CAT I Approach Lighting

CAT I Approach Lighting is a precision approach lighting system designed to support Category I instrument approaches. 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. CAT I systems typically have a configuration length of 900 metres under the ICAO precision approach CAT I standard, or 731 metres (2,400 ft) – extendable to 914 metres/3,000 ft on shallow-slope sites – under the FAA MALSR/ALSF‑1 standard, depending on the airfield requirements and the specific standards applied. They support approaches with a decision height of 200 feet and RVR down to 550 metres (1,800 ft).

CAT I approach lighting exists to make instrument approaches down to a 200-foot decision height and reduced visibility genuinely flyable, by combining horizontal alignment cues (barrettes, centreline) with directional depth cues (sequenced flashing lights) into a single coherent system rather than relying on either type of guidance alone. Its purpose is calibrated specifically to CAT I minimums: providing enough visual structure that a pilot can reliably transition from instruments to visual reference at 200 feet and confirm the runway environment is in sight, without the additional redundancy, red side-row bars, and enhanced intensity that CAT II/III demand for their lower minimums. This makes it the standard configuration wherever an airport wants to support routine low-visibility operations without the substantially higher infrastructure and reliability requirements of the more advanced categories.

CAT I approach lighting 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 minimum)
• Regional and international airports that handle commercial air traffic
• Military airbases requiring all‑weather precision approach capability
• Runways with moderate traffic volumes that still require reliable low‑visibility operations
• Airports transitioning to higher CAT (CAT II/III) where CAT I serves as the baseline

Fixtures are set out along the extended runway centreline beginning at the threshold, on frangible light stands graded to follow the required approach slope over the 720 m or 900 m configuration length. Barrette stands are positioned at the specified 60 m or 100 m spacing, with the sequenced flashing light heads on their own dedicated stands in line with the centreline. Where the approach area crosses roads, drainage, or uneven terrain, fixtures are elevated 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. Setting-out is tied to the extended centreline and threshold survey to keep the array true to the final approach course.

The CAT I approach lighting 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 — 30m (100 ft) on FAA ALSF‑1 or the ICAO barrette section, 61m (200 ft) on FAA MALSR — 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. Higher intensities are used during low visibility.

A CAT I approach lighting installation is really two control systems sharing one approach corridor. The steady-burning elements — barrettes, crossbar, and centreline barrette — sit on a constant current series circuit: the ALCMS selects the required brightness step from RVR data or ATC instruction, the CCR regulates the whole loop to a single fixed current, and every steady fixture on that loop, fed through its own isolating transformer, burns at the same intensity simultaneously. The sequenced flashing lights are driven separately by a flasher/sequencer controller that discharges through each SFL lamp in strict outer-to-threshold order to create the rolling effect; the ALCMS commands its intensity tier in step with the steady loop, but the two subsystems are electrically independent, so a fault on one does not affect the other. Fault detection on the steady loop remains loop-based: the CCR monitors loop voltage against commanded current, and a voltage rise flags an open circuit or failed isolating transformer as a loop fault rather than a specific fixture.

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 terrain, stands are raised on piers or piled steel structures engineered to keep the barrette bars level and coplanar. 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.