Taiwan and China in January 2015 consulted over the M503 flight route, which runs northeast to southwest about 8 nautical miles (14.8km) west of the median line of the Taiwan Strait.
At the time, China said that overland civil aviation routes from Shanghai to Guangzhou had become so congested that it needed to open a new air artery over the Strait.
Taiwan countered that air routes over the Strait were too congested for such a move, so the two sides compromised on a southbound-only opening and agreed that China would not seek additional northbound routes without prior consultation.