Acars

ACARS On-Board Components

ACARS (Aircraft Communications Addressing and Reporting System) is made up of several aircraft systems that work together to send and receive digital messages between the aircraft and the ground.

1. ATSU / ACARS Communications Management Unit (CMU)

• The core router of the ACARS system.
• Chooses the most cost-effective communication link (VHF, HF, or SATCOM).
• Sends and receives all ACARS messages between aircraft and ground.

2. Flight Management System (FMS)

• Often the main source and destination of ACARS messages.

• Sends:

  • Position reports
  • Flight plan change requests

• Receives:

  • ATC clearances
  • Controller instructions

3. Control Display Unit (CDU / MCDU)

• Pilot interface for ACARS messages.
• Displays uplink messages from the ground.
• Allows pilots to create and send downlink messages.
• Provides visual alerts when new messages arrive.

4. Data Communications Display Unit (DCDU)

• Used mainly on older aircraft.
• Stand-alone ACARS display when integration into the CDU is not practical.
• Performs similar functions to a CDU but dedicated to datalink.

5. Printer

• Can automatically print uplink messages from the ground.
• Pilots can print messages as needed (e.g., clearances, weather).

6. Maintenance Computer

• Sends diagnostic and fault data to the ground.

• Allows ground technicians to:

  • Run diagnostics remotely
  • Perform in-flight troubleshooting

• Results are automatically downlinked.

7. ATC Message Visual Annunciator

• Alerts the flight crew when an ATC message is received.
• Often integrated into existing FMS alerting systems.
• DCDUs typically include their own annunciator.