Controller–Pilot Datalink Communication (CPDLC)

Once the handshake is made (article 2), the datalink core is CPDLC (Controller–Pilot Datalink Communication): pilot and controller exchange ATC messages by typing instead of speaking. This article covers the backbone — what it is, how messages are built (predefined + freetext), how uplink messages are answered, how requests are sent, how automatic replies are modified, and how an emergency message is sent. The timing/delay/position-report "safety valves" are held for article 4.

1. What CPDLC is — predefined messages plus freetext

The FCOM gives the definition and how messages are built. Per FCOM DSC-46-10-30-20:

The CPDLC application enables an exchange of specific ATC messages between the flight crew and air traffic controller. [...] The flight crew and air traffic controller use a set of predefined messages to build datalink messages. The flight crew and air traffic controller can also send freetext datalink messages. Freetext messages can only be used, when pre-formatted datalink messages do not allow to send the information.

The cost of freetext. Per FCOM DSC-46-10-30-20:

Note: The freetext message does not allow the datalink system to generate automatic pre-formatted answer. The freetext message must contain standard ATC phraseology and standard abbreviations.

[!warning]- Use the predefined message if you can; freetext only when you must Freetext feels flexible, but it costs two things: the system cannot generate an automatic response to it (the other side must type a reply — slow and error-prone), and it must still use standard phraseology (free is not casual). So: prefer predefined message frames; use freetext only when the predefined set cannot express the information. This is CPDLC's design intent — standardising ATC communication into a finite message set to cut ambiguity and error.

2. Answering uplink messages — via the DCDU

The uplink messages CPDLC can answer fall into four types. Per FCOM DSC-46-10-30-20:

The CPDLC application enables the flight crew to answer following uplink messages via the DCDU: Clearance, Report request, Negotiation message, Information message.

Answering is done on the DCDU (the WILCO / UNABLE / STANDBY answer keys are detailed in article 8). CPDLC supports six functions: connection (article 2), request, report (article 4), message modify, MAX uplink delay (article 4), and emergency.

3. Requests — lateral / vertical / negotiation / other

To send a request to ATC, the MCDU builds four page types. Per FCOM DSC-46-10-30-20:

The MCDU enables the flight crew to create: Lateral Request [ATC LAT REQ] [...] Vertical Request [ATC VERT REQ] [...] WHEN CAN WE EXPECT Request [...] Other Request [ATC OTHER REQ]. [...] The flight crew can prepare messages to request clearance, own separation and voice contact on the ATC OTHER REQ page.

A boundary. Per FCOM DSC-46-10-30-20:

Note: Requests of the departure clearance, oceanic clearance and ATIS are not part of the CPDLC application.

Those are ATS 623 applications (article 6), on their own pages, separate from CPDLC requests.

4. Message Modify — editing an automatic reply

CPDLC drafts an automatic reply to some uplink requests, which the crew may edit. Per FCOM DSC-46-10-30-20:

The Message Modify function enables the flight crew to modify automatic replies that the CPDLC generates, in accordance with specific requests from the ATC. Via the DCDU, the CPDLC application may automatically propose a response to: Confirm request (position, speed, heading, altitude, ...), Report request, Open negotiation request, A combination of the above requests.

Pressing MODIFY on the DCDU opens the MESSAGE MODIFY page on the MCDU; on first opening, modifiable fields are pre-filled with FMGES data. The crew check/edit, then send. This is the deep FMS–datalink coupling — many replies need no typing, the system fills them from flight-management data and you only verify.

5. Emergency

The emergency message is linked to ADS-C emergency. Per FCOM DSC-46-10-30-20:

The EMERGENCY page enables the flight crew to: Create an emergency datalink message, Activate the ADS-C Emergency. The flight crew can switch the ADS-C to emergency mode manually via the EMERGENCY page of the MCDU. The ADS-C switches to the emergency mode automatically, when the flight crew sends a MAYDAY message.

[!note]- Emergency: send MAYDAY, ADS-C enters emergency automatically Datalink emergency has two legs: the CPDLC emergency message (typed notice) and ADS-C emergency mode (automatic, intensified position/intent reporting so the ground watches you closely). Send a MAYDAY and ADS-C enters emergency automatically — no separate action. But remember the master rule (article 1): in a real emergency voice is the primary means (revert to voice on emergency/doubt); datalink emergency is the aid.

Self-test

[!note]- Q1. How are CPDLC messages built, when is freetext used, and what are its two costs? From a predefined message set, with freetext only when the predefined set cannot express the information. Freetext gets no automatic response and must still use standard phraseology.

[!note]- Q2. Which four uplink message types can the crew answer, and on which interface? Clearance, report request, negotiation, information — answered via the DCDU.

[!note]- Q3. What are the four request page types, and why are DCL/OCL/ATIS requests not CPDLC? Lateral, vertical, WHEN CAN WE EXPECT, and other request. DCL/OCL/ATIS are ATS 623 applications on their own pages.

[!note]- Q4. What does MESSAGE MODIFY do, and where do the auto-reply fields come from? It lets the crew edit CPDLC's automatic replies; the modifiable fields are pre-filled with FMGES data.

[!note]- Q5. How is an emergency message sent, and what does ADS-C do on a MAYDAY? On the EMERGENCY page; sending a MAYDAY automatically switches ADS-C to emergency mode.

Key takeaways

References

• FCOM DSC-46-10-30-20 — CPDLC definition, predefined + freetext, four answerable uplink types, four request pages, MESSAGE MODIFY, EMERGENCY with ADS-C emergency.

Independent study material, not an Airbus publication and not endorsed by the manufacturer. Always defer to the current operator FCOM, FCTM, and QRH for operational use.