What is a VASI?
The Visual Approach Slope Indicator. Two bars of light units show the pilot, by red and white, whether the aircraft is above, on or below the approach slope.
A VASI is a visual slope indicator built from light units arranged in bars across the runway approach. Each unit sends out a split beam: white above its slope and red below it, with a narrow pink band between. The pilot needs no instrument, only the colours of the bars.
In the common two-bar VASI there is a near bar and a far bar, installed on one side of the runway (usually the left) or on both sides in the larger layouts. The far bar is aimed slightly higher than the near bar. Above both slopes the pilot sees white over white. Between them, on the intended path, the far bar is red and the near bar is white: red over white. Below both, it is red over red. Pilots remember this as “red over white, you’re all right.”
A three-bar VASI adds a third bar so the pilot can fly either a lower or a higher path, the higher one suiting aircraft whose pilots sit well above the wheels. The upper path is normally about ¼° above the lower one, which is normally set at 3°. VASI has largely given way to the PAPI on new work, but many VASI installations remain in service and need to be maintained and aligned correctly. The two- and three-bar VASI is described in FAA guidance. ICAO deleted the two- and three-bar VASIS from Annex 14 in 1995, and the PAPI and APAPI are now its standard visual approach slope indicators.
Frequently Asked Questions
Q1What does VASI stand for and what does it do?
VASI stands for Visual Approach Slope Indicator. It is a set of light units arranged in bars beside the runway. Each unit shows white to a pilot who is above its slope and red to a pilot who is below it, so the colours of the bars tell the pilot whether the aircraft is high, on slope or low.
Q2How does a pilot read a two-bar VASI?
Both bars white means the aircraft is above the slope. The far bar red and the near bar white, often remembered as red over white, means the aircraft is on the slope. Both bars red means the aircraft is below the slope.
Q3Why is the far bar red when the aircraft is on the slope?
The far bar is aimed at a slightly higher angle than the near bar. An aircraft on the intended slope is therefore below the far bar's slope, which shows red, and above the near bar's slope, which shows white. The narrow band between the two slopes is the path the pilot is meant to fly.
Q4What is the difference between a 2-bar and a 3-bar VASI?
A 2-bar VASI gives one glide path. A 3-bar VASI adds a third bar so the pilot can choose between a lower and a higher path, the higher one suiting aircraft with a long eye-to-wheel distance. The upper path is normally about a quarter of a degree above the lower, which is normally set at 3 degrees. The number of bars, their spacing and their angles come from the applicable standard.
Q5Is a VASI the same as a PAPI?
No. Both are visual slope indicators, but a VASI is read by comparing the colours of separate near and far bars, while a PAPI is read from how many of four units in a single bar show red. PAPI is the more common choice for new installations, and it is the system ICAO Annex 14 specifies, while the two- and three-bar VASI is an FAA-described system. Many older VASI systems are being replaced.
See It. Understand It. Then Learn How to Engineer It.
Recognising an AGL system visually is the first step. The next step is knowing how to design, install, test, commission and maintain them correctly.
Build Practical AGL Engineering Skills
Go deeper with structured training from Precision AGL Academy, covering:
Learn how each light, sign and circuit works as part of a complete airport lighting system, not just as an isolated diagram.