Sanya FIR communication procedures
Coming out of Hong Kong westbound:
I understand there is a need to call 15-20 minutes piror to entering PRC airspace. Is Hong Kong FIR considered to already be in PRC airspace? Is it expected and/or required to call ahead to Guangzhou or Sanya 15-20 minutes prior to entering their FIR when coming from VHHH airport (yes I have been attempting to do so, with mixed success)?
In regard to Sanya: There is a requirement on the enroute charts to call 5 minutes prior to entering their FIR. I understand this to apply regardless of where you are coming from, even from within the PRC (Guangzhou). What is the reason for this 5 minute call ahead? Is Hainan a Special Administrative region like Hong Kong?
Also, how can I determine the correct frequencies for these call aheads to Sanya? Often I get no response on the primary frequencies listed on the jeppessen enroute charts, and when I am handed off by Hong Kong westbound or Hanoi eastbound, the frequency I am issued is for an entirely different sector than the one I am flying into. I suppose I could just ask Hong Kong Radar for the correct frequency but given language barriers and frequency congestion I would prefer not to. At least the frequencies listed for the call aheads to Hong Kong are established and published on the charts clearly.
I have tried to research the answers in the jeppessen airway manual reference sections but haven't had success.
Thanks in advance for any help that can be offered. My employer is not very proactive in providing us with this sort of information and we are left to fend for ourselves.