CAT III Approach Lighting

CAT III Approach Lighting is the most advanced precision approach lighting system available, designed to support Category III instrument approaches — the lowest visibility operations in commercial aviation. The system features a comprehensive 900‑metre configuration of high‑intensity barrettes, sequenced flashing lights, red side‑row bars, and enhanced centreline lighting. CAT III systems are distinguished by their exceptional reliability, redundancy, and light output, with multiple independent power feeds and backup systems to ensure continuous operation in the most critical conditions. They support operations with decision heights as low as 50 feet (CAT IIIa) or zero feet (CAT IIIb and CAT IIIc), and RVR down to 200 metres (CAT IIIa) or as low as 75 metres (CAT IIIb) and even zero (CAT IIIc).

CAT III approach lighting exists to support the narrowest margins in commercial aviation, where decision heights as low as 50 feet or zero and RVR as low as 75 metres or zero leave essentially no room for a lighting failure or a momentarily degraded visual cue to go unnoticed. Its comprehensive combination of high-intensity barrettes, sequenced flashing lights, red side-row bars, and enhanced centreline lighting exists to give the pilot (or, at the lowest minimums, the autoland system and any last-second visual cross-check) the maximum achievable visual information in the shortest possible viewing window before touchdown. The purpose behind its redundant power feeds and backup systems is equally central: at these minimums, a lighting outage isn't a degraded-but-workable situation, it is potentially the loss of the only visual reference available, so the system is engineered specifically to keep functioning through failures that a less critical system could simply tolerate.

CAT III approach lighting systems are installed at the world's busiest and most advanced airports. They are typically used on:

• Precision approach runways with CAT IIIa/b/c capability (50ft or zero DH, 200m RVR or less)
• Major international hub airports in regions with frequent fog, heavy rain, or snow
• Airports that serve as critical diversion points for long‑haul and transoceanic flights
• Military airbases with all‑weather operational requirements
• Runways with the highest traffic volumes where operational continuity is paramount
• Airports implementing CAT IIIb/c for zero‑visibility operations (with appropriate taxiway guidance)

Fixtures are set out along the extended runway centreline for the full 900 m length, on frangible stands graded to the approach slope; structures crossing roads, water, or depressions are elevated on piers or catwalks to keep the array on a consistent plane. Given the zero-tolerance for outage at CAT III minima, every stand is fed from dual, independently routed circuits in separate ductbanks, each backed by its own constant current regulator and standby power source, so that loss of a single feeder or regulator cannot take down the array. The red side-row bars in the final 300 m are tied into the runway's own lighting so the approach-to-runway transition reads as one continuous system. Setting-out follows the extended centreline and threshold survey to the same tolerance as the runway lighting it feeds into.

The CAT III approach lighting system incorporates all the features of CAT II with enhanced reliability and performance for the lowest visibility:

• High‑Intensity Barrettes: A series of light bars (typically 5 lights per bar) positioned at 30m/100 ft intervals along the full 731m/914m (2,400 ft/3,000 ft) approach path, with significantly higher luminous intensity than CAT I/II for penetration of dense fog.
• Sequenced Flashing Lights (SFL): Ultra‑high‑intensity lights that flash in rapid sequence from the outer end toward the threshold, creating a highly visible "rolling" effect that indicates the approach direction. The SFL in CAT III has the highest intensity and fastest flash rate for maximum visibility in fog.
• Red Side‑Row Bars: Red lights on both sides of the approach path in the final 300m segment, providing a critical visual warning of lateral deviation in very low visibility.
• Enhanced Centreline Lighting: More frequent centreline barrettes and lights, providing precise lateral guidance with reduced spacing for improved continuity in low visibility.
• Decision Bar: A row of lights at the decision height point, providing a clear visual reference for the pilot's go/no‑go decision.
• Redundancy and Reliability: CAT III systems feature redundant power feeds, dual CCRs, and automatic failover to ensure uninterrupted operation. Many systems also include remote monitoring and diagnostics for proactive maintenance.
• Automatic Intensity Control: The system automatically adjusts intensity based on real‑time RVR data, ensuring optimal visibility without blinding pilots in varying conditions.

At its core, CAT III control logic follows the same two-path architecture as CAT I and CAT II: steady-burning elements — high-intensity barrettes, red side-row bars, and enhanced centreline lighting — sit on a constant current series circuit, with the ALCMS selecting the intensity step and the CCR regulating the loop to a single fixed current shared by every fixture via its own isolating transformer, while the sequenced flashing lights and decision bar are driven by a separate flasher/sequencer controller discharging through each lamp in outer-to-threshold order at the highest rate and peak intensity in the approach lighting family. What sets CAT III apart is that this entire control chain — ALCMS, CCR, and sequencer — is duplicated rather than run as a single string, with automatic real-time adjustment against live RVR data on both subsystems. The mechanics of that duplication and the automatic failover between feeds are covered in the next section.

Barrette stands are frangible structures carrying multiple lamp housings on a rigid bar, graded in height to keep the array on the approach slope; die-cast aluminium housings behind toughened, gasket-sealed lenses achieve an IP67/IP68 rating on each high-intensity LED array or 200W+ halogen lamp. The sequenced flashing heads use the highest-output xenon strobe tubes and trigger electronics in the approach lighting family, housed in weatherproof domes with correspondingly larger capacitor banks to sustain the ultra-high-intensity, rapid sequence required at CAT III minima. Red side-row bar fixtures in the final 300 m share construction with the runway's own edge lighting, tying the approach hardware directly into the runway system. Given the zero-tolerance for outage at CAT III minima, each stand and fixture is wired to accept dual, independently routed feeder cables in separate ductbanks, each terminating at its own isolating transformer, so the loss of a single feeder or transformer does not extinguish the fixture; stands crossing roads, water, or depressions are raised on piers or catwalks engineered to the same redundancy standard. Stainless-steel fasteners and corrosion-resistant finishes are used throughout to meet the reliability demands of CAT III operation.