MIAMI — United Airlines will introduce daily non-stop flights between San Francisco and Singapore, becoming the first and direct service between both cities, and the only non-stop one between the United States and Singapore. The flights, to be operated by the carriers' Boeing 787-9 Dreamliner fleet, are expected to start on June 3, subject to government approval.The flight will take about 16 hours, 20 minutes to Singapore, saving about four hours of travel time when compared to the airline’s current service via Tokyo's Narita. The return flight to San Francisco will be approximately 15 hours, 30 minutes.Measured by distance, the San Francisco - Singapore service will be the longest scheduled 787 flight operated by any airline, and the longest scheduled flight operated by any U.S. carrier, at 8,446 miles (13,593 km)."This long-awaited service has been made possible by the newest version of our customer-pleasing Boeing 787 Dreamliner" said Marcel Fuchs, United's vice president of Atlantic and Pacific sales.United joins Middle East carriers Emirates, Qatar Airways and Etihad, as well as Australia's Qantas, among the airlines offering ultra long-haul flights, taking advantage of the operational economics of last-generation aircraft such as the Airbus A350 XWB, the A380-800 and the Boeing 787 Dreamliner. Also, the current decrease in fuel prices has been a leading factor for airlines to exploit long, thin routes.A spike in fuel prices and poor demand caused Singapore Airlines to cancel the world's longest non-stop service from Singapore to Newark in 2013, as well as its non-stop service to Los Angeles. However, the airline announced plans to bring these flights back in 2018, once it takes delivery of its ultra-long-range Airbus A350-900ULR variant.Currently, the longest non-stop scheduled airline flight is Qantas Flight QF8 from Sydney to Dallas/Ft. Worth, operated by an Airbus A380-800, with a distance of 8,577 miles (13,804 km).