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Aircraft Communications Addressing and Reporting System (or ACARS) is a digital datalink system for transmission of small messages between aircraft and ground stations via radio or satellite. more...

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The protocol, which was designed by ARINC and deployed in 1978, uses telex formats. It will be superseded by the Aeronautical Telecommunications Network (ATN) protocol.

History of ACARS

Prior to the introduction of datalink, all communication between the aircraft (i.e., the flight crew) and personnel on the ground was performed using voice communication. This communication used either VHF or HF voice radios, which was further augmented with SATCOM in the early 1990s.

In many cases, the voice-relayed information involves dedicated radio operators and digital messages sent through an Aeronautical Telecommunications Network (ATN) to an airline teletype system or its successor systems.

Introduction of ACARS Systems

The airlines, in an effort to reduce crew workload and improve data integrity, introduced the ACARS system in the late 1980’s. (A few initial ACARS systems were introduced before the late 1980s, but ACARS did not start to get any widespread use by the major airlines until the later part of the 1980s.) Although the term ACARS is often taken into context as the datalink avionics Line-replaceable unit installed on the aircraft, the term actually refers to a complete air and ground system. On the aircraft, the ACARS system was made up of an avionics computer called an ACARS Management Unit (MU) and a CDU (Control Display Unit). The MU was designed to send and receive digital messages from the ground using existing VHF radios. On the ground, the ACARS system was made up of a network of radio transceivers, which would receive (or transmit) the datalink messages, as well as route them to various airlines on the network.

Note that the initial ACARS systems were designed to the ARINC standard 597. This system was later upgraded in the late 1980’s to the ARINC 724 characteristic. ARINC 724 addressed aircraft installed with avionics supporting digital data bus interfaces. This was subsequently revised to ARINC 724B, which was the primary characteristic used during the 1990s for all digital aircraft. With the introduction of the 724B specification, the ACARS MUs were also coupled with industry standard protocols for operation with flight management system MCDUs using the ARINC 739 protocol, and printers using the ARINC 740 protocol. The industry has defined a new ARINC characteristic, called ARINC 758, which is for CMU systems, the next generation of ACARS MUs.

OOOI Events

One of the initial applications for ACARS was to automatically detect and report changes to the major flight phases (Out of the gate, Off the ground, On the ground and Into the Gate); referred to in the industry, as OOOI. These OOOI events were determined by algorithms in the ACARS MUs that used aircraft sensors (such as doors, parking brake and strut switch sensors) as inputs. At the start of each flight phase, the ACARS MU would transmit a digital message to the ground containing the flight phase, the time at which it occurred, and other related information such as fuel on board or origin and destination. These messages were primarily used to automate the payroll functions within an airline, where flight crews were paid different rates depending on the flight phase.

Read more at Wikipedia.org