Understanding Runway Approach Lighting Systems: Category I, II, and III
Approach lighting systems (ALS) are essential components of a runway’s visual aids, helping pilots transition from instrument flight to visual landing. These lighting systems are especially crucial during low-visibility conditions. They are designed to support various precision approach categories defined by the International Civil Aviation Organization (ICAO) and FAA: Category I (CAT I), Category II (CAT II), and Category III (CAT III). Each category has different lighting requirements based on minimum visibility and decision height for landing.
Purpose of Approach Lighting Systems
Approach lights provide visual cues that help pilots align with the runway centerline and judge the correct approach angle during landing. These lights are positioned symmetrically along the extended centerline of the runway and include a combination of steady-burning lights, sequenced flashing lights, and visual guidance elements like crossbars and barrettes.
Category I Approach Lighting (CAT I)
CAT I approaches are designed for operations where visibility is relatively good. They allow for a decision height (DH) not lower than 200 feet (60 meters) and runway visual range (RVR) not less than 550 meters.
- Lighting System: Commonly uses an ALSF-1 or SSALR (Simplified Short Approach Lighting System with Runway Alignment Indicator Lights).
- Length: Typically extends 420–900 meters from the runway threshold.
- Visual Elements: Includes a centerline of steady white lights, crossbars, and sometimes a sequenced flashing light system (RAIL).
Category II Approach Lighting (CAT II)
CAT II systems are intended for lower visibility than CAT I, supporting operations with DH down to 100 feet and RVR not less than 300 meters.
- Lighting System: Often uses ALSF-2.
- Length: Generally extends up to 900 meters.
- Visual Elements: More complex than CAT I; includes steady-burning centerline lights, barrettes, crossbars, and sequenced flashing lights (RAIL) to enhance visibility and alignment.
Category III Approach Lighting (CAT III)
CAT III approaches are used in very low visibility conditions with DH lower than 100 feet or even no decision height (CAT IIIB and IIIC). RVR requirements are as low as 75 meters.
- Lighting System: Uses ALSF-2, sometimes in conjunction with High-Intensity Runway Lights (HIRL) and touchdown zone lighting (TDZL).
- Length: Full 900 meters.
- Visual Elements: Maximized visual aid, including closely spaced barrettes, intense centerline lighting, and full sequenced flashing lights to provide precise alignment under foggy or zero-visibility conditions.
Comparison Table: CAT I vs CAT II vs CAT III Approach Lighting
| Feature | CAT I | CAT II | CAT III |
|---|---|---|---|
| Decision Height (DH) | ≥ 200 ft (60 m) | 100–200 ft (30–60 m) | < 100 ft or no DH |
| Runway Visual Range (RVR) | ≥ 550 m | ≥ 300 m | As low as 75 m |
| Lighting System | ALSF-1 / SSALR | ALSF-2 | ALSF-2 + HIRL + TDZL |
| System Length | 420–900 m | Up to 900 m | Full 900 m |
| Centerline Lights | White, steady-burning | More intense and tightly spaced | Highly visible, intense, continuous |
| Crossbars/Barrettes | Yes | Multiple, more frequent | Dense, closely spaced |
| Sequenced Flashing Lights | Optional | Required | Required (for precise alignment) |
| Usage Conditions | Moderate visibility | Low visibility | Very low to zero visibility |
Conclusion
Runway approach lighting systems are engineered to match the level of precision and visibility required for safe landings. As the approach category increases, so does the complexity and intensity of the lighting system. Understanding the differences among CAT I, II, and III helps airport planners, engineers, and pilots align their operations with international aviation safety standards. Whether it's a basic ALS for moderate conditions or a fully equipped CAT III system for zero-visibility landings, each lighting configuration plays a crucial role in safe aircraft operations.