ACARS explained: receiving and decoding airline data link

At the fence you hear the engines, and on a scanner perhaps the tower. What you do not hear: while the aircraft is being pushed back from the gate, it is already sending a short text message to its airline. At takeoff the next one follows, and in cruise weather requests, fuel levels and maintenance reports are added. This data link is called ACARS, and since the late 1970s it has run almost unnoticed alongside voice radio. In this article, you learn how ACARS came about, how a message is structured, which paths it travels and what you need to know before you start a decoder yourself.

ACARS in one paragraph

ACARS stands for Aircraft Communications Addressing and Reporting System. It is not voice radio but a data service for short telegrams between aircraft and ground. An onboard computer, the so-called Management Unit, collects reports from the flight management system, engine monitoring, door sensors or the cockpit keyboard and sends them over a radio network to ground stations. From there, a service provider forwards the message to the airline. The return path works the same way: the operations center can send weather, schedule changes or free text to the cockpit, where they end up on a display or a small printer.

How ACARS came about: a time clock for the cockpit

The idea behind ACARS was sober. In the late 1970s, crews reported their duty and block times by voice radio, and the airline entered them by hand. That cost time in the cockpit and was error-prone. ARINC – a radio service provider for US airlines founded in 1929 – therefore developed a system that records these times automatically. ACARS went into operation in July 1978, the avionics came from Teledyne Controls, and the launch customer was Piedmont Airlines.

This “automatic time clock” gave rise to the OOOI reports:

• Out – the aircraft leaves the parking position, detected for example by closed doors and a released parking brake
• Off – the aircraft takes off, detected by the landing gear struts
• On – the aircraft touches down
• In – the aircraft is at the gate and the doors open

These four timestamps are still the basis today for punctuality statistics, crew pay and rotation planning. Later, SITA also built its own ACARS ground network. ARINC was acquired by Rockwell Collins at the end of 2013 and now belongs to Collins Aerospace. In technical language, these large providers are called datalink service providers – they operate the ground stations and pass the messages on to the airlines.

What runs over ACARS today

The time recording system has become a general-purpose channel. Roughly, three groups can be distinguished:

Group	Who communicates	Typical content
Airline operational communications (AOC)	Aircraft and the airline’s operations center	OOOI times, fuel, delays, weather for destination and alternate airports, flight plan data for the FMS
Administrative communications	Aircraft and airline administration	Crew matters, passenger and catering notes, connecting flights
Air traffic control (ATC)	Aircraft and air traffic control	Clearances and instructions via CPDLC, automatic position reports via ADS-C, departure clearances

For an airline’s technical department, ACARS is especially valuable: many aircraft report fault codes and engine parameters automatically, so the right spare part can already be waiting at the destination airport. How maintenance is organized is explained in Aircraft maintenance: A to C check.

In air traffic control, ACARS plays a role mainly over the oceans. The FANS-1/A package uses ACARS as the transport path for CPDLC, that is clearances by text, and for ADS-C, in which the aircraft reports its position automatically according to an agreed pattern. How CPDLC works and why Europe uses a different path for it is covered in CPDLC explained.

How an ACARS message is structured

Anyone who runs a decoder for the first time first sees a block of abbreviations and numbers. That looks cryptic but follows a fixed pattern. Every message consists of a header with administrative information and the actual text. The fields that decoders such as acarsdec output are these:

Field	Meaning	What spotters can read from it
Mode	Operating mode over which the connection runs	rather technical
Address (Aircraft)	Registration of the aircraft	the registration in plain text, such as D-AIxx
Ack	Acknowledgment of a previous message	whether a telegram arrived
Label	Two-character code for the message type	what kind of report it is
Block ID	Running identifier of the transmission block	whether uplink or downlink, order
Message number	Counter of the message	long messages in several parts
Flight number	The airline’s flight ID	which rotation is currently flying
Text	Content of the message	from plain text to columns of numbers

For you as a spotter, the address field is the most exciting part. While with Mode S you only see a 24-bit hex address that you first have to match to an aircraft, with ACARS the registration is right in the header. How registrations are built is shown in Understanding aircraft registrations, and the difference from the transponder is explained in Mode S and transponders. And why the flight number in the header does not always match the callsign on voice radio is covered in Callsign and flight number.

Labels: the drawer for every message

The label is a two-character code by which ground systems recognize where a message belongs. A large share of the labels is standardized, but many are used airline-specifically. Some examples that community documentation describes:

• H1 – messages from and to onboard systems, such as the flight management computer or maintenance computer
• 44 – position reports and OOOI times
• _d – pure keep-alive messages without content, used to maintain the connection
• SQ – so-called squitters, with which ground stations announce their presence
• :; – instruction to change the frequency of the onboard radio

The meaning of individual labels, and especially of the contents, differs by airline, aircraft type and service provider. Free projects have been collecting these formats for years but are not complete anywhere. So expect a large share of the messages to remain incomprehensible to you.

What is in the text

The range goes from readable free text through weather reports in METAR format to long, airline-specific number sequences. You quickly recognize weather data – how to decode it is shown in Reading METAR and TAF. For air traffic control applications such as CPDLC and ADS-C, there are binary formats of their own. Libraries such as libacars translate part of them into readable fields, including FANS-1/A messages and so-called media advisories, with which the aircraft reports which data links are currently available.

The transport paths: VHF, VDL2, shortwave and satellite

ACARS primarily describes the message format and addressing. Which radio path a message travels is decided by the aircraft depending on its equipment and position. Modern avionics automatically choose the best path currently available.

Classic VHF ACARS. The ancestor works in the upper aviation band around 131 MHz with amplitude modulation and a data rate of 2.4 kbit/s. As with voice radio, range is limited to line of sight – at high altitude that is roughly up to 200 nautical miles. In Europe, the common channels are at 131.525 and 131.725 MHz, plus, depending on the source, further channels just above. How the aviation band is divided is explained in Understanding aviation radio frequencies.

VDL Mode 2. The digital successor on VHF works at 31.5 kbit/s, much faster than the classic method. In Europe, a large share of ACARS traffic runs over it today, and the common signaling channel is on 136.975 MHz. Details are in Receiving VDL Mode 2.

HFDL. Over oceans and in polar regions, there are no VHF ground stations. There, the data link on shortwave helps, which bridges very large distances but achieves only low data rates. How to receive it is shown in HFDL on shortwave.

Satellite. Via Inmarsat or Iridium, aircraft can be reached almost everywhere. Especially over oceans, where VHF ground stations are missing, the satellite is often the most important path. The signals arrive in the L band and require different receiving technology than VHF.

Path	Frequency	Range	Strength
VHF ACARS	around 131 MHz	line of sight	simple, widespread
VDL Mode 2	around 136.9 MHz	line of sight	faster, standard in Europe
HFDL	shortwave	thousands of kilometers	ocean and polar coverage
SATCOM	L band	nearly worldwide	independent of ground stations

For spotters in central Europe, this means: anyone who only listens to the classic ACARS channels sees only a section. The abundance only comes with VDL Mode 2.

Receiving and decoding ACARS: the principle

Technically, getting started is manageable. You need four building blocks:

The decoder turns the short radio bursts into text and outputs it line by line, verbosely or as JSON. acarsdec decodes several channels at the same time as long as they lie within 2 MHz, and with libacars it can also break down FANS-1/A content. Frequencies, gain, frequency correction and the setup of ACARS Hub are described step by step in the practical guide Receiving ACARS messages with SDR. If you are working with SDR for the first time, it is best to start with Receiving aviation radio with an SDR stick and the comparison of SDR sticks.

Legal situation: technically open, legally not

The fact that ACARS messages are mostly sent unencrypted does not make them public. They are addressed to airlines, service providers and air traffic control, not to the general public. In Germany, they therefore fall, according to the prevailing view, under the same regime as voice radio: Section 5 TDDDG prohibits listening to messages that are not meant for you, as well as passing on their content or even the fact that you received them. Violations are punishable under Section 27 TDDDG. Buying and owning an SDR stick, on the other hand, is allowed.

This is covered in detail in Listening to aviation radio: technology and legal situation. Why ADS-B is assessed differently is explained in Is receiving ADS-B data allowed?. Austria, Switzerland and other countries have their own rules, which you should check before operating.

ACARS and spotting

For everyday life at the fence, ACARS is not a tool you need. Which aircraft is behind a flight and when it lands, you find out more conveniently and without legal issues through ADS-B and flight tracking – for example the way Tracking an aircraft by registration describes it. ACARS is more for tech fans who want to understand how modern flight operations work in the background: how the aircraft reports times on its own, how maintenance is prepared remotely and how controllers over the Atlantic issue clearances by text message.

If you want to dive deeper into the technical language, you will find terms such as squawk, callsign or block-off time in the plane spotting glossary.

Conclusion

ACARS is aviation’s silent text messaging service. What began in 1978 as an automatic time clock for block times now carries operational data, maintenance reports, weather and, via FANS-1/A, even air traffic control clearances. Every message carries a fixed header with registration, flight number and label, travels via VHF, VDL Mode 2, shortwave or satellite and can be made visible with an SDR stick and free software. It is exciting mainly for tech fans – as long as the content stays where it belongs: with you.