ACARS FAQ

Frequently Asked Questions

Over the years I get e-mails from people asking roughly the same questions, so I try here to list the most asked ones in the hope of helping some people out.

• What is 'ACARS'?
• What is a 'Hoppie ID'?
• What is a 'logon code'?
• What is a 'call sign'?
• What is 'CPDLC'?
• Who is 'Hoppie'?
• Can I send and receive real ACARS messages?
• What are those networks? Can I have one?
• Why does my callsign not show online?
• Is there a Discord channel for assistance?
• How can I test the ACARS connection?
• Can I get a flight plan or dispatch report from ACARS?

What is 'ACARS'?

ACARS is mostly old. The real system started in the 1970s as a way for airborne airplanes to relay operational information about their flight back to their airline. Events such as Out of the gate, Off the runway, On the runway, and In the gate (OOOI) were all manually gathered and telephoned to the airline's central ops center. ACARS automated this.

ACARS is built mainly with VHF radio stations, about one station per 200 km, but many (many!) more in busy areas. It can roughly be compared to the modern cellular system, but then with 1970 radios on VHF using AM and a data transfer rate of about 33 kilobits per second for all airplanes in that area together.

Over time, ACARS got used for more and more things, and today it is a very heavily (over)used system that carries millions of messages. But most are short, a few hundred bytes. It is not suitable for more than about four kilobytes per message. This is also why you see so many coded messages such as weather reports: there simply is no bandwidth for more.

What is a 'Hoppie ID'?

A Hoppie ID is the wrong name that some people give to a logon code for Hoppie's ACARS.

What is a 'Logon Code'?

A logon code for Hoppie's ACARS is a simple key that gives you access to all features of the system, and allows the system to figure out which of your programs can access the same callsign. Without a logon code, it would be possible for multiple people to accidentally use the same callsign, which would be cumbersome.

Everybody can request a free logon code for Hoppie's ACARS using the link in the left margin of this page, Get Logon Code. You need to give your name and a working e-mail address to send the code to. These data won't be sold or exposed, see my privacy policy.

There also is a CPDLC logon code. Unfortunately the name clash happened because the two systems were not initially designed together. This CPDLC logon code is the four-letter code of the ATC station you want to connect to for CPDLC services. Remember that CPDLC services, just as voice services, are offered by ATC networks like IVAO or VATSIM, and that the station may not even be online.

What is a 'callsign'?

A callsign is the ATC code of your station. For airplanes it usually is a tail number (N12345, PH-WOW) or a flight identifier (KLM123). Airplane operator stations, like dispatchers, usually use the three-letter code of their operator (KLM). For ATC stations it usually is a four-letter code (LPPT). In real-world ACARS, callsigns are limited to 7 characters, but Hoppie's ACARS does not restrict this.

Take care that callsigns with underscores "_" cannot be typed by pilots that use an MCDU. The underscore is not part of the ACARS character set, not in the simulator and not in the real world.

What is 'CPDLC'?

CPDLC (Controller-Pilot Data Link Communication) is a system that since about 1990 has been in use to allow routine air traffic control reports and clearances over a datalink, instead of over voice radio.

There are a few very specific things that make CPDLC totally different from plain ACARS messages!

• CPDLC is only between controllers and pilots (that's in the name). It is not used, at all, ever, for communication between airlines and pilots, or anything else except safety services by ATC.
• CPDLC is primary where offered, but pilots always need to also have a working voice link with ATC. As soon as a CPDLC message is unclear or needs to be urgently amended, ATC will switch to voice. CPDLC is not, never was, never will be, just texting with ATC.
• CPDLC is a very restricted set of ATC clearances, pilot acknowledgements, and pilot requests. It is designed around coded messages that you cannot change, except for some values.
• Free text in CPDLC, although possible for clarification of clearances or requests, is very much discouraged.
• Every CPDLC clearance from ATC to a pilot needs to be acknowledged, and there are always very restricted ack options. For example, an ATC "do this" instruction can be acked by either wilco or by unable. If a pilot really (really) needs more time to decide, they can return standby and then asap rsn send the real answer. ATC will not wait very long for the answer before taking corrective action.

CPDLC was initially introduced in the 1990s for oceanic control, to significantly improve the HF radio that then was the main means of long range communication. The older Inmarsat Classic Aero satellite system was used for this, next to voice links, but those were really expensive. Note well that HF radio and satellite voice communications never are between a pilot and a controller. Never. They are always to a commercial radio station, where operators work through telexed lists of messages to send and receive from the procedural (no airplane contact!) controllers at the oceanic control centers. Sending an HF message to an airplane could easily take 20 minutes. CPDLC finally gave the (procedural) controllers a way to automate most of their routine communications and also receive automated position updates back (ADS-C). It never was a radar, it required airplanes on tracks in trail with fixed speeds. But it was a huge relief compared to the bad quality endless HF attempts to get anything through.

Only in the 2010s, domestic CPDLC was slowly introduced in Europe and the US. "Of course" both continents use a different system... but for pilots they look and feel almost the same. It is mandatory to always have a working VHF voice link with the controller that is on the other end of your CPDLC link. And just as with oceanic CPDLC, it will never be used for truly time-sensitive ATC matters. In practice, only at the top flight levels where nearly everybody just drones on, following their filed flight plan route, CPDLC is used.

The exception is that for ground clearance deliveries, a subset of CPDLC called CPDLC-DCL (Departure Clearance) may be used. Of course, the people that invented this acronym had humour. DCL tremendously relieves the frequency congestion of clearance delivery positions, and often includes loadable routes, that can be directly imported into an FMC, without the long and error-prone readbacks and manual keying.

Who is 'Hoppie'?

That's me, Jeroen Hoppenbrouwers. It has been my nickname since about 1979. I occasionally drink coffee.

I have been interested in ATC since I was about 12, and when I finally got myself into the emerging flight simulator scene, I found it striking that there was no sign of ACARS. This was in the time of RogerWilco, where VATSIM's predecessor (SATCO) did not yet universally offer voice connections between pilots and controllers, and everybody was texting to each other instead. Few people saw what ACARS would add to this, everybody was getting excited to not need to text and just push PTT and yell at each other. However the real world used ACARS very, very much and was also pushing to introduce CPDLC. So I set out to build exactly that, simulator-independent, mostly for fun.

It got a bit out of hand when this home-brew system attracted the attention of the avionics industry and I got an actual job offer from a small avionics company in Miami. Suddenly I found myself building the actual thing for actual airplanes... and I still do.

With nobody else really interested in ACARS/CPDLC for simulators, Hoppie's ACARS remained the only viable solution for simulation for decades, and still is the major player in the game. Not that I really push for this or make any effort to increase my user count. It just is what it is.

Can I send and receive real ACARS messages?

Hoppie's ACARS is a hobby system, simulating an ACARS network for simulator enthusiasts. Several training facilities also use it, and I know of a few demonstration setups that hook into it as well. But no, with Hoppie's ACARS you cannot send messages to real aircraft or even listen in to real ACARS. If you have the right HAM radio equipment, it's not difficult to eavesdrop on real ACARS, see for example the RTL-SDR.com web site (there are many more). To send real ACARS messages you would illegally use a VHF radio, and it would be as much an offense as stepping onto tower transmissions with your own radio near an airfield. Don't. Only the two big international duopolists Collins and SITA currently provide commercial access to ACARS (and a few national organisations in some countries).

What are those networks? Can I have one?

Hoppie's ACARS originally started as one single network, where all callsigns shared the same "name space". When more and more IVAO and VATSIM users started to use ACARS, callsign clashes began to appear, especially for ATC stations during busy hobby evenings. So I introduced the sub-networks as callsign name spaces. Just imagine your callsign, such as "EDDF", is prefixed by your network, like "VATSIM:EDDF", and you have understood it.

For those people who don't want to fly online with IVAO or VATSIM, but still like to have ACARS access to weather, or to play around with their own ground station, I created the "None" network. This is nothing special, just a namespace not IVAO and not VATSIM.

As ACARS only works if enough people provide ATC and other services, it turned out that literally all other networks, started by good-willing people, never grew out of a handful of users. So I decided no longer to accept new network requests. If you absolutely must have your own network, show me the users ... start on NONE and gather data. I can be convinced.

To change your Network Affiliation, in other words to change the namespace in which your callsign will live and be visible, use the My ACARS Account form.

Why does my callsign not show online?

If you think everything should work, but you don't get any messages, check the online status display. If your callsign is not on this list, the ACARS system has not seen you yet and therefore will not be able to deliver any messages you may have pending.

Some ACARS software intentionally does not bring you online until you have sent a message out yourself. This may be a simple weather request. There are no cases, also not in the real world, where an airplane spontaneously starts receiving ACARS messages, as nobody would send anything until they know the airplane is active. So make yourself known to the world. This also locks down your callsign so nobody else can step on it.

In nearly all other cases, there is an issue with your ACARS connection. This may be an invalid logon code, or a network problem. Your ACARS software usually gives relevant suggestions when its attempt to connect fails.

Is there a Discord channel for assistance?

In case you would like to ask other users and developers for assistance or just to exchange thoughts, there is a Discord text channel available.

How can I test my ACARS connection?

If your ACARS client is not very vocal about whether it works or not, the easiest way to test it probably is to request a weather report. Usually this option goes under the name "WXR" or "METAR". If it works, you get a weather report of the airport in a few seconds. Good airport codes to try are the larger ones, such as "EHAM" or "KJFK".

You can also send a free text ("telex") message to your own callsign. It may take up to a minute before you receive it, so give it the time it needs.

You can look on the ACARS web site whether your callsign appears online. This also verifies whether you have made yourself member of the correct subnet, such as VATSIM or IVAO. Only members of the same subnet can communicate, so the same callsign can exist on more than one subnet (which is handy for airport stations).

Take note that some ACARS clients intentionally don't make you appear online until you actually did something, such as asking for a weather report. This is to avoid having many airplanes online that don't even realize they have activated ACARS.

Lastly you can telex somebody who you know is online and ask for confirmation.

Can I get a flight plan or dispatch report from ACARS?

Well, yes and no. ACARS is not a dispatch service, there is no machine that can generate flight paperwork for you in this system. It can get at some limited publicly available resources exactly like the real ACARS system, such as weather reports, but for the rest, ACARS depends on other people (or machines) delivering services. Just as in the real world.

In practice, you probably want to join one of the many Virtual Airlines that offer dispatch services via ACARS. Some even have dedicated custom ACARS software for in your simulator.

If you want to provide dispatch services, you may want to look at the jurassic software I wrote around 2000 for ideas. Since most simulators do not share the same loadable file formats, the software does not provide anything like that, but it can be used as a ground station for a virtual airline. It's relevant for gathering details from airplanes out there and relay things back to the crews.