How often does U.S. Air Force cross China's Air Defense Identification Zone?

As often as they wish to as long as they remain in international airspace.  The USA has operated air defense zones off its own coasts for decades.  Aircraft transiting such a zone are under no obligation to the country declaring the zone since freedom of navigation rules apply in international airspace.  The Chinese are making it as if they have control over this area and their permission is required to operate within it.  They do not have that authority. Like the Soviets and Americans did frequently during the Cold War, they would send aircraft into each others air defense zones to test them.  The other country would sent up aircraft to say "Hello" and fly alongside the "intruders".  As long as the "invading" aircraft in the zone stayed in international airspace and did not otherwise act aggressively, it was simply a way to test each other and gather intelligence.  Over the years it really did become a game for both sides.  This is no different.  China is free to send fighters out to see who's flying there, fly alongside, say "Hello, we see you!" and so on.  As long as they don't act aggressively towards such "intruders", it is really a form of diplomatic sabre rattling in an attempt to ratchet up their desire to impose some kind of legitimacy claim on the control or ownership of the islands in the region owned by Japan. China's trying to play with the big boys.  A game the USA is very good at.  As long as the Chinese play nice and don't try to use the zone as an excuse for "incidents" such as the one over Hainan Island in 2001, the US will play too.  At least it will be a nice change of pace for US forces.  I guarantee the USA will be reinforcing "freedom of navigation" exercises should the Chinese be trying to use this area as a lever to force others out or control them.  The US Navy and US Air Force won't be having any of that.  Especially if the Chinese opt to try to interfere with freedom of navigation by a carrier battle group in international waters.

There's a ton of misinformation about China's new Air Defense Identification Zone (ADIZ) right now, and a lot of commentators talking about a subject that they have very little knowledge about. So let's start with a few facts. What is an ADIZ? An ADIZ is a unilateral declaration by a country's military or security service that any aircraft entering a specified airspace over water must identify itself.  Unlike the rules about territorial waters or exclusive economic zones, it's not a matter of international law.  It's simply a unilateral declaration/warning.  Therefore, a country can say whatever it wants and these zones often overlap with each other.  Each operator of aircraft be it civilian or military, can choose whether or not to follow the rules set forth, although in practice only military aircraft would have any incentives to violate the rules.  In practice, most ADIZs follow NORAD's example: http://www.faasafety.gov/files/gslac/library/documents/2011/Jan/49877/ADIZ%20TFR%20Intercepts%20w%20answers.pdf What happens if an aircraft enters an ADIZ and doesn't declare itself? Nobody knows for sure.  Once again, it's not a matter of international law.  There are no set rules.  Here's what the FAA/NORAD says: "Any aircraft flying in these zones without authorization may be identified as a threat and treated as an enemy aircraft, potentially leading to interception by fighter aircraft."  Notice the words "may" and "potentially."  Basically, what they're saying is fair warning, if you don't declare yourself, then we may make your life uncomfortable.  Or we may not.  But do you want to take the risk?  Once again, for civilian aircraft, the answer is most likely no but for military aircraft the answer may be yes. How often will US aircraft fly into China's ADIZ? The answer is probably fairly often.  As you've noticed the US has several bases in the area, and does conduct exercises in the area regularly, much to China's chagrin.  US aircraft also fly into and out of Japan's ADIZ, Taiwan's ADIZ, South Korea's ADIZ, etc.  I highly doubt that during the exercises US military aircraft are going to carry transponders on them and voluntarily give away their position.  In fact the chances of this are practically nil.  Once again, just setting up a ADIZ doesn't mean other countries aren't allowed to fly in the region.  It's just a warning: if you don't declare yourself, we may send up fighters to intercept you. What will happen if foreign aircraft fly into China's ADIZ? In the short term, probably nothing.  As multiple articles have pointed out, this is a very highly trafficked area, and it's unlikely China has the will or resources to send up aircraft to intercept every single aircraft that traverses the zone.  The most I could see them doing in the short term is every once in a while scrambling a fighter to tail some other military aircraft entering the area, although even that would require that they are able to properly identify the aircraft they are looking to tail. Why did China setup the ADIZ? Since an ADIZ is not recognized under international law, there are relatively few sovereignty implications.  Obviously, relatively few doesn't mean none, and that's the primary reason the Japanese government is so upset over this.  The more practical reason is, as stated above, commercial aircraft will generally as a matter of course declare their presence in any defined ADIZs and register their flight plans.  That does mean that the Chinese government, by simply excluding the commercial aircraft, will be able to better track military aircraft in the area.  This may be a reason why the US is against this move. Is this an unexpected or provocative act? Probably not.  Most countries have setup ADIZs and in the era of modern aircraft being able to travel over a thousand miles an hour it was basically inevitable that China would have setup one too.  In fact, it's highly likely that they will setup more in the future.  This current one could be essentially viewed as a test or pilot run.  However, given the sensitivities about sovereignty in the East China Sea, the controversy may have also been inevitable.