ALSF‑1

The ALSF‑1 (Approach Lighting System with Sequenced Flashing lights – Category I) is a precision approach lighting system designed to provide visual guidance to pilots during the final phase of approach. It consists of a series of light bars (barrettes) and sequenced flashing lights (SFL) extending from the runway threshold into the approach area. The system provides both horizontal and vertical guidance, helping pilots maintain the correct approach path and align with the runway centreline. ALSF‑1 supports Category I precision approaches with decision heights as low as 200ft and RVR as low as 550m (1,800 ft).

ALSF-1 exists to combine horizontal alignment guidance and directional depth cues into a single integrated system purpose-built for CAT I precision approaches, rather than relying on either barrettes or sequenced flashing lights in isolation. The barrettes give the pilot the fine lateral and distance reference needed to track the centreline precisely, while the sequenced flashing lights add a directional, rolling cue that reinforces correct alignment and improves depth perception, particularly as visibility becomes marginal. Its purpose is calibrated to CAT I's specific operating envelope — a 200-foot decision height and RVR as low as 550m (1,800 ft) — providing enough combined visual structure to make that minimum reliably achievable without the additional red side-row bars and redundancy that only become necessary at CAT II/III's lower minimums.

ALSF‑1 systems are installed at airports that support Category I precision approaches. They are typically used on:

• Precision approach runways with CAT I capability (200ft DH, 550m (1,800 ft) RVR)
• Major international airports requiring reliable low visibility operations
• Military airbases supporting precision approach operations
• Runways with approach speeds requiring enhanced visual cueing
• Airports transitioning to higher CAT (as a baseline system)

Fixtures are set out along the extended runway centreline beginning at the threshold, on frangible light stands whose height is graded to follow the required approach slope. Barrette stands are positioned at the specified 60 m or 100 m spacing, with the sequenced flashing light heads mounted on their own dedicated stands in line with the centreline. Where the approach area crosses roads, drainage, or uneven ground, fixtures are raised on piers or piled structures to keep the array on a consistent plane. Steady-burning and SFL circuits are cabled separately in duct back to their own constant current regulators, so a fault on one circuit does not extinguish the other. Setting-out is tied to the extended runway centreline and threshold survey to keep the array true to the final approach course.

The ALSF‑1 system operates through a combination of steady-burning barrettes and sequenced flashing lights:

• Barrettes: A series of light bars (typically 3 to 5 lights per bar) positioned at regular intervals (typically 30m (100 ft)) along the 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.
• Centreline Barrette: A short row of lights on the approach centreline, providing precise lateral guidance and helping pilots align with the runway.
• Crossbar Lights: Additional lights perpendicular to the approach path, providing lateral and distance reference.
• Intensity Control: The entire system is controlled by CCRs with multiple intensity steps (typically 5 steps), allowing the ATC to adjust brightness for visibility conditions.

The ALSF‑1 array is actually two control systems working side by side. The steady-burning elements — barrettes, crossbar, and centreline lights — are wired onto a constant current series circuit like any other AGL fixture: an Airfield Lighting Control and Monitoring System (ALCMS) selects the intensity step, the CCR regulates the whole loop to a single fixed current, and every steady fixture on that loop burns at the same brightness simultaneously via its own isolating transformer. The sequenced flashing lights (SFL), however, are not part of that steady-current loop at all. Each SFL lamp is fired individually by a flasher/sequencer controller that discharges a capacitor bank through the lamp in strict timed order, outer light first, working inward toward the threshold, to create the rolling effect. The ALCMS commands the sequencer's flash rate and intensity tier much as it commands the CCR's current step, but the two subsystems are electrically independent: a fault on the steady loop does not affect the flash sequence, and vice versa.

Barrette stands are frangible structures, each carrying multiple lamp housings mounted side-by-side on a rigid bar, with the bar height and stand base graded to keep the whole array on the required approach slope; die-cast aluminium housings behind toughened, gasket-sealed lenses achieve an IP67/IP68 rating on each lamp. The sequenced flashing light heads are a separate assembly — a xenon strobe tube and trigger electronics in a weatherproof dome, backed by a capacitor bank at the base of the stand, engineered to deliver the discharge pulse for each flash in the outer-to-threshold sequence. Where the approach area crosses roads, drainage, or uneven ground, stands are raised on piers or piled steel structures engineered to keep the barrette bars level and coplanar despite the terrain below. Internal wiring for both steady and SFL circuits terminates at L-823 primary connectors sealed for direct burial, feeding separate isolating transformers and regulators so a fault on one circuit cannot extinguish the other. Stainless-steel fasteners and corrosion-resistant finishes are used throughout to withstand prolonged outdoor exposure along the extended approach.