What are the differences between ACARS and CPDLC?

Question

What are the differences between Aircraft Communication, Addressing and Reporting System (ACARS), and Controller Pilot Data Link Communication (CPDLC)?

Can anyone please let me know in short, what is the difference between ACARS and CPDLC? So far, I have learned that both use data link communication. A ground station is necessary for ACARS and I am not sure weather ground station in necessary for CPDLC or not. ACARS can works on VHF and HF frequency also on SATCOM. Does CPDLC also works on these or it requires different system to work on? CPDLC gives the messages on a digital display. Does ACARS also provide information on a digital display or it is like fax machine where information come out on a paper from the machine.

I become confuse while figure out the differences between ACARS and CPDLC. Can anyone help me out?

Answer 1

Each of those terms refers to a few things, so they depend on the context.

Some examples:

• Getting CPDLC-DCL (departure clearance) in USA?
  That's FANS over ACARS

• Automatically reporting position over the Atlantic?
  ADS-C using FANS over ACARS

• Requesting climb over the Pacific?
  CPDLC using FANS over ACARS

• Receiving reroute over Europe?
  PM-CPDLC over ATN B1

• Receiving load sheet?
  AOC over ACARS

Depending on the plane and region, not all systems and functions may be available. I think it helps to know them top-down: what each data link system offers.

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Aircraft data link systems

1. ACARS ATS

Application	Description	Network
DCL	departure clearance not used in USA (see CPDLC-DCL under FANS)	ACARS
OCL	oceanic clearance	ACARS
D-ATIS	data link ATIS	ACARS
PDC	pre-departure clearance in use since 1991; primarily in USA; not a direct link to ATC not under 'ACARS ATS' the system but is worth mentioning here	AOC over ACARS

Further reading: honeywell.com and code7700.com

Pilot about to select the OCL page aboard a 747-400 (YouTube)

2. FANS 1/A+

Application	Description	Network
AFN	ATS facilities notification the application used to log on the ATC network	ACARS mainly oceanic
CPDLC	the application used for communicating with ATC	ACARS mainly oceanic
ADS-C	automatic contract position reporting uses set intervals and events	ACARS mainly oceanic
AOC	airline operational control the airlines' own applications (e.g. uplinking load sheets) predates and prerequisite to FANS	ACARS
CPDLC-DCL	departure clearance used in USA; utilizes the FANS system and requires logging on the flight plan can indicate whether this DCL or PDC is preferred or second best	ACARS terrestrial only

Further reading: boeing.com

Successful logon using AFN aboard a 747-400 (YouTube)

3. FANS 1/A ADS-C

The same as the above minus the CPDLC capability.

4. ATN B1

Application	Description	Network
CM	context management: data link initiation capability (DLIC) FANS' AFN equivalent	ATN B1 Continental Europe
CPDLC	the application used for communicating with ATC AKA PM-CPDLC (protected mode)	ATN B1 Continental Europe

Accepting a frequency change using ATN B1 aboard an A320 (YouTube)

5. FANS 1/A - ATN B1

AKA FAN 2/B, and A+B (in Airbus lingo); combines FANS 1/A and ATN B1, and is found on newer aircraft (e.g. 787 and A350), and can be retrofitted to some older aircraft (e.g. A380).

Related: What are the differences between FANS A, B, and C? Is FANS C commissioned?

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The subnetworks

ATN B1 uses VDL M2: very high frequency data link - mode 2 (project's name: LINK2000+).

FANS 1/A uses VDL M0/A (AKA Plain Old ACARS – POA), VDL M2 (AOA), HFDL (high frequency data link), and SATCOM (Inmarsat and Iridium).

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Main reference used: ICAO's Global Operational Data Link (GOLD) Manual (PDF)

Answer 2

While @ymb1 has provided a comprehensive answer, I’ll offer a simpler one:

CPDLC is an application, which can run over multiple networks (including ACARS).

ACARS is a network, which supports multiple applications (including CPDLC) and runs over multiple data links.

Applications tend to be screen- and keyboard-oriented, but terminals have the ability to print data or pass it between applications. For instance, if the pilot receives a new route in CPDLC (maybe via ACARS), they can press a button to send it to the FMS, rather than have to print it out and then retype it.