CPDLC explained: how controllers and cockpits communicate by data link

If you listen in on the upper airspace over Central Europe, you eventually notice it: on some frequencies it is surprisingly quiet. Aircraft check in, get handed off a few minutes later – and in between you hardly hear a word, even though climb clearances, direct-to instructions and frequency changes are definitely taking place. They just no longer come by radio but as a text message on the display in the cockpit. The system behind it is called CPDLC, Controller–Pilot Data Link Communications. In this article you learn how it works, which variants exist, why Europe went its own way and what it means for you as a spotter and radio listener.

CPDLC: the short answer

CPDLC is a two-way digital data link between air traffic controller and cockpit. Instead of speaking an instruction into the microphone, the controller selects a predefined message, such as a new flight level, a direct clearance to a waypoint or a frequency change. The message appears in the cockpit, the crew checks it and replies at the push of a button. Conversely, the crew can request clearances, make reports and, in an emergency, even declare a distress situation.

The ranking is important: CPDLC supplements voice radio but does not replace it. Under ICAO procedures, the data link is intended for routine traffic where seconds don’t matter. Everything that has to be acted on immediately – an avoidance instruction, traffic information, a conflict – still goes by radio. How this voice radio is structured is explained in the article Radio phraseology.

Why CPDLC exists at all

Voice radio has a basic problem: on a frequency, only one party can transmit at a time. If two pilots press the transmit button at the same time, the transmissions overlap and everything has to be repeated. Every clearance also needs a readback. In a dense sector in the upper airspace, the frequency is therefore often the real bottleneck – not the controller and not the airspace.

The obvious way out, making sectors ever smaller, has limits. Every additional sector means more handoffs between controllers and another voice frequency, and those are exactly what is scarce in the aviation radio band. Why that is so and how the 8.33 kHz channel spacing created additional channels is covered in separate articles.

CPDLC takes the pressure off the frequency by moving routine messages to a separate channel. Simulations by the US Federal Aviation Administration (FAA) for heavily loaded en route airspace concluded that this can cut voice channel occupancy by about three quarters. There are further benefits:

• Fewer mix-ups: Similar-sounding callsigns are a known source of error on the radio. A text message is unambiguously addressed to a specific aircraft.
• No mishearing: “Flight level two niner zero” or “two five zero”? On the display, the number is in black and white and can often be loaded straight into the flight management system.
• Safer frequency changes: If the change is instructed by data link, the risk drops that a crew checks in on the wrong frequency and becomes temporarily unreachable for the controller.
• Parallelism: A controller can handle several requests without each one blocking the frequency.

How a CPDLC connection works

Before a single clearance can flow, the aircraft has to log on with air traffic control. After that come messages, replies and finally the handover to the next control facility.

The handover between the current and next data authority has a safety background. It prevents a crew from accidentally logging on with the wrong control facility – much like tuning the wrong frequency on voice radio. After all, a controlled flight may be handled by only one facility at any given time.

The most important replies

Free text is possible in CPDLC, but the exception. The normal case is standardized message elements from a fixed ICAO catalog. The same goes for the replies:

Reply	Meaning	Typical use
WILCO	Will comply	Clearance or instruction is being carried out
UNABLE	Cannot comply	Instruction cannot be followed, for example because of performance or weather
STANDBY	Reply to follow	Crew or controller needs time to check
ROGER	Understood	Information noted, with no obligation to comply
AFFIRM / NEGATIVE	Yes / No	Answer to a question

One peculiarity: a message from the ground can consist of several elements, for example a climb clearance plus a speed restriction. But the crew always replies to the whole message. If it cannot comply with one part, the reply is UNABLE for everything. That is why the recommendation is to use multi-part messages only when the parts really belong together. In addition, per aircraft there may be only one open dialogue for each kind of instruction at a time – so not two open heading changes at once.

FANS-1/A and ATN: two systems, one name

When people talk about CPDLC, in practice two different technical worlds are meant. They serve the same purpose but don’t speak the same language.

FANS-1/A: over oceans and remote regions

FANS stands for Future Air Navigation System. In the early 1990s, Boeing developed FANS-1 as software for the flight management computer of the Boeing 747-400. The target was the South Pacific, where over long stretches there is neither radar nor VHF radio coverage and communication ran over crackling HF radio – often through intermediaries as well. A 747-400 of Qantas received certification for the system in June 1995, and the first commercial flight followed immediately afterward.

Airbus developed a comparable package called FANS-A for the A340 and A330. Together they are referred to as FANS-1/A. Technically, FANS-1/A rides on ACARS, the airlines’ text messaging system. Alongside CPDLC, ADS-C is part of the package: the aircraft concludes a “contract” with the ground station and reports its position automatically – regularly at fixed intervals or as soon as it deviates from its route. Over the ocean, that partly replaces the missing radar coverage.

The messages travel by satellite, VHF or HF. How the HF route works and how to receive it is shown in the article HFDL explained. Today FANS-1/A is the standard on the big oceanic routes, such as over the North Atlantic and the Pacific, where air navigation service providers require data link equipment in certain altitude bands.

ATN-CPDLC: the European way

Europe bet on a different concept: the Aeronautical Telecommunication Network (ATN) to the ICAO standard, transmitted over VDL Mode 2 in the VHF band. On flight plans it appears as J1, in technical jargon also as ATN B1. Airbus calls its ATN solution for the A320 family FANS-B.

The pioneer was the Maastricht Upper Area Control Centre of Eurocontrol, which controls the upper airspace over the Benelux countries and northwestern Germany. ATN-CPDLC has been in regular operation there since 2003. Under the Eurocontrol program Link 2000+, the service was extended across Europe; today many control centers offer it, including the upper airspace control center of the DFS (German air navigation service provider) in Karlsruhe. The radio side is provided by data link networks from providers such as ARINC (today part of Collins Aerospace) and SITA. How VDL Mode 2 is technically built is described in the article Receiving VDL Mode 2.

Feature	FANS-1/A	ATN-CPDLC (ATN B1)
Origin	Boeing FANS-1, Airbus FANS-A, from the mid-1990s	ICAO ATN standard, Link 2000+
Transport	ACARS over satellite, VHF or HF	ATN over VDL Mode 2 in the VHF band
Typical area of use	Oceans and remote regions	upper airspace in Europe
Position reports	ADS-C in the same package	not part of the basic service
Flight plan identifier	J2 to J7	J1

Many modern long-haul aircraft such as the A350 can use both systems. On a flight from Europe to North America, it is therefore possible to communicate with air traffic control via ATN-CPDLC over the continent and via FANS-1/A over the Atlantic.

The European data link mandate

On January 16, 2009, the European Commission issued Regulation (EC) No 29/2009 on data link services in the Single European Sky. It obliges air navigation service providers and aircraft operators alike. The core: in the airspace above flight level 285, aircraft in general air traffic must be reachable by CPDLC, and the control facilities must offer the services. There are exceptions for older aircraft and some categories.

Four basic services are mandated:

• DLIC – the logon, without which nothing works.
• ACM – support for handing over voice radio and data link between sectors, that is, mainly frequency changes.
• ACL – clearances, instructions, requests and reports.
• AMC – the “microphone check,” with which a controller can ask all aircraft on a frequency to check their microphone, for example when a stuck transmit button is blocking the frequency.

A bumpy start: the provider aborts

The schedule didn’t hold. In operation, random connection drops, so-called provider aborts, appeared in growing numbers. For crews and controllers that meant extra work and uncertainty, and in the worst case losing track of which message had arrived. Some air navigation service providers reacted with “white lists” that allowed the data link only for certain avionics combinations.

An investigation commissioned by EASA concluded that the cause was a mixture of radio environment and the single-frequency infrastructure in use at the time – a network with several frequencies was needed. With Regulation (EU) 2015/310, the deadlines were therefore postponed, to February 2018 for the ground side and February 2020 for equipping aircraft. In 2023, the old regulation was replaced in a consolidation by Regulation (EU) 2023/1770; the equipage requirement above FL285 remained in substance.

What CPDLC can’t do

As elegant as text communication is, it has clear limits, and the procedures take them into account.

The last point is often underestimated. On voice radio, listening in creates a shared picture of the situation: crews hear that the traffic ahead of them received a deviation heading because of a thunderstorm, and plan accordingly. With text communication, this “party-line principle” is missing. That is also why voice remains the medium of choice as soon as something is unusual.

For the fallback, there are fixed radio phrases. If a controller wants to correct a faulty data link message by radio, he tells the aircraft to disregard the CPDLC message and then gives the correct instruction. If the whole system fails, the control facility informs all aircraft by broadcast that CPDLC is disrupted, and later when normal operation resumes. In an emergency, voice stays in front anyway – more in the article Mayday and Pan-Pan.

Data link on the ground: the departure clearance

CPDLC in the narrower sense is an en route service. But you also notice a related procedure at the airport: the departure clearance by data link (Departure Clearance, DCL). Instead of picking up the route clearance before engine start over the clearance delivery frequency, the crew requests it as a message and confirms it electronically. In the US, the FAA’s Data Comm program delivers departure clearances by data link at many major airports and has also introduced CPDLC in the en route centers.

For you at the fence, this means: if on the delivery frequency of a major airport you hear fewer long-haul flights than are standing on the apron, that is not a reception problem. Many clearances simply no longer go over the radio. Which frequencies at the airport are for what is explained in Understanding aviation radio frequencies.

What CPDLC means for spotters and radio listeners

For most spotters CPDLC is invisible – and that is exactly the point. A few practical consequences are still worth knowing.

If you want to dig deeper, you can make the data packets visible with an SDR stick and free software. Decoders such as dumpvdl2 recognize ATN and FANS messages in VDL Mode 2 traffic, and HFDL decoders pick up FANS messages on HF. How to get started is shown in Receiving aviation radio with an SDR stick and Receiving VDL Mode 2.

The data link is not entirely without use for spotters, though. Long-haul flights over the Atlantic check in with the oceanic control facilities via ADS-C and CPDLC, and their positions show up even where there is no radar. In addition, space-based ADS-B now closes these gaps for flight tracking services. When long-haul flights arrive in Europe and when it is worth standing at the fence is shown in the article Long-haul arrival times in Europe.

Terms at a glance

Term	Meaning
CPDLC	Controller–Pilot Data Link Communications, text communication between controller and cockpit
FANS-1/A	ACARS-based package of CPDLC and ADS-C, mainly over oceans
ATN B1	ICAO network for CPDLC over VDL Mode 2, standard in Europe’s upper airspace
VDL Mode 2	digital VHF data link, transport route for ATN and many ACARS messages
ADS-C	contract-based automatic position reporting to air traffic control
DLIC	logon of the aircraft with the data link service
CDA / NDA	current and next data authority, that is, the responsible and the following control facility
DCL	departure clearance by data link
Provider abort	unintended termination of the data connection by the network

Conclusion

CPDLC moves routine communication between controller and cockpit from the radio to the screen. Over the oceans, that began in the mid-1990s with FANS-1/A and fundamentally improved communication there. In Europe, ATN-based CPDLC over VDL Mode 2 is mandatory in upper airspace, after a bumpy start with connection drops and postponed deadlines. Voice nevertheless remains the backbone of air traffic control: everything urgent, unclear and unusual still goes by radio – and that is exactly what you hear on the scanner.