How to start CPDLC?
 

I'm not a pilot but I enjoy listening on liveatc.net.
Here I have a question on how to start CPDLC, switching from voice communication.

Once an aircraft enters CPDLC area, like Pacific Ocean, does controller tell pilots "Contact XXX center on CPDLC"?

Are pilots assigned HF frequency for backup voice communication?

Then how do they end CPDLC as an aircraft comes close to the the end of the ocean?

I hope some answer my question here.
Thank you.

It's digital data link via satellite channels. You can't listen.

The pilot "logs on" to the "data authority", i.e. the controlling agency., usually the Oceanic Area Control, bug could be over land. Then all comms are via satellite data link. HF is available and the crew checks the SELCAL, but that's it.

Or via VHF if over land, for example if using CPDLC in the maastricht control area.

Just to add to the above - not sure of your level of knowledge, but keeping it simple it's a bit like using SMS or text messaging.

You normally have the option of displaying a CPDLC page on one of the flight deck displays.

Logon is usually done by simply entering the 4 letter centre code on a log in page and selecting "send". FWIW many places expect you to log in at least 15 minutes before entering their airspace unless it's an automated handover (see below), though as you leave VHF cover you may get a final reminder e.g." continue with Shannon now on CPDLC". FWIW there's usually only one code per centre and they're in our documentation, so in many busy parts of the world ATC won't read out the 4 letters by voice unless asked.

Having sent the log in there then usually follows text confirmation from the ground that it has worked. You usually also have a HF or sometimes VHF backup.

As you cross from one CPDLC equipped centre to another CPDLC equipped centre (e.g. Mid Atlantic from Shannon to Gander) the switch between the two is automatic but you do get a "text" confirmation and usually also there's a requirement to do a check in by voice on VHF or HF with the new centre.

Once you leave a CPDLC area and enter a non CPDLC area the system usually automatically logs off ( again you get confirmatory "text message"). You also always have the option at any time to log off manually from the flight deck.

Thank you
 

Thank you for your inputs, everyone.

Now I think I understand the procedure in detail.

Thanks,

From my experience in the Pacific area, after logging in as per ATC page in the FMC you receive a MSG req pos rep. After pos rep received the system starts doing its CPDLC thing.

That's increasingly common, e.g. going into the likes of YMML FIR from the north you'll be logged on and established on to CPDLC well before the boundary (probably already talking to them on HF as well). However despite already being logged on CPDLC and checked in via voice you always get the CPDLC "request posn report msg" after you have entered the FIR. I believe that is due to ADS-C protocol, rather than being a requirement for CPDLC to start working.

The ICAO GOLD document lists the procedures at the FIR boundaries. For the Pacific area centers I send one manual position report crossing the boundary. However the North Atlantic centers specifically say not to send a position report. You get an uplink, "This is an automated message to confirm..."