What are taxiway centreline lights?

Taxiway centre line lights are provided along the centre of a taxiway to give the pilot continuous directional guidance, keeping the aircraft aligned with the pavement centre while taxiing — particularly valuable through curves, at complex junctions, and in low visibility where surface markings alone are hard to see. They are required on exit taxiways, taxiways, de-icing/anti-icing facilities and aprons intended for use in runway visual range below 350 m (unless traffic is light and edge lights and marking give adequate guidance), and recommended for night use in better visibility, particularly at complex intersections and on exit taxiways. Because the aircraft nose gear follows the centreline, taxiway centre line lights are normally inset (flush-mounted) fixtures, unlike taxiway edge lights, which may be elevated.

The lights are normally green, spaced more closely than taxiway edge lights: on straight sections not more than 30 m apart (not more than 15 m on a taxiway intended for use below 350 m RVR), against up to 60 m for edge lights, with the spacing reduced further through curves. On an exit taxiway the lights alternate green and yellow from near the runway centre line out to the perimeter of the ILS/MLS critical or sensitive area (or the lower edge of the inner transitional surface, whichever is farthest from the runway), with the first light green and the light nearest that perimeter always yellow; beyond it all lights are green. A pilot vacating the runway therefore sees green first, then alternating colours, and the last yellow marks the perimeter: all green beyond it means the aircraft is clear of the area. The yellow lights warn the pilot that the aircraft is still inside the runway environment. FAA practice (AC 150/5340-30) also alternates green and yellow on exit taxiways, but ends the pattern relative to the runway holding position or ILS critical area boundary rather than by the ICAO limit.