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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?

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  • $\begingroup$ Nice question! I think that CPDLC is the generic term: one implementation of the CPDLC is through the ACARS system (that is a trademark name). But I wait for a complete answer, since I am interested like you as well. $\endgroup$ – Gianni Alessandro Oct 4 '16 at 9:07
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    $\begingroup$ @GianniAlessandro That is incorrect. CPDLC is much newer than ACARS. Neither is an implementation of the other. $\endgroup$ – Ralph J Oct 4 '16 at 11:39
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CPDLC

CPDLC is used for communication to/from ATC facilities. As the name suggests, it's a controller—pilot link. An aircraft with CPDLC has one of the following panels/interfaces:

787 example:

enter image description here

  1. The pilot can accept, cancel, or reject an instruction.

  2. Uploaded instructions are automatically fed into the different MCP windows, for example UL (uplink) 250 for the speed. And a transfer (XFR) button to activate it. The RL in the heading window is the turn direction.

  3. For route changes, those are fed into the CDU. There's also a text reply option via the CDU (multi function display on the 787). The message also appears on the aux display (outboard side of the display unit for the PFD):

  4. The pilots can also send requests.

A330 example:

enter image description here
(wikimedia.org) The Datalink Control and Display Unit (DCDU) on an Airbus A330, the pilot interface for CPDLC messages.

Similar system but without integration into the autopilot control panel.

ACARS

ACARS is accessed via the control display unit, the same unit for controlling the FMC. Messages can be printed. It's mainly for company communication (between an airline and its pilots). The company uses the ACARS for Aeronautical Operational Control.

  • Sending wind aloft data, expected delays, METAR, load sheets, etc.
  • Receiving position reports, systems status, etc.

Also, messages can be relayed to/from ATC via the company during communication difficulties, for receiving delivery clearances, submitting oceanic position reports, etc.


Summary:

  • CPDLC is an ATC tool, and ACARS is a company tool but it can be used to relay messages to/from ATC.
  • CPDLC coverage is not global yet, it depends on the FIR to have it.
  • CPDLC utilizes VHF Data Link (VDL) and satcom. ACARS uses VHF, VDL, HF, HFDL, and satcom.
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    $\begingroup$ Good description, but a minor clarification. ACARS is an acronym for Aircraft Communications Addressing and Reporting System, which was the original airline datalink system. The service you describe is known as AOC, Airline Operational Control. Crews tend to use the terms interchangeably. $\endgroup$ – Gerry Oct 4 '16 at 11:52
  • $\begingroup$ Interesting answer, just why "It takes 10-15 minutes for relaying (one-way) a message"? $\endgroup$ – mins Oct 4 '16 at 13:32
  • $\begingroup$ ACARS messages may be of three broad types: Air Traffic Control messages[4] are used to request or provide clearances. Aeronautical Operational Control Airline Administrative Control Control messages are used to communicate between the aircraft and its base, with messages either standardized according to ARINC Standard 633, or user-defined in accordance with ARINC Standard 618.[5] The contents of such messages can be OOOI events, flight plans, weather information, equipment health, status of connecting flights, etc.@mins , @ ymb1. Just to clarify few points, source Wikipedia...... $\endgroup$ – Mirajul.Pias Oct 6 '16 at 15:47

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