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CPDLC

Controller Pilot Data Link Communications (CPDLC) is a two-way data-link system by which controllers can transmit non-urgent 'strategic' messages to an aircraft as an alternative to voice communications. The message is displayed on a flight deck visual display.

CPDLC was first implemented in Europe by Eurocontrol's Maastricht Upper Airspace Control Centre. Since then, many other ANSPs worldwide have implemented CPDLC in their ACCs. Nowadays, most aircraft flying as GAT above FL285 within the Single European Sky (SES) must be CPDLC equipped (with a few exceptions). 

Benefits

Multiple studies describe the following benefits of CPDLC:

  • Less communication on the ATC frequency
  • Increased sector capacities
  • More pilot requests can be dealt with simultaneously
  • Reduced probability of miscommunication (e.g. due callsign confusion)
  • Safer frequency changes, hence fewer loss of communication events

Main Principles of CPDLC

In general, the following main principles exist for the use of CPDLC:

  • Voice and data link shall co-exist. Pilots are required to monitor ATC's frequency and have to make an initial call after changing frequencies. Additionally, ATC can always use voice communications when necessary. 
  • CPDLC shall only be used in the context of non-time-critical communications. Time-criticality is mainly determined by the following factors: ATC traffic situation, end-to-end performance (systems and flight crew/controller response time) and recovery time. Users should be aware that while a voice response is generally expected in a few seconds the latency of CPDLC is usually much longer (up to several minutes).
  • Only one open dialogue of the same type (climb/descent, direct, speed, heading, squawk) with the same aircraft at any given time is allowed. For example, it is not allowed to issue two climb instructions without a response from the pilot between both messages. This is to avoid confusion and misinterpretation of ATC clearances. 
  • The decision to use either voice or CPDLC shall be at the discretion of the controller and/or pilot involved.
  • The provision of CPDLC shall respect the following standard as provided in ICAO Annex 11, Chapter 3, par. 3.5.1: “A controlled flight shall be under the control of only one air traffic control unit at any given time”.

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