What is a T-VASI?
The T Visual Approach Slope Indicator. A white wing bar with fly-down and fly-up lights forms a T, so the pilot reads the slope as a shape.
A T-VASI (T-VASIS) is a visual slope indicator whose light units are laid out as a T. A wing bar of white lights lies across the approach, and a stem of fly-down and fly-up lights runs above and below its centre. The pilot reads the slope from the pattern the lights make. The full T-VASIS has 20 light units: two wing bars of four units each, one on each side of the runway, each split by a line of six stem lights (three fly-down above and three fly-up below).
On the slope only the white wing bar is seen, as a horizontal line. Above the slope, white fly-down lights appear above the bar, and more appear the higher the aircraft is. Below it, fly-up lights appear below the bar, white at first, and more appear the lower the aircraft is. Well below the slope, the wing bar and all three fly-up lights are red. The pilot’s job is to fly the aircraft until only the wing bar is left.
Because the size of the error shows as the number of lights, the T-VASI gives a clear cue for how far off the slope the aircraft is, not just which side. The price is more light units and more space than a PAPI. A reduced layout on one side of the runway is called an AT-VASI (AT-VASIS): 10 light units, one wing bar of four and a stem of six. ICAO Annex 14 Vol. I (5.3.5.4) recommends that T-VASIS and AT-VASIS stop being used as standard visual approach slope indicator systems from 1 January 2020, with PAPI as the standard choice, so many remaining installations are legacy ones. Check the current edition.
Frequently Asked Questions
Q1What does T-VASI stand for?
T-VASI stands for T Visual Approach Slope Indicator, often written T-VASIS with the word System added. It is called a T-VASI because its light units form a T-shaped pattern that changes as the aircraft moves above or below the slope.
Q2How does a pilot read a T-VASI?
On the slope, only the white wing bar is seen, as a horizontal line of lights. Above the slope, white fly-down lights appear above the wing bar, and more appear the higher the aircraft is. Below the slope, fly-up lights appear below the wing bar, white at first, and more appear the lower the aircraft is. Well below the slope, the wing bar and all three fly-up lights are red.
Q3What do fly-down and fly-up mean?
The names tell the pilot what to do. White lights above the wing bar mean the aircraft is high and should fly down toward the slope. Lights below the wing bar mean it is low and should fly up toward the slope. The fly-up lights are white at first. When the aircraft is well below the slope they turn red, and so does the wing bar.
Q4What is an AT-VASI?
An AT-VASI, or abbreviated T-VASI, uses the same idea with only half of the layout, on one side of the runway: 10 light units instead of 20. It gives the same fly-down, on-slope and fly-up indications with fewer light units.
Q5How is a T-VASI different from a PAPI?
A PAPI is read by counting how many of its four units show red. A T-VASI is read from the shape the lights make, and from how far the fly-down or fly-up lights extend from the wing bar. It needs more units and more space, which is one reason PAPI is more common on new installations. ICAO Annex 14 recommends that T-VASIS and AT-VASIS no longer be used as standard systems from 1 January 2020.
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