China has sparked an increase in tensions with neighboring countries by establishing an air defense identification zone over waters and islands off its east coast also claimed by Japan and Taiwan. When it announced the zone November 23, China's Defense Ministry said aircraft entering the zone must identify themselves and report flight plans to its government.
The move was denounced by Japan, the US and South Korea, with all three nations sending military planes into the area. China flew its military jets into the zone in response.
Here are answers to some frequently asked questions about the air-defense identification zone, or ADIZ. The information is drawn from industry groups, analysts and aviation websites.
Q. What is an air-defense identification zone?
It's an area above land or water in which aircraft have to identify themselves in the interest of national security. Many countries have them over their own territories, and governments such as the U.S. extend them into the waters around their lands. More than 20 countries have such zones, according to China's defense ministry.
Q. What does this mean for plane passengers in the region?
The zone is in the midst of some of the busiest passenger routes in Asia - connecting Japan to Hong Kong, Taipei, and Southeast Asian cities like Singapore. That means passengers may end up in the middle of the dispute, whether they realize it or not. Commercial airlines in Japan and South Korea are defying China's requests to hand over flight information, while US, Australia, Hong Kong, Taiwan and Singapore carriers are complying.
The US State Department urged American airlines to notify Chinese authorities before flying through the new air-defense zone, though the Obama administration made it clear that doesn't mean it accepts China's authority over the zone.
While there's no direct effect on passengers so far, there could be an impact if airlines choose to take an alternative route, thereby lengthening flights.
Q. What are China's plans for the zone?
China has sent mixed signals about how aggressively it's enforcing its authority over the zone. After saying aircraft must identify themselves to Chinese authorities, the government appeared to back off, with officials saying it won't affect normal international flights and China hoped civilian airlines would actively cooperate. Then on Nov. 28, China sent warplanes through the area, in a sign its determination to exert its rights isn't waning.
China could also disrupt commercial flights by trying to enforce the zone and can complain to the airlines. It can send fighter jets for closer visual verification of passenger planes, according to Tim Huxley, executive director at the International Institute for Strategic Studies Asia in Singapore.
Q: What are the precedents for such interactions?
Military and commercial aircraft share the skies all the time without incident. However, the situation gets more complex when airspace is contested and communications aren't clear.
In September 1983, the former Soviet Union, in the midst of the Cold War, shot down a Korean Air Lines plane, killing all 269 people aboard the jet. The Soviets said the airplane had flown into their airspace.
Though no one expects such an outcome now, some people are concerned about worst-case scenarios as tensions rise.
"Tragedies can happen," said Huxley. "I think it's not an ideal situation."
Q. How do airlines usually handle such situations?
When commercial flights transit through an airspace, they are required to file their flight plans and establish two-way radio communication with the relevant air traffic control (ATC) authorities. Airlines do this routinely around the world.
China's establishment of the new ADIZ means airlines are being asked for information on flights across a wider region, including between neighboring countries such as Japan and Taiwan.
Q. What are the options for airlines?
They can cooperate, fly alternate routes, or wait to see how China reacts. If the tension between China and Japan rises further, it could hamper tourism and lead to a decline in airline passengers in the region, according to Siyi Lim, an aviation analyst at OCBC Investment Research in Singapore.