China’s Civil Aviation Administration (CAA) in early January announced the expansion of its heavily-trafficked flight route M503. Authorities announced that the route, which previously accommodated only southbound flights over the Taiwan Strait, would be expanded into a north- and south-bound route and accompanied by the establishment of three extension routes servicing the cities of Xiamen, Fuzhou, and Dongshan.

Authorities in Taipei quickly condemned the announcement as a unilateral and destabilizing violation of a previous cross-strait agreement, and demanded an immediate halt to air traffic using the route. The United States also weighed in, with the American Institute in Taiwan expressing concern that the announcement was made without consultation with Taipei and unilaterally altered the cross-strait “status quo.” But why would the expansion of a previously-accepted route that falls on the Chinese side of the median line in the Taiwan Strait (albeit by just 4 nautical miles) garner such a strong response from Taipei? The dispute offers insights into the messy nature of air traffic in Asia, how the lack of a robust international flight regulation system contributes to that disorder, and the often-overlooked difficulties Taiwan faces due to its exclusion from most international bodies.

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The CAA first announced M503 in 2015 as a two-way route servicing the eastern China seaboard. The route was proposed as a measure to ease crowded airspace over the Chinese mainland, and facilitate travel from cities in the Yangzi River Delta like Nanjing and Shanghai to points south including Guangzhou and Hong Kong, and vice versa. At the time, the most congested air route along China’s eastern seaboard was carrying 1,200 flights a day, including 840 just along the southeast coast, and the volume was increasing by 10 percent a year. But the CAA’s announcement triggered an immediate outcry from Taipei, which worried that the flights over the strait could jeopardize safety in nearby Taiwanese airspace. In response, China agreed to only operate southbound flights along the proposed route, and to hold off on using the three extension routes, W121, W122, and W123, until negotiations over those routes were complete.

For almost three years, the southbound M503 route operated smoothly. But on January 4, without consulting Taipei, the CAA announced that the extension routes would begin operating and Chinese planes would begin using M503 for northbound travel. Taiwan’s protests this time were entirely dismissed by Chinese government officials, who said they did not need permission to adjust the flight route, since it “involves no Taiwan flight route or destination and will not affect Taiwan flight safety” and, crucially, falls entirely within Shanghai’s flight information region (FIR).

An FIR is an area in which a country (or, where warranted, a single city) coordinates air traffic, information, and alert services for aircraft in flight. This information includes weather conditions, position of nearby aircraft, and any potential dangers to flight. Planning flight routes that exist entirely within one FIR generally falls to the state administering that FIR, according to the guidelines of the International Civil Aviation Organization (ICAO). M503 runs along the easternmost edge of the Shanghai FIR, as little as 4 nautical miles from Taipei’s FIR, and so it is technically China’s right to set up the route to service flights along the coast. But section 4.2.6 of ICAO’s Air Traffic Services Planning Manual stipulates that establishing and changing flight routes should be done “only after they have been coordinated with all parties concerned.” Authorities in Taiwan protested that M503 “concerns” them because it hews so closely to the border of the Shanghai and Taipei FIRs and crosses preexisting routes from Taiwan to the Kinmen and Matsu island groups, meaning they should have been consulted on its expansion.

The Taipei FIR services over 1.5 million flights per year carrying 58 million travelers annually, yet Taiwan is not a member of ICAO, a UN agency. Taiwan was expelled from the United Nations in favor of the People’s Republic of China in 1971 and is therefore not afforded member status in most of its agencies. As recently as 2013, China asked the ICAO president to invite Taiwan as a guest to its triennial assembly, where the organization determines its worldwide policy agenda. But in a sign of colder relations between Beijing and the administration of President Tsai Ing-wen, Taiwan was snubbed in 2016. Taiwan’s exclusion from the international body means it has no formal avenue for registering complaints against China’s aviation actions.

Further complicating the matter are restricted airspace blocks set up by the Taiwanese air force. The Republic of China Air Force (ROCAF) maintains a series of patrol areas on the western border of Taipei’s FIR, just a few miles from the median line over the Taiwan Strait. The ROCAF maintains these blocks as a buffer zone against intrusions by Chinese military aircraft, and any plane entering them without clearance could be confronted by fighter jets scrambled from Taiwan. With M503 flights regularly operating just a few miles west, ROCAF alert teams could be hard-pressed to determine whether a plane violating the restricted space is engaged in an intentional incursion, or just a passenger plane veering slightly off-course.

The potential for aerial encounters raises another thorny issue in the airspace over the East China Sea—the overlapping air defense identification zones (ADIZs) declared by Japan, Taiwan, South Korea, and China. ADIZs are areas declared around the national airspace of a country in which aircraft are required to comply with a series of regulations, which vary greatly. In most cases, planes travelling through the ADIZ intending to enter a nation’s air space, or having just left it, are required to share their flight plan and provide regular position updates. China’s ADIZ, declared in 2013, drew U.S. condemnation for seeking to apply regulations to all aircraft passing through the zone, even those en route to another country with no intention of entering or leaving Chinese airspace.

China’s ADIZ has been widely criticized not only for this, but also for overlapping with those of its neighbors in the East China Sea. But the other ADIZs in the region overlap and cause friction as well. Taiwan’s ADIZ crosses over Japan’s Yonaguni Island and overlaps Japan’s ADIZ there; China entirely ignores the Taiwan ADIZ, which crosses deep into mainland China; and South Korea extended its ADIZ south more than 100 nautical miles in retaliation against China’s 2013 declaration.

A major problem with ADIZs is that there is little basis for them in international law—even less than for FIRs. States create them unilaterally, and civilian aircraft generally abide by them to ensure that they do not fall afoul of the declaring state, not because of any widely agreed-upon basis in law. Foreign military planes, meanwhile, acknowledge or ignore ADIZs based largely on whether they agree with the precedents being established. For instance, in the East China Sea, U.S., Japanese, and South Korean aircraft all violated China’s ADIZ almost immediately after its declaration to signal their disagreement.

In retaliation for not being consulted on the expansion of Beijing’s M503 air route, Taipei canceled 176 flights from China to Taiwan over Lunar New Year operated by carriers using the route. The cancellations did not result in any significant drop-off in overall travel for the holidays and was undertaken to send a diplomatic message given Taipei’s lack of other options. Now with M503 fully operational, there is little Taiwan can do other than try to mitigate the dangers of miscalculation should a plane veer over the median line into the Taiwanese FIR and air force patrol areas. Meanwhile, the entire incident highlighted the messy, largely bootstrapped system managing civilian air traffic internationally, and especially in Asia. It has also cast a light on the difficult balancing act Taiwan faces in engaging the world without access to the international institutions meant to manage those interactions.

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