The Visual Approach Slope Indicator
(VASI) is a system of lights
on the side of an airport runway
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.
Basic visual approach slope indicators consist of two sets of lights. One set marks the start of the runway, while the other is set up some seven meters (twenty feet) along the runway. Each set of lights is designed so that the lights appear 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 he is on the glide slope, the first set of lights appears white and the second set appears red. When both sets appear white, he is flying too high, and when both appear red he is flying too low. This is the most common type of visual approach slope indicator system.
PAPI ''(Precision Approach Path Indicator)
Precision Approach Path Indicator 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
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 4.5° in order to have proper obstruction clearance.
PVASI (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 that 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.
Pilots often use mnemonics to help them remember basic information. In English, the following are common:
- Red over white, you're all right.
- White over white, you're out of sight.
- Alternative: White over white, you'll fly all night.
- Alternative: White over white, you're high as a kite.
- Alternative: White over white, you remain in flight.
- Red over red, you're dead.
- Alternative: Red over red, watch your head.
- Red over Red: hit the bed.
- Red red you're dead
- Red and white you're alright
- White and white you're as high as a kite
- Two whites, you're light; two reds, you're dead.
FAA Aeronautical Information Manual, Chapter 2 (Aeronautical Lighting and Other Airport Visual Aids), Section 1 (Airport Lighting Aids)
Alternative: White over White - too much height