Operations resources


General information


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.


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.

Approach slope indication
Eye height above threshold
3 lights fly up
0 to 7 ft
2 lights fly up
7 to 25 ft
1 lights fly up
25 to 41 ft
On glide slope
49 ft
1 light fly down
57 to 75 ft
2 lights fly down
75 to 94 ft
3 lights fly down
94 to 176 ft

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).


On glide slope
Slightly high Slightly low
High Low
Very high Very low


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.


On correct approach path (3 degrees)
Slightly high (approximately 3.3 degrees) Slightly low (approximately 2.7 degrees)
Too high (>3.5 degrees) Too low (<2.5 degrees)

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