What is Controller Pilot Datalink Communication (CPDLC)?

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  • Post last modified:July 24, 2020

What exactly is CPDLC?

CPDLC (Controller Pilot Datalink Communications) is an air to ground datalink that allows pilots and controllers to communicate via text messages. These text messages are displayed to pilots on a screen in the cockpit and allow the pilot to communicate with air traffic controllers through a series of preformatted text exchanges.

How does CPDLC work?

CPDLC involves the use of pre-formatted messages which allows communication from the controller to the crew (uplink) and from the pilot to the controller (downlink). The use of pre-formatted communications increase the efficiency, reduces the workload and reduces the chance of an input error.

In addition to the pre-formatted messages there is the option for “free text” to allow normal communication between an ATC unit and a pilot. 

Prior to reaching the relevant FIR pilots must logon to the relevant center using the 4 letter ICAO code. For example:

Gander Oceanic CZQX

New York Oceanic KZWY

Reykjavik BIRD

Santa Maria LPPO

Shanwick EGGX

U.S. Domestic KUSA

What is CPDLC-DCL and when is it used?

CPDLC-DCL (Controller-Pilot Data Link Communications – Departure Clearances) allow crews to request and receive air traffic control departure clearances in text form via data link.

Approaching departure time crews will input a departure request into the FMS/MCDU and this is sent to the airport of departure, which in turn replies with the departure clearance.

The initial request from the crew may include: callsign, aircraft type, gate number and the code of the ATIS that has been copied. Some systems will allow an amended departure clearance to be loaded directly into the FMS/MCDU, mitigating input errors. 

What are the advantages of CPDLC?

Typically communication between ATC (Air Traffic Control) and pilots is done through radio calls either over VHF for shorter ranges comms (e.g. operating over the U.S. or Europe) or HF for long range communications (e.g. over oceanic areas). Radio communication suffers from a number of serious drawbacks:

  • It is limited to one radio call at a time and if two stations call at the same time often neither are decipherable – you will often hear pilots call “blocked” on the frequency and the calls have to repeated.
  • Voice calls take a certain amount of time – for example a clearance that is issued by ATC has to be “read back” by the pilot and that read back in turn has to be confirmed to be correct by the ATC controller.
     
  • Poor reception and signal distortion in some areas (this is particularly relevant for long-range HF (High Frequency) radio calls. (Any of you familiar with the joy of trying to make a position report halfway across the ocean on a night of bad HF reception will know what I mean.)

The advantages that CPDLC provides are:

  • Increased ATC efficiency by reducing required communications and reducing controller workload. 
  • A reduction in possible voice communication errors by both pilots and controllers due to language barriers.
  • Reducing the chance of VHF frequencies being “blocked” by simultaneous transmissions, thereby increasing airspace utilisation.
  • Allowing different communications at the same time thereby increasing the speed of communication.
Aircraft cockpit/flight deck in flight.
Image: John Christian Fjellestad/Flickr

Example of a typical flight using CPDLC

Taking an Airbus A380 flight from London Heathrow (LHR) to John F Kennedy (JFK) as an example. 

The MEL(Minimum Equipment List) must be checked to ensure all required components are operational.

When the crew are ready to receive their pre-departure clearance (PDC) they select the clearance page of the MCDU (Multi-Function Control and Display Unit – similar to Boeing’s FMS) and would enter the flight number, aircraft model, stand and latest ATIS code. Usually the reply would be received almost immediately and be displayed on the DCDU (Datalink Control and Display Unit) which is in front of both pilots. One pilot will read out the received clearance while the other cross checks with what has been programmed, checking the SID, cleared altitude, transponder squawk code and any other information.

Once both crew are happy to accept the clearance they would send an acknowledgement. Lastly they would receive a final message confirming the clearance from ATC.

Learn how transponders work here.

After departure, at a convenient time above 10,000’ the crew would “logon” to the relevant ATC unit EGTT and after a short time would see the active ATC unit appear in the DCDU.

Automatic transfer usually occurs between adjacent FIRs (Flight Information Regions) and as our flight continues westward we would see an automatic handover to Shannon Control (EISN).

At a convenient time in the cruise the crew populate the Ocean Request (OCX) section of the MCDU with their requested North Atlantic crossing clearance from Shanwick (EGGX) – flight number, entry point, time at entry point, requested speed, level and any other information (often maximum altitude they are capable of at entry point). Their request would be automatically acknowledged with an expected clearance time – if none is forthcoming Shanwick operate a VHF frequency the crew can call to relay their request by voice.

Traditionally on an Atlantic crossing crews had to make regular position reports crossing every 10º of longitude however CPDLC, together with ADS-C (Automatic Dependent Surveillance-Contract), means crews are only required to make a HF “SELCAL” (Selective Calling) check with Shanwick control, and then passing 30º West with Gandar Control. 

(SELCAL is a system where ATC can call a specific aircraft on a particular HF frequency which causes a buzzer to sound on the flight deck alerting to the fact they are being called on HF).

Once SELCAL checks ok, HF radio is used as a backup and CPDLC is the form of primary ATC communication. Requests our flight may make over CPDLC during the crossing include requests for level or speed changes, or for weather avoidance for example.

We continue to use CPDLC over Canada but once passing south to the U.S. en-route CPDLC is not currently used. On arrival on stand in JFK the crew have access to a complete log of all CPDLC communications which can be printed if necessary (for example, if a safety report about the system needed to be filed).

Video Example of CPDLC

Useful further reading:

Learn about the differences between CPDLC and ACARS.

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