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In aviation, ACARS (//; an acronym for Aircraft Communications Addressing and Reporting System) is a digital datalink system for transmission of short messages between aircraft and ground stations via airband radio or satellite. The protocol was designed by ARINC and deployed in 1978, using the Telex format. More ACARS radio stations were added subsequently by SITA.
Prior to the introduction of datalink in aviation, all communication between the aircraft and ground personnel was performed by the flight crew using voice communication, using either VHF or HF voice radios. In many cases, the voice-relayed information involved dedicated radio operators and digital messages sent to an airline teletype system or successor systems.
In an effort to reduce crew workload and improve data integrity, the engineering department at ARINC, introduced the ACARS system in July 1978. The first day of operations saw about 4,000 transactions, but ACARS did not experience widespread use by the major airlines until the 1980s. The original ARINC development team was headed by Crawford Lane and included Betty Peck, a programmer, and Ralf Emory, an engineer. The terrestrial central site, a pair of Honeywell Level 6 minicomputers, and their software were developed by a subcontractor, Eno Compton of ECOM, Inc.
Although the term ACARS is often understood as the data link avionics line-replaceable unit installed on aircraft, the term actually refers to a complete air and ground system. The original expansion of the abbreviation was "Arinc Communications Addressing and Reporting System". Later, it was changed to "Aircraft Communications, Addressing and Reporting System".
On the ground, the ACARS system is made up of a network of radio transceivers managed by a central site computer called AFEPS (Arinc Front End Processor System), which handles and routes messages. Generally, ground ACARS units are either government agencies such as the Federal Aviation Administration, an airline operations headquarters, or, for small airlines or general aviation, a third-party subscription service. Usually government agencies are responsible for clearances, while airline operations handle gate assignments, maintenance, and passenger needs.
The ACARS equipment on the aircraft is linked to the that on the ground by the datalink service provider. Because the ACARS network is modeled after the point-to-point telex network, all messages come to a central processing location to be routed. ARINC and SITA are the two primary service providers, with smaller operations from others in some areas. Some areas have multiple service providers.
ACARS messages may be of three broad types:
Control messages are used to communicate between the aircraft and its base, with messages either standardized according to ARINC Standard 633, or user-defined in accordince with. The contents of such messages can be OOOI events, flight plans, weather information, equipment health, status of connecting flights, etc.
A major function of ACARS is 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 are detected using input from aircraft sensors such as doors, parking brake and strut switch sensors. At the start of each flight phase, an ACARS message is transmitted to the ground describing the flight phase, the time at which it occurred, and other related information such as the amount of fuel on board or the flight origin and destination. These messages are used to track the status of aircraft and crews.
ACARS interfaces with flight management systems, acting as the communication system for flight plans and weather information to be sent from the ground to the FMS. This enables the airline to update the FMS while in flight, and allows the flight crew to evaluate new weather conditions or alternative flight plans.
ACARS is used to send information from the aircraft to ground stations about the conditions of various aircraft systems and sensors in real-time. Maintenance faults and abnormal events are also transmitted to ground stations along with detailed messages, which are used by the airline for monitoring equipment health, and to better plan repair and maintenance activities.
ACARS interfaces with interactive display units in the cockpit, which flight crews can use to send and receive technical messages and reports to or from ground stations, such as a request for weather information or clearances or the status of connecting flights. The response from the ground station is received on the aircraft via ACARS as well. Each airline customizes ACARS to this role to suit its needs.
ACARS can send messages over VHF if a VHF ground station network exists in the current area of the aircraft. VHF communication is line-of-sight propagation and the typical range is up to 200 nautical miles at high altitudes. Where VHF is absent, an HF network or satellite communication may be used if available. Satellite coverage may be limited at high latitudes (trans-polar flights).
The sound of an ACARS VHF transmission made on 130.025 MHz, recorded at Petaluma, California on 15 August 2006
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In the wake of the crash of Air France Flight 447 in 2009, there was discussion about making ACARS an "online-black-box" to reduce the effects of the loss of a flight recorder. However no changes were made to the ACARS system.
In March 2014, ACARS messages combined with satellite ping times played a very significant role in tracking the whereabouts of Malaysia Airlines Flight 370.
In 2002, ACARS was added to the NOAA Observing System Architecture. Thus commercial aircraft can act as weather data providers for weather agencies to use in their forecast models, sending meteorological observations like winds and temperatures over the ACARS network. NOSA provides real-time weather maps.