Mori Building City Air Services Co., which runs a helicopter service between downtown Tokyo and Narita airport, may add a third chopper now that tieups with airlines have boosted traffic.

The unit of Mori Building Co., Japan's biggest closely held developer, may add an eight-seat helicopter to its two existing four-seaters, Koichi Ueno, a managing director, said Wednesday in Tokyo, without giving a time frame. The helicopter service pares travel times from downtown Tokyo to Narita to about 30 minutes, he said.

Mori has doubled monthly passenger numbers since starting flights in September after American Airlines, Deutsche Lufthansa AG, Alitalia SpA and Japan Airlines Corp. joined All Nippon Airways Co. in offering the service to business and first-class passengers. East Japan Railway Co.'s Narita Express train takes about an hour to make the approximately 80-km trip from Tokyo Station to the nation's busiest international airport.