Touchdown Zone Light (TDZ)

A Touchdown Zone Light (TDZ) is a flush-mounted inset light installed in the touchdown zone of precision approach runways. These white lights are arranged in rows parallel to the runway centreline, covering the first 900 metres (3,000 feet) of the runway from the threshold. The TDZ lights provide pilots with a clear visual indication of the optimal touchdown area during the final approach and landing phase, particularly critical for Category II and III precision approaches where visibility is severely limited.

TDZ lights exist to give a pilot a positive visual indication of the runway surface itself, directly beneath and ahead of the aircraft, during the final seconds of the flare and touchdown when peripheral references like edge and centreline lights are no longer enough on their own to judge height and closure rate accurately. This matters most on Category II and III approaches, where cloud ceiling and visibility may be so limited that the runway surface only becomes visible in the last moments before touchdown, leaving very little margin to correct a misjudged flare. By illuminating the specific zone where touchdown should occur, rather than the runway generally, the lights let the pilot confirm they are landing in the intended area rather than long, short, or off-centre, which is critical when there is no time left to go around and re-attempt the approach.

Installed on precision approach runways (CAT II and CAT III) where enhanced touchdown zone visibility is required. The lights are arranged in rows on both sides of the runway centreline, typically at 30-metre (100-foot) intervals along the runway length. The touchdown zone extends from the threshold for 900 metres (3,000 feet), providing continuous visual guidance during the flare and landing phase. TDZ lights are used in conjunction with centreline lights, edge lights, and approach lighting systems.

Fixtures are installed in pairs on either side of the runway centreline, in rows spaced at approximately 30 m intervals extending from the threshold for 900 m, with each pair set a fixed offset either side of the centreline so the row reads as two parallel white light lines rather than a single line. Each unit mounts on an L-868 base can recessed flush with the runway pavement, rated for the wheel loads and jet-blast environment of the touchdown zone, with cabling run in pavement ducting back to the touchdown zone lighting circuit and its own isolating transformer bank, allowing TDZ intensity to be stepped independently of the centreline and edge circuits. Setting-out is tied to the runway centreline survey so the TDZ pattern remains symmetrical and continuous with the threshold and centreline lighting, and the full 900 m installation is typically staged in sections to limit runway closure time during construction or retrofit.

Each TDZ light is powered from the series circuit via an isolating transformer. The light projects a uni-directional beam towards the approaching aircraft using a precision optical system with a reflector and prism. The white colour provides maximum visibility against the runway surface. The intensity is controlled by the CCR's current steps, with higher intensities used during low visibility operations. The lights are flush-mounted in the pavement to withstand heavy aircraft loads and provide a low-profile installation.

TDZ lights are not individually addressable — control is exercised at the loop level through their own dedicated constant current series circuit. An Airfield Lighting Control and Monitoring System (ALCMS), operated by the tower or an automated visibility-linked routine, selects the required brightness step based on prevailing RVR, ATC instruction, or (at smaller aerodromes) pilot-controlled lighting input. This command is passed to the CCR serving the TDZ circuit, which regulates the entire 900 m loop to a single fixed current. Because TDZ lights typically run on a separate loop from the centreline and edge circuits, the touchdown zone can be stepped in intensity independently of the rest of the runway lighting, but every fixture within the TDZ loop itself still burns at the same intensity simultaneously — there is no way to dim or brighten one row, or one part of the 900 m zone, independently of the rest of the loop. Fault detection is likewise loop-based: the CCR continuously monitors loop voltage against the commanded current, and a voltage rise at constant current signals increased circuit impedance, typically an open-circuit lamp or a failed isolating transformer, which is reported to the ALCMS as a loop fault rather than pinpointed to an exact fixture without further sectionalised testing.

The fixture housing — FAA Type L-850B — is a die-cast aluminium body machined to seat into an L-868 base can and rated for the full wheel-load and jet-blast environment of the touchdown zone, with the top plate set flush with the pavement and crowned slightly to shed water. The white uni-directional beam is produced by an LED array or 45W/65W halogen lamp behind a toughened glass or high-strength polycarbonate lens, gasket-sealed to the housing with an O-ring or silicone seal to achieve an IP67/IP68 rating against water and de-icing fluid ingress. Internal wiring terminates at L-823 primary connectors sealed for direct burial, which mate to the secondary leads of the dedicated touchdown zone isolating transformer bank, keeping TDZ circuit joints separate from the centreline and edge circuits. Stainless-steel fasteners are used throughout, and the base can assembly includes a drainage provision to prevent water pooling beneath the fixture in the heavily trafficked touchdown zone pavement.