proper descent profile. An active pulsing white light is seen when well above or pulsing red when well below. This allows the pilot to determine his position and rate of deviation or correction within the signal format and therefore determine the corrective action needed to return to the proper descent profile. Although PVASI is a single box system, its signal was evaluated by the U.S. Air Force and found to be much more accurate than VASI and equivalent to the four box PAPI.
This is a single light that appears amber above the glide slope, green on the glide slope and red below it. It is rarely used, partly because pilots who are unfamiliar with them have been known to misinterpret the lights, causing them to 'correct' in the wrong direction. Despite this shortcoming, it is (reportedly) in widespread use in Eastern European countries, especially Russia and Ukraine.
There is a stabilised version for use on moving landing platforms such as aircraft carriers. The SGSI projects a beam of light, with coloured sectors, from the aft face of the ship. This beam is stabilised to remove the effects of the ships’ roll and pitch and provides the pilot with visual information relating to his approach angle. The coloured sector of the beam seen by the pilot will indicate to him if his approach is above, below or on the correct glide path. There are various beam configurations available, to suit different naval requirements.