Visual approach slope indicator
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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 approach. These lights may be visible from up to 8 kilometres (5.0 mi) during the day and up to 32 kilometres (20 mi) or more at night.
Basic visual approach slope indicators consist of one set of lights set up some 7 metres (23 ft) from the start of the runway. Each light is designed so that it appears as either white or red, depending on the angle at which it is 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 aircraft is too high, and when both appear red it is too low. The first set of lights appearing red and the second set white is not possible lest the plane were upside down or the approach was coming from above, which is virtually never. This is the most common type of visual approach slope indicator system. A mnemonic to remember the colors and their meaning is :
White over White, you're high as a kite. / you'll fly all night
Red over White, you're alright.
Red over Red, you're dead.
White over Red, unsaid / you're under head
Precision approach path indicator
A precision approach path indicator (PAPI) consists 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) indicate that the aircraft is significantly above glide slope (>3.5°).
Most large airports use 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. (London City Airport has PAPI set at 5.5 degrees on both runway 27 & 09)
Pulsating visual approach slope indicator
A 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 the aircraft's 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.
T-Visual Approach Slope Indicating System
A T-Visual Approach Slope Indicating System (T-VASIS) consists of 20 white lights, half on each side of the runway arranged in a crosswise wing bar of four lights and a longitudinal arm of six lights which bisect one another. The lights are visible within different approach slope angles so that a pilot on the correct path sees only the eight wing bar lights. When below the approach slope, the pilot sees a T-shape formed by the wing bar lights and one, two, or three "fly up" longitudinal lights, more being visible the greater the deviation from the correct position. When above the approach slope, the pilot sees an inverted T formed by the wing bar and "fly down" lights. When well below the approach slope, the wing bar and three fly up lights change from white to red. PAPI and T-VASIS, together with abbreviated versions of them, are the only approach slope indicating systems approved by the ICAO.
T-VASIS is used in Australia but is being replaced by cheaper PAPI systems.
A tri-colored VASI is a single light that appears amber above the glide slope, green on the glide slope and red below it. It has fallen out of widespread use, partly because pilots who are unfamiliar with them have been known to misinterpret the lights and "correct" in the wrong direction. These errors are increased due to a major design shortcoming of the tri-colored VASI. While on approach, the colour amber (above slope) can be seen at a very thin angle of approach between green (on slope) and red (below slope) due to the mixing of red and green which gives an amber colour. Pilots not familiar with this may see the amber light and think they are above glide slope and then descend rather than make the proper correction and ascending back to glide slope. Despite this shortcoming, it is (reportedly) in widespread use in Eastern European countries, especially Russia and Ukraine.
Stabilised glide slope indicator (SGSI)
The stabilised glide slope indicator is used 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 ship's roll and pitch and provides the pilot with visual information relating to the approach angle. The coloured sector of the beam indicates if the aircraft is above, below or on the correct glide path. There are various beam configurations available to suit different naval requirements.
- Pilot controlled lighting (PCL)
- Precision approach path indicator (PAPI)
- Runway end identifier lights (REIL)
- Runway edge lights (HIRL, MIRL, LIRL)
- Approach lighting system (ALS)
- Free Falcon wiki
- Pulsating Visual Approach Slope Indicator tested by the U. S. Air Force
- ICAO Manual of Aerodrome Standards
- T-Visual Approach Slope Indicator System (T-VASIS) versus Precision Approach Path Indicator (PAPI) – the debate revisited.