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