PAPI & VASI Lighting
Airport Runways & Infrastructure · 4 specs
Direct Answer
The Precision Approach Path Indicator (PAPI) and Visual Approach Slope Indicator (VASI) provide visual glide slope guidance on final approach using color-transition optical lenses: 4 White (Too High), 2 Red / 2 White (On 3.0° Slope), and 4 Red (Too Low).
Specs
| Standard Approach Angle | 3.0 degrees standard (up to 5.5° on steep approaches like London City EGLC) |
| PAPI Array Layout | Single row of 4 light units perpendicular to runway on left side |
| VASI Array Layout | 2 or 3 bars of lights in tandem (Upwind / Downwind bars) |
| Visible Range | 5 miles (8 km) by day, up to 20 miles (32 km) at night |
PAPI 4-Lamp Visual Glide Slope Optical Transition Angles
The Precision Approach Path Indicator (PAPI) consists of four sharp-transition optical projector units located on the left side of the runway threshold, calibrated to the standard 3.0° glide slope:
| Pilot Visual Indication | 4-Lamp Color Configuration | Glide Slope Angular Deviation | Pilot Cockpit Action Required |
|---|---|---|---|
| Too High | WHITE WHITE WHITE WHITE |
> 3.5° (> 3° 30') | Increase rate of descent; steep approach |
| Slightly High | WHITE WHITE WHITE RED |
≈ 3.2° (3° 10') | Gentle correction down to nominal path |
| On Correct Glide Slope | WHITE WHITE RED RED |
3.0° (2° 50'–3° 10') | Maintain established stable descent rate |
| Slightly Low | WHITE RED RED RED |
≈ 2.8° (2° 50') | Add thrust; shallow approach angle |
| Too Low (Danger) | RED RED RED RED |
< 2.5° (< 2° 30') | Immediate thrust increase / terrain risk |
Technical Comparison: PAPI vs. VASI vs. APAPI vs. T-VASIS
| System Type | Lamp Array Configuration | Standard Usable Range | Minimum Eye Height over Threshold (MEHT) |
|---|---|---|---|
| PAPI (Precision API) | 4 sharp multi-lamp units in single horizontal line | 4–5 NM (7–9 km) | 9.0–14.0 m (30–45 ft) |
| APAPI (Abbreviated PAPI) | 2 sharp multi-lamp units (Red/White pairs) | 3 NM (5.5 km) | 6.0–9.0 m (20–30 ft) (General Aviation) |
| VASI (Visual Approach Slope) | 2-bar or 3-bar red/white split box array | 3–5 NM (5–8 km) | 8.0–16.0 m (26–52 ft) |
| T-VASIS (T-Visual Approach) | 20-lamp 'T' inverted array (Fly-up / Fly-down) | 5 NM (9 km) | 15.0 m (50 ft) (Australia / Oceania) |