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

    Tips for Pilots

    • Log on with correct flight ID and ICAO code
    • Know how to retrieve and read D-ATIS, PDC, and CPDLC messages
    • Use LearnATC to simulate ACARS-based workflows and clearance scenarios