Clearly if it caused the collision, that is illegal. If the PRC caused the collision, that is likewise illegal. It issued a mayday call and then proceeded to land in Hainan. As I noted in the last message, it seems that even military planes have the right to make emergency landing without prior permission. International Legal Issues Arising from the Aerial Collision Following is a summary of my article to be published in Ming Pao Daily in Hong Kong.
Criticisms are welcome.
The PRC argument that US surveillance operation in China's EEZ is an abuse of freedom of overflight seems weak. The rights of a coastal state in EEZ are limited under LOS Convention., art. 56, and the obligations of other states in EEZ under art. 58 are probably limited in the same way. At any rate, the frequency of spying activities in foreign EEZs without evoking protests seems to belie the argument that reconnaissance operation in a foreign EEZ is contrary to international law.