A Visual Approach Slope Indicator System (VASIS) is a visual aid for pilots to maintain a safe glide slope (approach slope) to the runway during landing. This system is primarily used for VFR (Visual Flight Rules) and IFR (Instrument Flight Rules) approaches to confirm the correct descent angle.
How does a VASIS work?
VASIS systems use colored lights (usually white and red) that vary from the cockpit depending on the aircraft's position relative to the correct glideslope.
The basic rule for interpretation is:
- White above, red below → correct glide path (e.g. 3° descent)
- Too much white → too high
- Too much red → too low
VASIS is usually installed next to the runway and is visible from a distance of 8-10 kilometers during the day and 32 kilometers at night .
Types of VASIS systems
There are several variations of VASIS, including:
1. Standard VASIS
- Consists of two rows of two light boxes each , placed at a fixed distance from each other.
- Displays a combination of white and red lights to indicate the aircraft's position.
2. T-VASIS (T-Bar VASIS)
- A more advanced version with an additional T-shaped extension below the main indicators.
- Provides additional warning signals for too steep or too flat an approach , such as pulsations in the lights.
- Less commonly used due to the transition to PAPI systems.
3. PAPI (Precision Approach Path Indicator)
- Modern replacement for VASIS , consisting of four lights in a line next to the track.
- Lines:
- 4 white → too high
- 3 white, 1 red → light too high
- 2 white, 2 red → correct glide angle (usually 3°)
- 1 white, 3 red → light too low
- 4 red → dangerously low
PAPI is now more widely used than VASIS because it is more accurate and easier to interpret .
Why is VASIS important?
- Improves landing precision , especially in poor visibility.
- Reduces the chance of a hard landing or early touchdown.
- Important at airports without ILS (Instrument Landing System) or as visual confirmation when approaching with ILS.
- Aids in night and VFR landings , where a correct glide angle is more difficult to estimate.