ACARS Explained: How Airliners Text Their Airline

Somewhere over the Atlantic tonight, a printer the size of a paperback will hum quietly on an airliner's centre pedestal and spit out a strip of thermal paper. On it: a revised load sheet, a weather update, or a short note from an operations controller five thousand miles away. No radio call was made. No controller was interrupted. That is ACARS — the Aircraft Communications, Addressing and Reporting System — and it has been quietly running the airline world since 1978.

Most passengers have never heard of it. Most pilots use it a dozen times a flight without thinking about it. Here is what it actually is, how the messages travel, and why a system designed around 1970s telex is still the backbone of airline operational communication in 2026.

What ACARS actually is

ACARS is a digital data link system for exchanging short text messages between an aircraft and ground stations. It is not a single box or a single radio — the term covers the whole chain: the avionics on board, the radio and satellite network in the middle, and the ground processing systems at the other end.

The protocol was designed by ARINC and deployed in July 1978, using the Telex message format that airlines already understood. The avionics were built by Teledyne Controls and the launch customer was Piedmont Airlines. The original expansion of the acronym was in fact ARINC Communications Addressing and Reporting System; it was later changed to Aircraft Communications, Addressing and Reporting System as the technology spread beyond one provider.

Its first job was almost comically narrow. Flight and cabin crew pay depended on whether the aircraft was airborne, at the gate, or somewhere in between, and crews reported those times by voice to scattered radio operators. Airlines wanted the times reported automatically — partly to cut workload, partly to remove the possibility of inaccuracy, accidental or otherwise. The first day of ACARS operations saw roughly 4,000 transactions. Widespread adoption by the major airlines did not really arrive until the 1980s.

OOOI: the four letters that started it all

That original function still exists and is still one of the most valuable things ACARS does. It is known in the industry as OOOI:

• Out — the aircraft leaves the gate (door and parking brake sensors)
• Off — the aircraft lifts off (weight-on-wheels / strut sensors)
• On — the aircraft touches down (weight-on-wheels again)
• In — the aircraft arrives at the gate and the door opens

At the start of each phase, the system transmits a message describing the flight phase, the time it occurred, and related information such as fuel on board and origin and destination. Nobody presses anything. Those four timestamps drive crew pay, on-time performance statistics, gate planning, connection management and turnaround analytics across the entire industry.

The three-part architecture

Because ACARS was modelled on the point-to-point telex network, its architecture has three clean parts.

1. The aircraft equipment

On board, the heart of the system is the Management Unit (MU) or, in newer installations with more functionality, the Communications Management Unit (CMU). It behaves as a router: everything transmitted or received externally passes through it, and on advanced aircraft it routes internal traffic too. Crucially, a modern MU/CMU can automatically select the most efficient air-ground transmission method available at that moment — you do not choose VHF over satellite, the box does.

Crew access is normally through a control display unit, which on advanced aircraft can address up to seven different systems including the flight management system. A flight deck printer is provided, and a cabin crew terminal may also be fitted. On many modern aircraft an electronic flight bag can substitute for CDU access. The original avionics standard was ARINC 597, which defined a management unit with discrete inputs from door, parking brake and weight-on-wheels sensors, plus an MSK modem that pushed the reports out over the existing VHF voice radios. Later standards — ARINC 618 for the air/ground character-oriented protocol, ARINC 623 for ATS applications, ARINC 633 for standardised operational messages, ARINC 758 for the CMU Mark 2 — built the modern system on top.

2. The datalink service provider

A Datalink Service Provider (DSP) moves the messages over the radio link into its own ground routing system. The two primary DSPs are ARINC and SITA. Until fairly recently each part of the world was effectively covered by a single provider; competitive offerings are now increasingly common. Because the network is modelled on telex, every message routes through a central processing location before being delivered.

3. The ground processing system

At the far end sits either an air navigation service provider or the aircraft operator — and operators frequently contract the function out to a DSP or a specialist. Messages, especially automatically generated ones, are pre-configured by message type so that each lands in the right inbox: maintenance control, flight dispatch, crew scheduling, or an air traffic services unit.

How the message physically travels

ACARS messages can be sent over three media, using minimum-shift keying modulation on the radio links.

VHF / VDL. The original and still the cheapest path, and therefore the default whenever it is available. VHF is limited to line of sight; typical range is up to about 200 nautical miles (370 km) at high altitude. Early VHF datalink managed roughly 2.4 kbps; VDL Mode 2, which transfers bits rather than characters, raised that to about 31.5 kbps and was adopted for ACARS as narrowband capacity filled up.

SATCOM. When you are beyond VHF ground coverage, the message goes up instead of sideways. The satellite passes it to a Ground Earth Station, which hands it to the DSP, which delivers it to the airline or ATC. Inmarsat is the classic provider; Iridium's low-earth-orbit constellation is heavily relied on in polar regions, where geostationary coverage degrades badly at high latitudes.

HFDL. High Frequency Data Link was added specifically to fill the polar and remote-ocean gaps. It is the slowest of the three at roughly 1.8 kbps, message loss is not unheard of, and — counter-intuitively — it is the most expensive of the three paths.

What actually goes over the link

Messages fall into three broad categories: Air Traffic Control (ATC), Aeronautical Operational Control (AOC), and Airline Administrative Control (AAC).

ATC traffic includes pre-departure clearances, datalink ATIS and oceanic clearances. AOC and AAC traffic is everything between the aircraft and its own airline: final load and trim sheets uplinked to the flight deck, weather and NOTAM uploads, position and ETA downlinks, diversion notifications, spot weather observations straight from the aircraft's own sensors, automatically triggered exceedance and system-status reports for maintenance, and mundane housekeeping like catering uplift and special passenger advice. Free text is possible in both directions, which is why the system also functions as the airline's version of SMS. Cruise and descent wind data uplinked into the FMS lets the box recompute fuel and timings mid-flight — a genuine efficiency gain, not just a convenience.

If you enjoy this level of nuts-and-bolts detail, our free aviation tools and decoders cover the same territory for METAR, TAF and other everyday flight-deck data formats.

ACARS, FANS and CPDLC — how they relate

This is where most explanations get muddled. ACARS is the network. CPDLC — Controller Pilot Data Link Communications — is an application for talking to controllers by text instead of voice.

When ICAO was developing its Future Air Navigation Systems concept in the 1980s, the full Aeronautical Telecommunication Network was years and a great deal of infrastructure away. The pragmatic answer was to run air traffic services over the ACARS network in the meantime, even though it did not meet all of ATN's requirements. Boeing produced FANS 1, Airbus produced FANS A, and the pair are almost always referred to together as FANS 1/A. The later FANS 1/A+ added refinements such as a message latency monitor, so that a stale message cannot be acted on as if it were current.

The honest assessment from the safety community is that while ACARS fills a significant niche role in ATC communication, it was never the right long-term vehicle for widespread ATC datalink — that is CPDLC's job on a purpose-built network. In practice today, CPDLC is rarely used in continental airspace, where voice remains primary, and is heavily used oceanically, where VHF simply is not there. Alongside it, Automatic Dependent Surveillance–Contract (ADS-C) sends position, altitude, track and speed to the controlling unit automatically, with no crew input at all. Airbus flight decks add a dedicated Data Communications Display Unit for each pilot, used only for the ATC side of the conversation, never for airline traffic. Long oceanic sectors are exactly where a genuinely comfortable pilot headset earns its price, because the HF voice fallback is as unpleasant to listen to as it has always been.

The accident investigations that made ACARS famous

Most people outside aviation first heard the acronym during an investigation.

After the loss of Air France 447 in 2009, there was public discussion of turning ACARS into a kind of streaming "online black box" to reduce the consequences of losing a flight recorder. No changes to the system were made.

In March 2014, ACARS data became central to the search for Malaysia Airlines Flight 370. The primary ACARS reporting had been switched off, but a second satellite communications system remained active as long as the aircraft was powered, attempting to establish a connection with an Inmarsat satellite roughly every hour. Doppler analysis of those handshakes produced the approximate search corridor.

And on 19 May 2016, the ACARS unit aboard the EgyptAir Flight 804 A320 transmitted messages indicating smoke in a toilet and in the avionics bay before the aircraft was lost in the Mediterranean with 66 people on board — automated maintenance downlinks becoming, unintentionally, investigative evidence.

There is a quieter civic role too. In 2002 ACARS was added to the NOAA Observing System Architecture, so commercial aircraft routinely act as weather data providers, downlinking winds and temperatures that feed forecast models.

Why a 1978 system is still here

ACARS survives for the same reason the fax machine outlived several successors: universality. Every airline, every DSP, every dispatch system already speaks it. It is cheap, it degrades gracefully across three independent media, and it does not require a controller's attention. Newer, richer IP-based connectivity is steadily taking over the heavy lifting for cabin and aircraft data, but the short, structured, guaranteed-delivery operational message is still an ACARS message.

For a student pilot, the practical takeaway is this: understanding datalink is no longer optional airline knowledge. The difference between a PDC and a voice clearance, between CPDLC and ADS-C, between what your airline sees and what the controller sees, is line-check material. If you are working toward that first airline job, our own book Becoming an Airline Pilot ($20.99) walks through the training path, the interview process and the operational systems you will be expected to know on day one — written by a working A320/A321 captain rather than a course provider.

Next time you are in seat 14A and you hear a faint chirp from behind the cockpit door, that is probably it: a few hundred characters of text, routed through a network older than most of the aircraft it serves, doing a job nobody has yet found a reason to replace.

Sources: SKYbrary / EUROCONTROL — Aircraft Communications, Addressing and Reporting System; Wikipedia — ACARS (ARINC standards, deployment history, OOOI, MH370, EgyptAir 804, NOAA); Simple Flying — What Is ACARS & How Do Pilots And Airlines Use It?