Some updates from Chinese forums:
1) The Burmese jet responsible for the attack was a Q-5, not a MIG-29 as previously reported (my bad).
2) Chinese Air Force conducted air patrols over the border area with Burma since March 12th according to a PLAAF spokesperson.
How come they failed to intercept the attack jet?
"With" burma? Are we saying that the burmese were also patrolling their side of the border as well?
As for how it happened, I assume that PLAAF patrols cannot be in the air 24/7, and more importantly, I expect burmese planes to have been constantly flying over their own airspace close to the border on their own operations so it may be quite difficult to positively identify a plane that is going to cross the border until it already has.
More importantly I suspect this portion of air space is not a heavily monitored area compared to others, say, over the ECS, SCS, or yellow sea, or air space facing india and pakistan.
Anyway, SAMs are the right way to deal with this.
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By the way there is already a thread for this in the strategic defense sub forum, can we merge the two threads?