MARK II COMMUNICATIONS MANAGEMENT UNIT (CMU)——LINE MAINTENANCE GUIDE

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ACARS Messages are transmitted by VHF Radio to
ground stations maintained by ARINC, SITA, and others.
Messages are delivered to the Airline using a Telex
Network.
Messages May Also Be sent Via
Satellite and/or HF if These Systems are
Also Installed on the Aircraft.
Concept of Operation
The ACARS unit is used to send and receive data messages from the ground. The primary data link used by
the ACARS is the VHF3 radio. The ACARS may also be connected to a Satellite Data Unit (SDU) as well as
to an HF Data Unit (both of which are optional).
Avionics on board the aircraft such as DMU, CMC, FMC and Cabin terminals may be connected to ACARS
for the purpose of sending messages to the ground. In addition, ACARS displays MCDU screens to the crew
for the purpose of sending REQUESTS and Reports to the ground.
The ACARS also receives information such as Registration Number, from other avionics devices on the
ARINC 429 bus as well as from discrete inputs.
Messages from the ground are sent to the ACARS unit. The ACARS unit will examine the message and
either store the message or route it to other avionics depending on the destination encoded in the message.
Uplink messages maintained by the ACARS unit are displayed on a RECEIVED MESSAGES MCDU
screen. The crew can display or print the messages.
Typical ACARS configurations require little or no maintenance activity once installed with the appropriate
software downloaded into memory. Software is downloaded using either the Airborne data Loader or
Portable Data Loader or PCMCIA media.
Additional detailed information can be found in the ACARS CMU Component Maintenance Manual and
the CMU Requirements Document.