CPDLC (Controller Pilot Data Link Communications)

What is CPDLC?

Controller Pilot Data Link Communications (CPDLC) is a system that allows text-based communication between flight crews and air traffic controllers. Instead of relying solely on VHF or HF voice radio, CPDLC uses data links—either VHF datalink or satellite—to transmit messages. This is particularly valuable in oceanic or remote regions where voice radio coverage is limited or congested.

Types of CPDLC Messages

CPDLC supports a range of predefined message types, including:

• Altitude change requests and clearances
• Speed change requests and instructions
• Route modifications
• Frequency change instructions
• Acknowledgments and confirmations

These messages are selected and sent via the aircraft’s avionics interface, such as the Flight Management System (FMS) or a dedicated Control Display Unit (CDU).

Notification Phase (LOG ON)

Before using CPDLC, the crew must complete a notification or "LOG ON" phase. This procedure establishes the data link connection between the aircraft and the ATC ground system, ensuring both parties are ready to exchange messages.

Reverting to Voice Communication

If a CPDLC message is unclear, not acknowledged, or if the system fails, crews must revert to voice communication. Standard phraseology includes:

• "Disregard CPDLC [instruction], [new instruction]" to correct a message
• "CPDLC message failure" if a single message fails
• "CPDLC failure" if the entire system fails, followed by the required voice communication

All unacknowledged CPDLC messages at the time of failure must be retransmitted by voice.

CPDLC vs Voice Procedures

While CPDLC is efficient, certain situations require voice for speed or clarity. For example, urgent corrections, ambiguous messages, or system errors always demand a switch back to voice. Voice responses are required for voice-originated messages, and CPDLC responses for CPDLC-originated messages, unless operational circumstances dictate otherwise.