ALSF‑2

The ALSF‑2 (Approach Lighting System with Sequenced Flashing lights – Category II/III) is a high‑intensity precision approach lighting system designed for the most demanding low‑visibility operations. It extends 731 metres (2,400 ft) – or 914 metres (3,000 ft) on shallow-slope sites – from the runway threshold and includes all the features of the ALSF‑1 plus additional red side‑row bars and enhanced intensity. The system provides pilots with both horizontal and vertical guidance during CAT II and CAT III approaches, with decision heights as low as 100ft (CAT II) and 50ft or less (CAT III), and RVR down to 200m or lower. The ALSF‑2 is essential for airports that operate in regions with frequent fog, rain, or snow.

ALSF-2 exists to extend the ALSF-1 foundation specifically to the point where CAT II and CAT III minimums become achievable, adding red side-row bars and enhanced intensity precisely because the lower decision heights and shorter visual segments of those categories demand more visual information delivered with greater reliability than CAT I alone requires. The red side-row bars in particular give the pilot a distinct visual marker of entering the final, most critical approach segment, an added layer of positional awareness that CAT I operations don't need but that CAT II/III's tighter margins make valuable. Its purpose is to be the standard high-intensity system wherever an airport must support routine operations in the fog, rain, or snow-prone conditions that make CAT II/III capability operationally necessary rather than optional.

ALSF‑2 systems are installed at airports that support Category II and III precision approaches. They are typically used on:

• Precision approach runways with CAT II/III capability (100ft DH or lower, 200m RVR or lower)
• Major international hub airports requiring reliable low‑visibility operations
• Airports in regions with adverse weather (fog, heavy rain, snow)
• Military airbases supporting all‑weather operations
• Runways with high traffic volume where operational continuity is critical

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 and threshold-adjacent lights 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 ALSF‑2 supports CAT II/III operations, 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 ALSF‑2 system builds on the ALSF‑1 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 approach path. These provide distance and lateral guidance.
• Sequenced Flashing Lights (SFL): A row of 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 ALSF‑2 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 also includes automatic dimming based on RVR data.

As with ALSF‑1, the ALSF‑2 array runs two electrically independent control paths. Steady-burning elements — barrettes, red side-row bars, centreline barrette, and the decision bar — 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 are driven separately by a flasher/sequencer controller that discharges through each SFL lamp in strict outer-to-threshold order; in ALSF‑2 this sequencer is tuned for a higher flash rate and higher peak intensity than ALSF‑1 for better fog penetration. The ALCMS commands both subsystems in step together — raising RVR-linked brightness raises both the steady loop's current and the SFL's intensity tier — but a fault in one subsystem 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 along the stand run 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 higher-output xenon strobe tubes and trigger electronics than a CAT I system, housed in weatherproof domes with correspondingly larger capacitor banks at the base of each stand to sustain the faster, higher-intensity sequence. Red side-row bar fixtures in the final 300 m and the threshold-adjacent lights share the same die-cast, gasket-sealed construction as the runway's own edge and threshold lights, so the approach hardware transitions seamlessly into the runway lighting system. Where the approach crosses roads, water, or depressions, stands are raised on piers or catwalks engineered to keep the array coplanar, and because ALSF-2 supports CAT II/III operations, 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.