CAT II Approach Lighting

CAT II Approach Lighting is a precision approach lighting system designed to support Category II instrument approaches. It typically consists of a full 900‑metre configuration of barrettes, sequenced flashing lights, and additional red side‑row bars in the final approach segment. The system provides enhanced visual cues compared to CAT I, with extra centreline lights and barrettes to help pilots maintain precise alignment and glide slope during low‑visibility approaches. CAT II systems support operations with a decision height of 100 feet and RVR down to 300 metres (or 350 metres in some regions). They are essential for airports that experience frequent fog, rain, or snow.

CAT II approach lighting exists to push the decision height down to 100 feet and the RVR minimum down to roughly 300 metres, a regime where the pilot has meaningfully less time and visual distance to confirm the runway environment than under CAT I, so the system compensates with extra centreline lights, barrettes, and red side-row bars in the final segment to concentrate visual information exactly where the pilot needs it most. The red side-row bars specifically exist to mark the final approach segment distinctly from the earlier portion of the approach, giving the pilot an unambiguous cue that they are entering the most visually critical final stretch before the decision point. Its purpose sits between CAT I's routine low-visibility support and CAT III's near-zero-visibility redundancy, calibrated to airports that need reliable operations through frequent fog, rain, or snow without requiring the most extreme (and expensive) level of system redundancy.

CAT II approach lighting systems are installed at airports that support Category II precision approaches. They are typically used on:

• Precision approach runways with CAT II capability (100ft DH, 300m RVR minimum)
• Major international hub airports with high traffic volumes and frequent low‑visibility conditions
• Airports in regions with adverse weather (fog, heavy rain, snow)
• Military airbases requiring all‑weather operational capability
• Runways that serve as diversion airports for CAT II/III‑equipped aircraft

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. The red side-row bars in the final 300 m are tied into the runway's own lighting so the transition from approach to runway reads as one continuous system. Because CAT II operations demand higher reliability, stands and cabling are typically arranged on independently fed circuits so a single fault cannot extinguish the full array, with cable runs in duct back to redundant constant current regulators. Setting-out follows the extended centreline and threshold survey to the same tolerance as the runway lighting it feeds into.

The CAT II approach lighting system builds on CAT I with enhanced features for lower visibility:

• Barrettes: A series of light bars (typically 3 to 5 lights per bar) positioned at 30m/100 ft intervals along the full 731m/914m (2,400 ft/3,000 ft) approach path, providing distance and lateral guidance.
• Sequenced Flashing Lights (SFL): High‑intensity lights that flash in sequence from the outer end toward the threshold, creating a "rolling" effect that indicates the approach direction and helps pilots maintain the correct glide slope. The SFL in CAT II has higher intensity and faster flash rate for better visibility in fog.
• Red Side‑Row Bars: Additional red lights installed on both sides of the approach path in the final 300m segment. These provide a visual warning if the aircraft deviates laterally from the centreline and help define the approach corridor.
• Centreline Barrette: A short row of lights on the approach centreline, providing precise lateral guidance and helping pilots align with the runway.
• Decision Bar: A row of lights at the decision height point, providing a visual reference for the pilot's go/no‑go decision.
• Intensity Control: The entire system is controlled by CCRs with multiple intensity steps (typically 5 steps), with higher intensities used during low‑visibility conditions. The system may also include automatic dimming based on RVR data.

As with CAT I, a CAT II installation runs two electrically independent control paths across the same 731m/914m corridor. Steady-burning elements — barrettes, red side-row bars, and centreline barrette — sit on a constant current series circuit: the ALCMS selects the intensity step based on RVR, ATC instruction, or automatic visibility-linked logic, the CCR regulates the whole loop to a single fixed current, and every steady fixture on that loop burns at the same brightness via its own isolating transformer. The sequenced flashing lights and decision bar are driven separately by a flasher/sequencer controller that discharges through each SFL lamp in strict outer-to-threshold order; in CAT II this sequencer runs at a higher flash rate and higher peak intensity than CAT I for better fog penetration. The ALCMS commands both subsystems in step together, but a fault in one does not propagate to the other, and each is fault-monitored independently by its own controller.

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 lamp. The sequenced flashing heads use high-intensity xenon strobe tubes and trigger electronics housed in weatherproof domes, backed by capacitor banks at the base of each stand sized for the faster CAT II flash sequence. Red side-row bar fixtures in the final 300 m share the same die-cast, gasket-sealed construction as the runway's own edge lighting, so the approach hardware ties directly into the runway lighting system at the threshold. Where the approach crosses roads, water, or depressions, stands are raised on piers or catwalks engineered to keep the array coplanar, and because CAT II operations demand higher reliability, circuits are built with independent cabling and redundant isolating transformers so a single fault cannot take down the full array. Stainless-steel fasteners and corrosion-resistant finishes are used throughout.