The Precision Approach Path Indicator (PAPI) is a visual aid for pilots during landings. It consists of four lights next to the runway that help maintain the correct descent angle . PAPI is an essential tool for precision landings , especially at night or in poor visibility.
1. How does a PAPI system work?
The four PAPI lights emit a combination of red and white lights , depending on the pilot's viewing angle . This immediately indicates whether the aircraft is too high, too low, or at the correct approach angle .
| Light pattern | Meaning | Action for pilot |
|---|---|---|
| 🔴 🔴 🔴 🔴 | Too low | Increase descent angle |
| 🔴 🔴 🔴 ⚪ | A little too low | A little bit of a rise |
| 🔴 🔴 ⚪ ⚪ | Correct approach angle (usually 3°) | No adjustment required |
| 🔴 ⚪ ⚪ ⚪ | A little too high | Slightly descending |
| ⚪ ⚪ ⚪ ⚪ | Too high | Falling more sharply |
💡 Rule of thumb: More red = too low , more white = too high .
2. Benefits of a PAPI system
✈ Direct visual feedback → Pilots can maintain the correct descent angle without instruments .
🌫 Helps with low visibility and night landings → Especially useful in fog or darkness.
⚠ Prevents landings that are too steep or too flat → Improves safety and prevents hard landings .
🔧 Easy to install and maintain → Widely used in large and small airports .
3. Types of PAPI systems
📌 PAPI-4 → Standard PAPI with 4 lamps , most commonly used worldwide.
📌 PAPI-2 → Simplified version with 2 lamps , used at smaller airfields.
📌 APAPI (Abbreviated PAPI) → A shorter version for smaller airports with less space.
4. Difference between PAPI and VASI (Visual Approach Slope Indicator)
| System | Number of lights | Placement | Usage |
|---|---|---|---|
| PAPI | 4 lights in one row | On one side of the runway | More precise instructions |
| VASI | 2 rows of 2 lights | Both sides of the runway | Simpler, less accurate |
💡 In short:
The Precision Approach Path Indicator (PAPI) is a visual aid for pilots to maintain the correct descent angle during landing. Its four lights display a red-and-white pattern , allowing the pilot to immediately see if they are too high, too low, or perfectly on course .