Aircraft Communications Addressing and Reporting System (ACARS)

Definition

ACARS is a digital datalink communication system that enables the automatic exchange of messages between aircraft and ground stations (airlines, ATC, maintenance). It uses VHF, HF, or satellite networks to transmit text-based operational, logistical, and flight data, reducing the need for voice communication.

ACARS is the underlying network for services such as D-ATIS, CPDLC, OCEANIC clearances, and automated aircraft status reports.

Purpose

Support non-voice communication between aircraft and dispatch/ground units
Transmit flight and weather information, clearances, position reports, and maintenance data
Enhance flight efficiency, safety, and data tracking
Reduce radio congestion and manual reporting

ACARS Message Types

Category	Examples
ATC messages	D-ATIS, pre-departure clearances, CPDLC
Operational messages	Flight plan updates, weather data, NOTAMs
Administrative messages	Gate assignments, delay notices, crew instructions
Aircraft reports	Takeoff time, position reports, OOOI (Out, Off, On, In) status
Maintenance	System alerts, fault reports sent automatically to airline MRO teams

How ACARS Works

Aircraft logs on to ACARS via onboard FMS or ACARS control unit
Messages are sent to airline dispatch, ATC, or third-party providers
Communications can use:
- VHF (short range, line-of-sight)
- HF (long range, lower bandwidth)
- SATCOM (satellite link for global coverage)

Messages are automatically or manually composed and transmitted

Ground stations reply, forward, or act on messages (e.g., send gate changes)

Examples of ACARS Messages

“Request oceanic clearance via ACARS”
“D-ATIS received for EDDF: Runway 25L in use, wind 230/10…”
“Takeoff time 0914Z, FL330, estimating waypoint DITUM 0955Z”
“Fuel on board 9.2 tons, no MEL items”

OOOI Times

A common ACARS function is reporting OOOI times, which track the 4 key phases of flight:

Phase	Definition
Out	Off-blocks time (pushback starts)
Off	Takeoff time (wheels-up)
On	Landing time (touchdown)
In	On-blocks time (arrived at gate)

These times are used for billing, scheduling, crew duty logs, and fleet tracking.

ACARS vs. CPDLC

Feature	ACARS	CPDLC
General role	Messaging platform	Subset focused on ATC communication
Users	Airlines, maintenance, ATC	ATC and flight crew
Message format	Free-text or structured	ICAO standard phrase sets
Coverage	Global via VHF, HF, SATCOM	Specific FIRs and routes

Limitations

❌ Limited bandwidth – not suitable for voice or multimedia
❌ Not used for emergency or tactical control
❌ Not available on all general aviation aircraft
✅ Essential for modern airline operations, fleet tracking, and ATC integration

Benefits

✅ Improves communication reliability
✅ Reduces controller/pilot workload
✅ Supports automated reporting and flight tracking
✅ Integrates with flight planning, weather, and maintenance systems