Datalink
ACARS & CPDLC — ATC without the radio
Modern airliners get clearances as text messages. Here is how the pieces fit together, and how to use it in MSFS.
The building blocks
Aircraft Communications Addressing and Reporting System
The transport network — VHF, satellite and increasingly IP links between aircraft and ground. Everything else on this page travels over ACARS or its modern successors.
- Company ops (OOOI times, loadsheets)
- D-ATIS requests
- Clearance delivery
- CPDLC and ADS-C sessions
Pre-Departure Clearance
Your IFR clearance as a text message before pushback — also called DCL. No voice readback: you request it, review it, load it into the FMS and accept.
- SID, initial climb, squawk
- Loadable straight into the FMS
- Frees the clearance frequency
Controller–Pilot Data Link Communications
ATC instructions as structured text: clearances uplinked by the controller, answered with WILCO / UNABLE / STANDBY. Standard over oceans, growing in busy enroute airspace like Maastricht.
- Level clearances and directs
- Oceanic position reporting
- Frequency handoffs
- Weather deviations
Automatic Dependent Surveillance–Contract
Where radar ends, the aircraft reports itself: ATC sets a contract and receives automatic position, altitude and met reports. Runs silently alongside CPDLC — you normally never touch it.
- Oceanic tracking without radar
- Automatic — no pilot action
- Feeds ATC situational display
A CPDLC session, message by message
Fictional flight DLH123 under Maastricht UAC (EDYY) — the heaviest CPDLC user in Europe.
If the link drops — LOGON fails, a message times out — you simply revert to the voice frequency. Controllers expect it. Never wait on a datalink clearance while the clock runs; call on the radio instead.
The elements you'll actually see
| Dir | Element | Meaning |
|---|---|---|
| ↑ ATC | CLIMB TO [level] | Level clearance, climb |
| ↑ ATC | DESCEND TO [level] | Level clearance, descent |
| ↑ ATC | MAINTAIN [level] | Stop climb/descent at level |
| ↑ ATC | PROCEED DIRECT TO [fix] | Direct routing to a fix |
| ↑ ATC | CLEARED TO DEVIATE UP TO [dist] [dir] OF ROUTE | Weather deviation window |
| ↑ ATC | RESUME OWN NAVIGATION | Return to the cleared route |
| ↑ ATC | SQUAWK [code] | Transponder code change |
| ↑ ATC | CONTACT [unit] [frequency] | Voice handoff instruction |
| ↑ ATC | REPORT [condition] | Conditional report request (e.g. REPORT REACHING FL350) |
| ↑ ATC | NEXT DATA AUTHORITY [unit] | Datalink handoff notice |
| ↑ ATC | CHECK STUCK MICROPHONE [frequency] | Your mic appears keyed — check it |
| ↑ ATC | ERROR [info] | Protocol error — re-send or revert to voice |
| ↓ You | WILCO | Will comply — accepts the uplink |
| ↓ You | UNABLE | Cannot comply — expect a new clearance or voice |
| ↓ You | STANDBY | Need time — the clearance is not yet accepted |
| ↓ You | ROGER | Acknowledgement without accepting a clearance |
| ↓ You | REQUEST [level] | Level change request |
| ↓ You | REQUEST DIRECT TO [fix] | Direct routing request |
| ↓ You | REQUEST WEATHER DEVIATION UP TO [dist] [dir] OF ROUTE | Deviation request |
| ↓ You | REQUEST VOICE CONTACT | Ask to work it out on frequency |
| ↑ ATC | END SERVICE | Ground system closes the CPDLC session |
Two philosophies: Europe vs the US
Datalink is a cruise thing — and it is required by law
Above FL IFR CPDLC The aircraft connects by "logging on" to an airspace code — EDYY for Maastricht, EGTT for London. You can already log on at the gate, but the actual link only opens high up. Below FL 285 each country decides for itself — and on approach, everything stays on voice anyway.
Your clearance arrives before pushback
At around 65 big airports the IFR clearance pops up as a message while you are still parked — the FAA calls this DCL In cruise, the enroute centers uplink clearances above roughly 16,000 ft; approach control and tower always stay on voice. Only aircraft whose avionics are on an FAA approval list may join — airliners qualify, most small GA Over the ocean a separate satellite-based system takes over — the classic use case datalink was invented for.
Where it works in MSFS
Important distinction: Hoppie is just the pipe — a message relay, not an ATC service. The actual CPDLC clearances come from whoever is on the other end: a VATSIM controller or an AI service.
A free community ACARS relay — the pipe, not the service. Hoppie moves messages between your aircraft and ground stations; whether you actually get D-ATIS, PDC or CPDLC depends on a station being online at the other end (typically a VATSIM or IVAO controller).
Provides
The biggest live-ATC network — and its CPDLC rides on Hoppie. In Europe many controllers offer datalink clearances; in the US and Australia PDC often still arrives as a plain private message instead.
Provides
CPDLC uplinks from human controllers (mainly Europe) DCL/PDC at equipped airports D-ATIS via the aircraft interface Voice stays the baseline — datalink is opt-in
Setup
- 1.
Fly a CPDLC-capable add-on and select Hoppie as the CPDLC source (VATSIM traffic routes through it) - 2.
Connect to VATSIM normally, then log on to the controller callsign (e.g. EDYY_CTR) in the aircraft's CPDLC page - 3.
Uplinks arrive on the DCDU/MCDU — answer WILCO
AI-driven ATC covering the whole flight. Supported aircraft can select SayIntentions directly as the CPDLC source, so uplinks land in the cockpit instead of a chat window.
Provides
AI controller CPDLC uplinks Datalink inside supported aircraft Always available — no human controller required
Setup
- 1.
Subscribe and run the SayIntentions - 2.
In the aircraft EFB, pick SayIntentions as the CPDLC source (e.g. iniBuilds A340/A350) - 3.
Requests and uplinks flow through the normal FMS
AI air traffic control with a native datalink path: the aircraft must explicitly support BeyondATC as its ACARS/CPDLC provider — it is not plug-and-play.
Provides
PDC delivered to the aircraft DCDU CPDLC-style clearances in supported aircraft No external message client needed
Setup
Which aircraft support it
No built-in support? Standalone clients like EasyCPDLC connect over Hoppie for aircraft without a native implementation. Coverage changes fast — check your add-on's documentation.