Arrow Air flight APWP6L, a DC-8-62(F) cargo plane, touched down on runway 02L at Singapore-Changi Airport at 00:29 hours. The runway controller at Changi Tower instructed the aircraft to park at Bay 117, a remote aircraft parking bay. The ground movement planner at Changi Tower selected the taxiway centre line lights to guide the aircraft along Taxiway WA to Bay C7 (Bay 117 is the second parking bay after Bay C7). Instead of stopping at the end of its assigned taxi route, the aircraft continued taxiing past Bay C7 along a diverted portion of Taxiway WA. At about 00:37 hours, the aircraft called Changi Tower to indicate its position near Bay 106. Realising that the aircraft had missed its assigned parking position, the ground movement planner at Changi Tower reselected the taxiway centre line lights to guide the aircraft back to Bay 117. The aircraft followed the return route until it was abeam Bay 117 on the straight section of the diverted portion of Taxiway WA. At that location, the pilot saw the ground marshaller in position at Bay 117 on the aircraft’s right side. Instead of continuing to follow the taxiway centre line lights on the assigned taxi route, the aircraft turned right. In doing so, it left the Taxiway WA centre line and went onto a grass area between Taxiway WA and the parking apron. The nose gear of the aircraft went across a drain within the grass area. The aircraft came to a halt when its main landing gears went into the drain at about 00:44 hours. Contributory Factors – The runway controller was not aware of the requirement for aircraft to be towed into Bay 117. He instructed flight APWP6L to follow the green lights to Bay 117 in accordance with standard ATC procedures. – The flight crew did not notice on the Jeppesen charts that there was a turf island separating Taxiway WA from the parking apron where Bay 117 was located. – As taxiway centre line lights were provided along Taxiway WA, according to ICAO Annex 14, there was no requirement for taxiway edge lights to be provided. However, where there is a large unmarked paved area adjacent to a taxiway, the provision of taxiway edge lights or reflective markers (in addition to taxiway centre line lights) would provide an additional cue to pilots to stay within the taxiway. This may help to prevent pilots inadvertently straying off the taxiway. – There were no edge lights or markers to show the grass area between Taxiway WA and the parking apron where Bay 117 was located. There is no requirement in ICAO Annex 14 for edge lights or markers to show the presence of grass areas adjacent to taxiways. – The drain located within the grass area between the diverted portion of Taxiway WA and the parking apron was outside the taxiway strip. According to ICAO Annex 14, drains located outside a taxiway strip are not required to be covered.