The Visual Approach Slope Indicator (VASI) is a visual aid for pilots that helps maintain the correct descent angle during landing. The system consists of two rows of lights that display either red or white, depending on the pilot's viewing angle.
Goal:
✅ Helps pilots land safely and accurately .
✅ Provides instant visual feedback on approach angle.
✅ Especially useful for night-time and poorly visible landings .
1. How does a VASI system work?
A VASI installation usually consists of two rows of lights placed along the runway. The system works by changing color depending on the viewing angle .
| Top row | Bottom row | Meaning | Action for pilot |
|---|---|---|---|
| 🔴 🔴 | 🔴 🔴 | Too low | Increase descent angle |
| 🔴 🔴 | ⚪ ⚪ | Correct descent angle (usually 3°) | No adjustment required |
| ⚪ ⚪ | ⚪ ⚪ | Too high | Reduce descent angle |
💡 Rule of thumb:
- More red = too low
- More white = too high
- One row red, one row white = perfectly on track
2. Difference between VASI and PAPI
Although both VASI and PAPI indicate the angle of descent, there are some important differences:
| Feature | VASI | PAPI |
|---|---|---|
| Number of lights | 2 rows of 2 lights | 4 lights in one row |
| Placement | On both sides of the runway | On one side of the runway |
| Accuracy | Less accurate | More precise with 4 lights |
| Usage | Smaller airports and base landing systems | Major airports and precision landings |
📌 VASI is simpler and suitable for smaller airports , while PAPI is more precise and more commonly used at larger airports .
3. Benefits of VASI
✈ Direct and simple visual feedback → Pilots can correct their descent angle without instruments.
🌫 Helps with low visibility and night landings → Especially useful in fog, rain and night flying.
⚠ Prevents landings that are too steep or too flat → Reduces the chance of hard landings or a landing that is too long.
🔧 Simple and reliable → Less complex than PAPI, easy to install and maintain.
4. Variants of VASI
📌 Standard VASI → Consists of two rows of two lights each.
📌 Tri-color VASI → Shows green, yellow or red to indicate altitude.
📌 Pulsating VASI → Flashing light when the pilot flies too high or too low .
💡 In short:
The Visual Approach Slope Indicator (VASI) is a simple yet effective tool for pilots to monitor their descent angle . With a two-tone system (red/white), it helps pilots land safely and accurately , especially at smaller airports and airfields without precision instruments .