Does the ADS-C contract really stay active after CPDLC logoff?
Thread Starter
Joined: Mar 2011
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From: madrid
Does the ADS-C contract really stay active after CPDLC logoff?
I’ve noticed something odd on several long-haul flights (A350 fleet).
After a CPDLC disconnection at FIR exit, the ADS-C contract sometimes remains active for several minutes — even when no new contract has been established by the next ATC center.
I’m wondering what the intended logic is here.
Should the aircraft terminate the contract automatically once CPDLC is closed, or is the ground system responsible for sending a cancel message?
Some manuals imply ADS-C is fully independent of CPDLC, while others say it “rides” on the same data link session.
Has anyone seen official guidance from ICAO Doc 10037, GOLD 2020 ed., or manufacturer FCOM references clarifying this?
Curious to hear how different operators handle it and what the “correct” behavior should be.
After a CPDLC disconnection at FIR exit, the ADS-C contract sometimes remains active for several minutes — even when no new contract has been established by the next ATC center.
I’m wondering what the intended logic is here.
Should the aircraft terminate the contract automatically once CPDLC is closed, or is the ground system responsible for sending a cancel message?
Some manuals imply ADS-C is fully independent of CPDLC, while others say it “rides” on the same data link session.
Has anyone seen official guidance from ICAO Doc 10037, GOLD 2020 ed., or manufacturer FCOM references clarifying this?
Curious to hear how different operators handle it and what the “correct” behavior should be.
Joined: Aug 2009
Aviation Qualifications: ATPL
Posts: 615
Likes: 55
From: GPS L INVALID
I just had a look in the Lido COM chapter, according to that 15 minutes after leaving ADS-C airspace flight crew should terminate any open ADS-C contracts after coordinating with the current ATSU - since the two are technically independent systems the CPDLC logoff and the (ground initiated) termination of the ADS-C contract may not coincide.
Joined: Jun 2020
Posts: 10
Likes: 20
From: North
My info on this may be a bit dated now, but my understanding is as follows:
For CPDLC, there are two supported connections, these being the Current Data Authority (CDA) and the Next Data Authority (NDA). By only having these two connections for CPDLC, it ensures that there cannot be conflicting instructions issued to an aircraft from different locations, as in, only one Controller can issue control instructions. The NDA is so the aircraft knows that when the CDA disconnects, it has an established NDA to switch to and then accept control instructions only from that next unit.
Sometimes the switch doesn't flow smoothly for reasons, then it's SELCAL on HF to :"Disconnect CPDLC and log on to XXXX"
For ADS-C, there are several simultaneous connections supported, examples may be: Previous ATC unit, Current ATC unit, Next ATC unit, Company Operator, Airframe and Engine Monitoring etc. Current ATC unit is obvious. Next ATC unit ensures seamless connection and associated separation standards can be maintained across the border (some standards are ADS-C Reliant). Previous ATC unit for similar reasons, also, depending on the airspace, changes to track/level may need to be agreed upon by coordination between the current unit and the previous unit until a certain distance/time after the border crossing (10 minutes/15 minutes/50NM...).
TLDR: CPDLC only allows control instructions from one ATC unit. ADS-C, many units may have an interest.
For CPDLC, there are two supported connections, these being the Current Data Authority (CDA) and the Next Data Authority (NDA). By only having these two connections for CPDLC, it ensures that there cannot be conflicting instructions issued to an aircraft from different locations, as in, only one Controller can issue control instructions. The NDA is so the aircraft knows that when the CDA disconnects, it has an established NDA to switch to and then accept control instructions only from that next unit.
Sometimes the switch doesn't flow smoothly for reasons, then it's SELCAL on HF to :"Disconnect CPDLC and log on to XXXX"
For ADS-C, there are several simultaneous connections supported, examples may be: Previous ATC unit, Current ATC unit, Next ATC unit, Company Operator, Airframe and Engine Monitoring etc. Current ATC unit is obvious. Next ATC unit ensures seamless connection and associated separation standards can be maintained across the border (some standards are ADS-C Reliant). Previous ATC unit for similar reasons, also, depending on the airspace, changes to track/level may need to be agreed upon by coordination between the current unit and the previous unit until a certain distance/time after the border crossing (10 minutes/15 minutes/50NM...).
TLDR: CPDLC only allows control instructions from one ATC unit. ADS-C, many units may have an interest.
