ACARS — the Aircraft Communications Addressing and Reporting System — is a digital datalink between an aircraft and its airline's ground systems. It came into service in 1978 and is essentially text messaging for aircraft. It is not glamorous and it is not a tracking system, but it carries much of what makes an airline operate.
What it carries
Operational messages. Out, off, on and in times — pushback, takeoff, landing, gate arrival — sent automatically. These drive crew rostering, gate allocation and passenger connection management without anyone typing anything.
Engine and systems data. Periodic reports on engine performance and system status, used for trend monitoring. Manufacturers and airlines analyse this to detect a component drifting out of tolerance before it fails, which is the foundation of predictive maintenance.
Fault messages. When a system detects a fault, ACARS reports it to maintenance control on the ground, so parts and engineers can be waiting at the destination.
Crew messaging. Text between the flight deck and the operations centre — weather, diversions, connection information, clearances in some airspace.
How it gets there
ACARS uses VHF radio near land, HF radio for oceanic and polar regions, and satellite links where equipped. The aircraft's management unit selects whichever is available, preferring the cheapest.
Messages are short, sent in bursts, and cost money per message — airlines configure carefully what is sent and how often. It is a narrow, intermittent link, not a stream.
Air France 447
ACARS is why anyone knew what had happened to AF447 before the recorders were recovered two years later.
As the aircraft's systems detected faults during the event, they generated maintenance messages. Twenty-four of them were transmitted in about four minutes: inconsistent airspeed indications, autopilot disconnection, degradation to alternate control law, and warnings that followed.
Those messages did not explain the accident, and they were never meant to — they are maintenance data. But they established the pitot icing sequence within days, told investigators roughly where and when the aircraft was in trouble, and shaped the entire search. Without them, the search area would have been far larger.
Its limits
ACARS is not designed to locate aircraft and should not be relied on for it. It is intermittent, it can be disabled from the cockpit, and the airline can configure how much it sends. On MH370 it stopped reporting, and the ambiguity around whether that was deliberate is one of the unresolved elements of the case.
Its successor for air traffic purposes is Controller-Pilot Data Link Communications, which carries clearances as text and is now standard in oceanic airspace. ACARS itself remains in service for airline operations, forty-five years on, doing quietly what it has always done.