What is an AT-VASIS?
Ten light units on one side of the runway: a wing bar, with fly-down lights above it and fly-up lights below. The pilot flies the aircraft until only the bar is left.
The AT-VASIS is the abbreviated T-VASIS. Instead of the twenty units of a full T-VASIS, it keeps one side of the runway: a single wing bar of four light units, crossed by a longitudinal line of six lights. The pilot reads the slope from the pattern of lights, not from colour alone.
1Ten units, laid out as a T
The four units of the wing bar stand across the approach, and the line of six crosses their centre: three fly-up lights on the threshold side and three fly-down lights beyond the bar. Each unit is built and aimed so that it is seen only from its own range of angles.
2Fly the aircraft until only the bar is left
On the slope the pilot sees only the white wing bar. Above it, one, two or three white fly-down lights appear above the bar, and the more there are, the higher the aircraft. Below it, one, two or three white fly-up lights appear below the bar, and when well below, the bar and all three fly-up lights show red. No fly-up light is seen on or above the slope, and no fly-down light on or below it.
3Check every light and every sector
Because the number of lights carries the message, one failed lamp can give a wrong reading. Check that all ten units light, that each shows at the angles it was set for, that the wing bar is level and square to the approach, and that lenses are clean and the beams clear. A failed lamp, a shifted unit or a dirty lens changes the pattern and misleads the pilot.
4A legacy system
The AT-VASIS is no longer the standard choice. ICAO Annex 14 Vol. I recommends that T-VASIS and AT-VASIS no longer be used as standard systems from 1 January 2020, and PAPI or APAPI is specified instead. Existing installations still operate in places, so this page explains how to read and check one, but check your State’s rules and the edition in force before specifying it for new work.
| Pilot's position | Pattern seen | Meaning |
|---|---|---|
| Above the slope | Wing bar + 1, 2 or 3 white fly-down lights | More lights, higher above |
| On the slope | White wing bar only | On the approach slope |
| Below the slope | Wing bar + 1, 2 or 3 white fly-up lights | More lights, lower below |
| Well below | Wing bar and three fly-up lights, all red | Well below the slope |
Frequently Asked Questions
Q1What is an AT-VASIS?
An AT-VASIS is the abbreviated T-VASIS. It uses ten light units on one side of the runway: a single wing bar of four light units with a bisecting longitudinal line of six lights, three fly-down and three fly-up. The pilot reads the slope from the pattern the lights make.
Q2How is an AT-VASIS read?
On the slope the pilot sees only the white wing bar. Above the slope, one, two or three white fly-down lights appear above the bar, and more appear the higher the aircraft is. Below the slope, one, two or three fly-up lights appear below it, and when well below the bar and the three fly-up lights show red.
Q3How does AT-VASIS differ from T-VASIS and PAPI?
A T-VASIS has twenty units, with a wing bar and stem on both sides of the runway, while the AT-VASIS keeps only one side. A PAPI is read by the colour of four units, whereas the AT-VASIS is read by the pattern and number of lights, which shows how far the aircraft is off the slope but needs more units and space.
Q4Is the AT-VASIS still used?
Some existing installations remain in service, but ICAO Annex 14 Vol. I recommends that T-VASIS and AT-VASIS no longer be used as standard visual approach slope indicator systems from 1 January 2020. PAPI and APAPI are the standard systems, so check your State’s rules and the edition in force before specifying an AT-VASIS for new work.
Q5What happens if one light unit fails?
Because the number of lights carries the message, a failed lamp, a shifted unit or a dirty lens can make the aircraft look higher or lower than it is. All ten units, their setting angles and lens cleanliness should be checked in maintenance, and a fault that cannot be cleared quickly should be reported so the aerodrome can decide whether to withdraw the system and notify pilots.
Q6Why do the lights turn red when well below the slope?
Red is the warning step. When the aircraft is well below the slope, the wing bar and the three fly-up lights change from white to red, which adds a clear colour warning to the pattern. For the rest of the approach the pilot reads the slope from the pattern, not from colour.
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