Unidirectional Touchdown Zone Light

A Unidirectional Touchdown Zone Light is an inset fixture that projects white light in a single direction along the runway centreline, specifically oriented towards the approaching aircraft. These lights are installed in the first 900 metres (3,000 ft) of the runway from the threshold, arranged in rows or a barrette pattern, to clearly define the touchdown zone. The unidirectional design focuses the light output towards the approach path, providing pilots with a precise visual cue of the landing area during the final phase of approach. They are typically used on precision approach runways (CAT I/II/III) where enhanced visual guidance is required. Where both runway ends are used for landing, coverage for the opposite direction is provided either by a second, independently-oriented unidirectional set or by installing dedicated bidirectional TDZ fixtures, rather than by a single fixture serving both directions.

Touchdown zone lights exist to answer a question standard runway edge and centreline lighting alone can't precisely resolve: exactly where within the touchdown zone will the aircraft actually contact the runway. In marginal visibility, this is the difference between a pilot judging their flare and touchdown point by general runway perspective versus having a defined, lit target zone to aim for, which matters directly for landing distance margins on precision approach runways.

Making the light unidirectional, aimed specifically at the approaching aircraft, concentrates all of that visual guidance exactly where the approaching pilot needs it, rather than diffusing intensity in a direction where no one is looking. This focus is what allows TDZ lighting to remain effective even in the degraded visibility conditions CAT I/II/III operations are specifically designed to handle.

Unidirectional touchdown zone lights are installed on precision approach runways (CAT I, II, and III) to provide visual reference for the landing area. They are placed at 30‑metre intervals in the first 900 m of the runway from the threshold, typically in two parallel rows spaced 9 m apart on either side of the centreline, or as a single row of barrettes. They are used on runways with a predominant approach direction, where bidirectional guidance is not required. On bidirectional runways, unidirectional lights may be installed for each approach direction, with separate fixtures oriented towards each end. They are often supplemented by runway centreline lights and edge lights to provide comprehensive visual guidance.

Fixtures are recessed into the pavement in two parallel rows 9 m either side of the centreline (or as single-row barrettes), at 30 m intervals through the first 900 m of runway from the threshold, using a load-rated inset base can set flush with the surface. Because each fixture is unidirectional, orientation is set out to face the runway's predominant approach direction; on runways where both ends are used for landing, a second, independently-oriented set of fixtures is installed for the opposite direction rather than relying on one fixture to serve both. Cabling and isolating transformer arrangements follow the standard touchdown-zone-lighting convention, with position tied to the threshold survey for whichever approach direction each row serves.

Each unidirectional touchdown zone light is powered from the constant current series circuit via an isolating transformer. The fixture contains a single optical system – a precision reflector and prism – that projects a narrow, high‑intensity beam (typically 3° vertical × 6° horizontal) in the direction of the approaching aircraft. The light output is white, with intensity controlled by the CCR's current steps (up to 6.6 A) to match visibility conditions. The unidirectional design ensures that all light energy is directed towards the approach path, maximising brightness and range for pilots while minimising cost and complexity compared to bidirectional versions.

Unidirectional touchdown zone lights are not individually addressable — control is exercised at the loop level through the constant current series circuit dedicated to the touchdown zone lighting (TDZL) system, which is typically its own circuit distinct from the runway edge and centreline loops so it can be switched and stepped independently to match the approach category in use. An ALCMS, driven by tower instruction or an automated visibility-linked routine, selects the required brightness step for the TDZL loop, and the CCR regulates the entire loop to a single fixed current, up to 6.6A for the lowest RVR conditions the runway supports. Every fixture in both parallel rows, via its own isolating transformer, draws the same current and burns at the same intensity simultaneously — the CCR has no way to brighten one barrette independently of another on the same loop. Because each fixture is unidirectional, the loop only illuminates the approach in progress: on a runway landed in both directions, the two directions' TDZL fixtures are wired, oriented, and often switched as separate loops so the ALCMS activates only the set facing the runway direction currently in use. Fault detection remains loop-based: the CCR monitors loop voltage against commanded current, and a voltage rise — typically an open-circuit source or failed isolating transformer — is reported to the ALCMS as a loop fault, isolated to a specific barrette only through sectionalised field testing.

The fixture is built as a die-cast aluminium barrette housing containing two or three individual lamp or LED optical assemblies aimed in the same direction along the runway centreline, set flush into an L-852-series base can recessed into the runway pavement within the touchdown zone. Each optical position uses a white-filtered lens or LED array behind a toughened glass or polycarbonate top lens, load-rated for aircraft wheel loading and gasket-sealed with an O-ring to achieve an IP67/IP68 rating. The barrette's optical elements are all aimed toward the approach direction during installation, and internal wiring from all lamp positions is bundled within the housing before terminating at a sealed L-823 primary connector, which mates to the secondary lead of a nearby isolating transformer housed in its own base can. Stainless-steel fasteners and a drainage channel in the base can complete the assembly, consistent with other inset runway fixtures.