VASIS
Visual Approach Slope Indicator Systems (VASIS) AIP AD 1.1
Two types of VASIS are approved for use in Australia:
- T-VASIS – a high-intensity system for use by day or night
- PAPI – a colour discrimination system usable by day or night.
The standard installation aims to provide an obstacles clearance of at least 11 m above a 1.9° slope, within the azimuth splay of 7.5° either side of the runway centre line for a distance of a 5 nm from the threshold (7 nm for a runway equipped with an ILS).
When the installation differs from the standard, details are promulgated in the aerodrome documentation.
T-VASIS
The cross-bar indicates on-slope and deviations appear as one, two or three lights above or below the cross-bar. The sensitivity is similar to the ‘dot positions’ on an ILS glide path.
Increased eye height over the threshold can be achieved by flying the approach with one or more of the ‘fly-down’ lights visible.
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Approach slope indication
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Eye height above threshold
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3 lights fly up
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0 to 7 ft
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2 lights fly up
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7 to 25 ft
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1 lights fly up
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25 to 41 ft
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On glide slope
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49 ft
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1 light fly down
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57 to 75 ft
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2 lights fly down
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75 to 94 ft
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3 lights fly down
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94 to 176 ft
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Notes
The night azimuth splay is normally increased to 30° to permit T-VASIS to be visible on base leg. However, obstacle clearance is not guaranteed until the aircraft is within the runway approach obstacle limitation surface. Accordingly, T-VASIS should not be used for approach slope guidance until the aircraft is aligned with the runway.
The presence of a thin layer of ground fog or mist may produce abnormal T-VASIS indications, including erroneous fly-down or fly-up signals, or other fly-up or fly-down lights together with the correct lights (which are usually much brighter than the erroneous lights). Consequently, pilots should exercise caution when using the T-VASIS in ground fog or other conditions conducive to light reflection or refraction.
The above requirements may vary by 15 ft depending on the location of the system.
The intensity of the system may be varied at the request of the pilot.
An abbreviated version of T-VASIS, AT-VASIS, is used at some locations, with the equipment located on only one side of the runway (usually the left).
T-VASIS
| On glide slope | |
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| Slightly high | Slightly low |
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| High | Low |
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| Very high | Very low |
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PAPI
A PAPI (precision approach path indicator) installation consists of a set of four light boxes placed in a line at right angles to the runway, abeam the touchdown point and usually on the left-hand side. Each box radiates both red and white light. The transition between the red and white will appear instantaneous to the pilot (three minutes of arc); however, light changes between adjacent boxes will not occur unless the approach slope changes by about 0.25 degrees. A one degree progressive incremental spread from the outermost to the innermost light unit about the standard approach angle provides the visual guide shown below.
PAPI
| On correct approach path (3 degrees) | |
|---|---|
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| Slightly high (approximately 3.3 degrees) | Slightly low (approximately 2.7 degrees) |
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| Too high (>3.5 degrees) | Too low (<2.5 degrees) |
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