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Aircraft Communications Addressing and Reporting System (ACARS) is a digital datalink system for transmission of short, relatively simple messages between aircraft and ground stations via radio or satellite. The protocol, which was designed by Aeronautical Radio, Incorporated (ARINC) to replace their very high frequency (VHF) voice service and deployed in 1978, uses telex formats. SITA later augmented their worldwide ground data network by adding radio stations to provide ACARS service. Over the next 20 years, ACARS will be superseded by the Aeronautical Telecommunications Network (ATN) protocol for Air Traffic Control communications and by the Internet Protocol for airline communications.
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 involved dedicated radio operators and digital messages sent to an airline teletype system or its successor systems.
The engineering department at Aeronautical Radio, Inc (ARINC), in an effort to reduce crew workload and improve data integrity, introduced the ACARS system in July 1978. The first day of operations saw about 4,000 transactions. A few experimental ACARS systems were introduced earlier, but ACARS did not start to get any 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 (AFEPS) 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 Airline Communications, Addressing and Reporting System.
On the aircraft, the ACARS system was made up of an avionics computer called an ACARS Management Unit (MU) and a Control Display Unit (CDU). 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 managed by a central site computer called AFEPS (Arinc Front End Processor System), which received (or transmitted) the datalink messages as well as routed them to various airlines on the network.
The initial ACARS systems were designed to ARINC Characteristic 597. This was later upgraded in the late 1980s by the publication of ARINC Characteristic 724. ARINC 724 is intended for aircraft installed with avionics supporting digital data bus interfaces. ARINC 724 was updated to the current standard ARINC Characteristic 724B, which is the predominate standard for all digital aircraft.[clarification needed] With the introduction of ARINC 724B, the ACARS MUs were also coupled with industry standard protocols for operation with flight management system MCDUs (ARINC 739), and printers (ARINC 740 and ARINC 744). The ACARS MU has been since expanded to serve broader needs using a Communications Management Unit (CM) defined by ARINC Characteristic 758. Today, new aircraft designs integrate CM functions in Integrated Modular Avionics (IMA). ARINC Standards are prepared by the Airlines Electronic Engineering Committee (AEEC).
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 are determined by algorithms that use aircraft sensors (such as doors, parking brake and strut switch sensors) as inputs. At the start of each flight phase, a digital message is transmitted to the ground containing the flight phase, the time at which it occurred, and other related information such as amount of fuel on board or flight origin and destination. These messages are used to track the status of aircraft and crews.
In addition to detecting events on the aircraft and sending messages automatically to the ground, initial systems were expanded to support new interfaces with other on-board avionics. During the late 1980s and early 1990s, a datalink interface between the ACARS MUs and Flight Management Systems (FMS) was introduced. This interface enabled flight plans and weather information to be sent from the ground to the ACARS MU, which would then be forwarded to the FMS. This feature gave the airline the capability to update FMSs while in flight, and allowed the flight crew to evaluate new weather conditions, or alternative flight plans.
The introduction in the early 1990s of the interface between the FDAMS / ACMS systems and the ACARS MU resulted in datalink's gaining wider acceptance by airlines. The FDAMS / ACMS systems which analyze engine, aircraft, and operational performance conditions were now able to provide performance data to the airlines on the ground in real time using the ACARS network. This reduced the need for airline personnel to go to the aircraft to off-load the data from these systems. These systems were capable of identifying abnormal flight conditions and automatically sending real-time messages to an airline. Detailed engine reports could also be transmitted to the ground via ACARS. The airlines used these reports to automate engine trending activities. This capability enabled airlines to better monitor their engine performance and identify and plan repair and maintenance activities.
In addition to the FMS and FDAMS interfaces, the industry started to upgrade the on-board maintenance computers in the 1990s to support the transmission of maintenance-related information in real-time through ACARS. This enabled airline maintenance personnel to receive real-time data associated with maintenance faults on the aircraft. When coupled with the FDAMS data, airline maintenance personnel could now start planning repair and maintenance activities while the aircraft was still in flight.
All of the processing described above is performed automatically by the ACARS MU and other associated avionics systems, without flight crew intervention. As part of the growth of ACARS functionality, the ACARS MUs also interfaced directly with a control display unit (CDU), located in the cockpit. This CDU, often referred to as an MCDU or MIDU, provides the flight crew with the ability to send and receive messages similar to today’s email. To facilitate this communication, the airlines in partnership with their ACARS vendor defined MCDU screens that could be presented to the flight crew and enable them to perform specific functions. This feature provides the flight crew flexibility as to the types of information requested from the ground and the types of reports sent to the ground.
As an example, the flight crew could pull up an MCDU screen that allowed them to send to the ground a request for various types of weather information. After the desired locations and type of weather information were entered, ACARS transmitted this information to the ground. In response to this request message, ground computers sent the requested weather information back to the ACARS MU, which was then displayed and/or printed.
Airlines began adding new messages to support new applications (weather, winds, clearances, connecting flights, etc.) and ACARS systems were customized to support airline-unique applications, and unique ground computer requirements. This resulted in each airline having its own unique ACARS application operating on its aircraft. Some airlines have more than 75 MCDU screens for their flight crews, where other may have only a dozen different screens. In addition, since each airline’s ground computers were different, the contents and formats of the messages sent by an ACARS MU were different for each airline.
In the wake of the crash of Air France Flight 447, there has been discussion about making ACARS an "online-black-box." If such a system were in place, it would avoid the loss of data due to: (1) black-box destruction, and (2) inability to locate the black-box following loss of the aircraft. However the cost, due to the high bandwidth requirements, would be excessive and there have been very few incidents where the black boxes were not recoverable.
A person or a system on board may create a message and send it via ACARS to a system or user on the ground, and vice versa. Messages may be sent either automatically or manually.
A network of VHF ground radio stations ensure that aircraft can communicate with ground end systems in real-time from practically anywhere in the world. VHF communication is line-of-sight, and provides communication with ground-based transceivers (often referred to as Remote Ground Stations or RGSs). The typical range is dependent on altitude, with a 200-mile transmission range common at high altitudes. Thus VHF communication is only applicable over landmasses which have a VHF ground network installed.
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SATCOM can provide worldwide coverage. Depending on the satellite system in use, coverage may be limited or absent at high latitudes (such as needed for flights over the poles). HF datalink is a relatively new network whose installation began in 1995 and was completed in 2001. Aircraft with HF or global SATCOM datalink can fly polar routes and maintain communication with ground based systems (ATC centers and airline operation centers). ARINC is the only service provider for HF datalink.
ACARS messages may be of three types:
ATC messages are used to communicate between the aircraft and Air Traffic Control. These messages are defined in ARINC Standard 623. ATC messages are used by aircraft crew to request clearances, and by ground controllers to provide those clearances.
AOC and AAC messages are used to communicate between the aircraft and its base. These messages are either standardized according to ARINC Standard 633 or defined by the users, but in the latter case they must meet at least the guidelines of ARINC Standard 618. Various types of messages are possible, for example, relating to fuel consumption, engine performance data, aircraft position, in addition to free text .
A pilot wants to inform his flight operations department that departure has been delayed by Air Traffic Control. The pilot loads a CMU MCDU screen that allows him to enter the expected length of and reason for the delay. After entering the information on the MCDU, he depresses a “SEND” key on the MCDU. The CMU detects that the SEND key was pushed, and then generates a digital message containing the delay information. This message may include such information as aircraft registration number, the origination and destination airport codes, the current Estimated Time of Arrival (ETA) before the delay and the current expected duration of the delay. The CMU then sends the message to an existing radio (HF, SATCOM or VHF, with the selection of the radio based on special logic contained within the CMU). For a message to be sent over the VHF network, the radio transmits the VHF signals containing the delay message, which is then received by a VHF Remote Ground Station (RGS).
Most ACARS messages are only 100 to 200 characters in length. Such messages are made up of a one-block transmission from (or to) the aircraft. One ACARS block is constrained to be no more than 220 characters within the body of the message. For downlink messages which are longer than 220 characters, the ACARS unit splits the message into multiple blocks, transmitting each block to the RGS (there is a constraint that no message may be made up of more than 16 blocks). The RGS collects each block of such multi-block messages until the complete message is received before processing and routing the message. ACARS also contains protocols to support a retry of failed messages or retransmission of messages whenever the service provider changes.
Once the RGS receives the complete message, the RGS forwards the message to the datalink service provider's (DSP) main computer system. The DSP ground network uses landlines to link the RGS to the DSP. The DSP uses information contained in their routing table to forward the message to the airlines or other destinations. This table is maintained by the DSP and identifies each aircraft (by tail number) and the types of messages that it can process. (Each airline must tell its service provider(s) what messages and message labels their ACARS systems will send, and, for each message, where they want the service provider to route the message. The service provider then updates their routing tables from this information.) Each type of message sent by the CMU has a specific message label, which is contained in the header information of the message. Using the label contained in the message, the DSP looks up the message in the table and forwards it to the airline’s computer system. which then processes message.
This processing performed by an airline may include reformatting the message, populating databases for later analysis, or forwarding the message to other departments, such as flight operations, maintenance, engineering, finance, or other organizations within an airline. In the example of a delay message, it may be routed via the airline’s network to both their operations department as well as to a facility at the aircraft’s destination, notifying them of a potential late arrival.
The elapsed transmission time from the moment the flight crew presses the send key to the moment it is processed by an airline’s computer system varies, but is generally on the order of 6 to 15 seconds. The messages that are sent to the ground from the CMU are referred to as downlink messages.
For a message to be transmitted to the aircraft (referred to as an uplink message), the process is nearly a mirror image of how a downlink is sent from the aircraft. For example, in response to an ACARS downlink message requesting weather information, a weather report is constructed by the airline’s computer system. The message contains the aircraft registration number in the header of the message, with the body of the message containing the actual weather information. This message is sent to the DSP's main computer system.
The DSP transmits the message over their ground network to a VHF remote ground station in the vicinity of the aircraft. The remote ground station broadcasts the message over the VHF. The on-board VHF radio receives the VHF signal and passes the message to the CMU (with the internal modem transforming the signal into a digital message). The CMU validates the aircraft registration number, and processes the message.
The processing performed on the uplink message by the CMU depends on the specific airline requirements. In general, an uplink is either forwarded to another avionics computer, such as an FMS or FDAMS, or is processed by the CMU. For messages destined for the CMU (such as a weather report uplink), the flight crew refers to a specific MCDU screen which contains a list of all received uplink messages. The flight crew then selects the weather message for viewing on the MCDU. The ACARS unit may also print the message on the cockpit printer (either automatically upon receiving the message or in response to the flight crew's pressing a PRINT prompt on the MCDU screen).
Messages are sent to the ground from other on-board systems in a manner similar to the delay message example discussed above. As an example, an FDAMS system may have a series of active algorithms monitoring engine exceedance conditions during flight (such as checking whether engine vibration or oil temperature exceeds normal operating conditions). Upon detecting such an event, the FDAMS system automatically creates an engine exceedance condition message, with applicable data contained within the body of the message. The message is forwarded to the CMU, using what is referred to as ARINC 619 data protocols. The CMU then transmits the message to the ground. In this case, the service provider's routing table for an engine exceedance message is typically defined to have the message routed directly to an airline’s maintenance department. This enables airline maintenance personnel to be notified of a potential problem in real time.
There are three major components to the ACARS datalink system:
The heart of the datalink system on board the aircraft is the ACARS Management Unit (MU). The older version of MU is defined in ARINC Characteristic 724B. Newer versions are referred to as the Communications Management Unit (CMU) and are defined in ARINC Characteristic 758.
Aircraft equipment consists of airborne end systems and a router. End systems are the source of ACARS downlinks and the destination for uplinks. The MU/CMU is the router. Its function is to route a downlink by means of the most efficient air-ground subnetwork. In many cases, the MU/CMU also acts as an end system for AOC messages.
Typical airborne end systems are the Flight Management System (FMS), datalink printer, maintenance computer, and cabin terminal. Typical datalink functions are:
ACARS messages are transmitted over one of three air-ground subnetworks.
The router function built into the MU/CMU determines which subnetwork to use when routing a message from the aircraft to the ground. The airline operator provides a routing table that the CMU uses to select the best subnetwork.
The role of the datalink service provider (DSP) is to deliver a message from the aircraft to the ground end system, and vice versa.
Because the ACARS network is modeled after the point-to-point telex network, all messages come to a central processing location. The DSP routes the message to the appropriate end system using its network of land lines and ground stations. Before the days of computers, messages came to the central processing location and were punched on paper tape. The tape was then physically carried to the machine connected to the intended destination. Today, the routing function is done by computer, but the model remains the same.
There are currently two primary service providers of ground networks in the world (ARINC and SITA), although specific countries have implemented their own network, with the help of either ARINC or SITA. ARINC operates a worldwide network and has also assisted the Civil Aviation Administration of China (CAAC), as well as Thailand and South America with the installation of VHF networks. SITA has operated the network in Europe, Middle East, South America and Asia for many years. They have also recently started a network in the USA to compete with ARINC.
Until recently, each area of the world was supported by a single service provider. This is changing, and both ARINC and SITA are competing and installing networks that cover the same regions.
The ground end system is the destination for downlinks and the source of uplinks. Generally, ground end systems are either government agencies such as CAA/FAA, an airline operations headquarters, or, in the case of small airlines or general aviation consumers, a subscription based solution. CAA end systems provide air traffic services such as clearances. Airline and general aviation operations provide information necessary for operating the airline or flight department efficiently, such as gate assignments, maintenance, and passenger needs. In the early history of ACARS most airlines created their own host systems for managing their ACARS messages. Commercial off-the-shelf products are now widely available to manage the ground hosting.
Much of the processing performed by the CMU as well as basic requirements of the hardware are defined by ARINC specifications. The following is a list of the major ARINC specifications that define standards that govern many aspects of ACARS systems:
ARINC documents and their specifications
|ARINC 607||Design Guidance for Avionics Equipment. Includes definition of the Aircraft Personality Module (APM) required for ARINC 758 CMU installation.|
|ARINC 429||Specification for receiving and broadcasting ARINC 429 broadcast data (data transfer between avionics LRUs). ARINC 429 is the one-way communication data bus (one pair of data bus use for transmit data and another pair of data bus use for receive data).|
|ARINC 618||Defines the air/ground protocols for communicating between the ACARS/CMU and VHF ground systems. Also defines the format of the ACARS messages sent by the ACARS/CMU as well as received by the ACARS CMU. The format of this message is called a Type A message. This characteristic has been updated to define the future VDL Mode 2 AOA operation.|
|ARINC 619||Defines the protocols for the ACARS/CMU to use to transfer file data between other avionics in the aircraft. ARINC 619 covers file protocols that are used to interface with FMS, FDAMS, the cabin terminal, maintenance computers, SATCOM systems and HF voice data radios.|
|ARINC 620||Defines ground-to-ground communication protocols. This includes the format of messages routed between a service provider and an airline or other ground system. This is referred to as a Type B message (the air/ground Type A message is reformatted to a Type B message for ground transmissions).|
|ARINC 622||Describes the processing associated with sending ATC application messages over today’s ACARS links (including ARINC 623 ATC messages).|
|ARINC 623||This characteristic identifies ATC related messages that can be generated or received by an ACARS MU/CMU system (does not include FANS-1 or FANS-A messages that are processed by the FMS).|
|ARINC 629||Specification for a bi-directional data bus for sending and receiving data between multiple avionics LRUs. The specification was initially developed for use on Boeing 777 commercial airplanes, but was published as an ARINC industry standard in 1999.|
|ARINC 631||Specification for VHF Data Link (VDL) Mode 2. This specification provides general and specific design guidance for the development and installation of the protocols needed to exchange bit-oriented data across an air-ground VHF Digital Link (VDL) in an Open System Interconnection (OSI) environment.|
|ARINC 724B||Specification for an ACARS MU for ARINC 724B wiring.|
|ARINC 739||Specification for interfacing with Multi-purpose cockpit display units.|
|ARINC 740||Specification for interfacing to cockpit printers.|
|ARINC 758||Specification for a CMU relative to ARINC 758 wiring. This specification identifies various levels of functionality, these in turn defining future growth phases for the CMU. Initial CMU systems which perform today’s ACARS functions are classified as Level OA.|
|ARINC 823||Two-part specification that defines a security framework for protecting ACARS datalink messages exchanged between aircraft and ground systems. Security services include confidentiality, data integrity, and message authentication. Part 1, ACARS Message Security (AMS), specifies the security protocol, and Part 2, Key Management, specifies life-cycle management of the cryptographic keys necessary for secure and proper operation of AMS.|
It has been rumored that the introduction of datalink into the airline industry originated as part of a contest to see how many acronyms could be developed around a specific technology. Whether this is true or not, the industry is at the point where acronyms are now nested within acronyms. For example, AOA is an acronym for ACARS Over AVLC, where AVLC itself is an acronym for Aviation VHF Link Control and VHF is also an acronym for Very High Frequency.