Aircraft Communication Addressing and Reporting System

Aircraft Communication 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. The protocol was defined in the 1970s and 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 It will be superseded by the Aeronautical Telecommunications Network (ATN) protocol.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.

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 1980's, but ACARS did not start to get any widespread use by the major airlines until the later part of the 1980's.) 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.

Flight Management System Interface

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 alternate flight plans.

Maintenance Data Download

It was the introduction in the early 1990s of the interface between the FDAMS / ACMS systems and the ACARS MU that resulted in datalink gaining a 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 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.

Interactive Crew Interface

All of the processing described above is performed automatically by the ACARS MU and the associated other avionics systems, with action performed by the flight crew. As part of the growth of the ACARS functionality, the ACARS MUs also interfaced directly with a control display unit (CDU), located in the cockpit. This CDU, often times referred to as an MCDU or MIDU, provides the flight crew with the ability send and receive messages similar to today’s email. To facilitate this communication, the airlines in partnership with their ACARS vendor, would define MCDU screens that could be presented to the flight crew and enable them to perform specific functions. This feature provided the flight crew flexibility in 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 a MCDU screen that allowed them to send to the ground a request for various weather information. Upon entering in the desired locations for the weather information and the type of weather information desired, the ACARS would then transmit the message to the ground. In response to this request message, ground computers would send the requested weather information back to the ACARS MU, which would be displayed and/or printed.

Airlines began adding new messages to support new applications (Weather, Winds, Clearances, Connecting Flights…) and ACARS systems became customized to support airline unique applications, and unique ground computer requirements. This results in each airline having their own unique ACARS application operating on their aircraft. Some airlines have more than 75 MCDU screens for their flight crews, where other airlines 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.

How it works

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 and manually.

VHF Subnetwork

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.

SATCOM and HF Subnetworks

SATCOM provides worldwide coverage, with the exception of operation at the 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. HF datalink is responsible for new polar routes. Aircraft with HF 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.

Datalink Message Types

ACARS messages may be of three types:

Air Traffic Contol (ATC)
Airline Operational Control (AOC)
Airline Administrative Control (AAC)

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 defined by the users, but must meet the guidelines of ARINC Standard 618. Various types of messages are possible and these include fuel consumption, engine performance data, and aircraft position as well as free text data.

Example Downlink: Departure Delay

A pilot may want to inform his flight operations department that departure has been delayed by Air Traffic Control (ATC). The pilot would first bring up a CMU MCDU screen that allows him to enter the expected time of the delay and the reason for the delay. After entering the information on the MCDU, the pilot would then press a “SEND” key on the MCDU. The CMU would detect the SEND key being pushed, and would then generate 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 ETA before the delay and the current expected delay time. The CMU would then send the message to one of the existing radios (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 would transmit the VHF signals containing the delay message. This message is then received by a VHF Remote Ground Station (RGS).

It should be noted that the majority of ACARS messages are typically 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 that 220 characters within the body of the message. For downlink messages which are longer than 220 characters, the ACARS unit will split 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). For these multi-block messages, the RGS collects each block until the complete message is received before processing and routing the message. The ACARS also contains protocols to support retry of failed messages or retransmission of messages when changing service providers.

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 and forwards to the airline’s computer system. The message is then processed by the airline’s computer system.

This processing performed by an airline may include reformatting the message, populating databases for later analysis, as well as 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, the message 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 transmission time from when the flight crew presses the send key to send the message, to the time that it is processed within 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 a downlink message.

Example Uplink: Weather Report

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 frequency. 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 which the CMU is the destination, such as a weather report uplink, the flight crew can go to a specific MCDU screen which contains a list of all of the received uplink messages. The flight crew can then select the weather message, and have the message viewed on the MCDU. The ACARS unit may also print the message on the cockpit printer (either automatically upon receiving the message or upon flight crew pressing a PRINT prompt on the MCDU screen).

Example downlink: FDAMS Message

Messages are sent to the ground from other on-board systems in a similar manner as the delay message example discussed previously. As an example, a FDAMS system may have a series of algorithms active to monitor engine exceedance conditions during flight (such as checking engine vibration or oil temperature exceeding normal operating conditions). The FDAMS system, upon detecting such an event, 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 would then transmit the message to the ground. In this case, the service provider 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 3 major components to the ACARS datalink system:

Aircraft equipment
Service provider
Ground processing system

Aircraft equipment

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:

FMS - sends flight plan change requests, position reports, etc. Receives clearances and controller instructions.
Printer - as an end system, can be addressed from the ground to automatically print an uplink message.
Maintenance Computer - downlinks diagnostic messages. In advanced systems, in-flight troubleshooting can be conducted by technicians on the ground by using datalink messages to command diagnostic routines in the maintenance computer and analyzing downlinked results.
Cabin Terminal - Often used by flight attendants to communicate special needs of passengers, gate changes due to delays, catering, etc.

ACARS messages are transmitted over one of three air-ground subnetworks.

VHF is the most commonly used and least expensive. Transmission is line-of-sight so VHF is not available over the oceans.
SATCOM provides worldwide coverage (except in polar regions) by means of the INMARSAT satellite network. It is a fairly expensive service.
HF is the most recently established subnetwork. Its purpose is to provide coverage in the polar regions where SATCOM coverage is unreliable.

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.

Datalink Service Provider

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 would come in to the central processing location and be punched to paper tape. The tape would be 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 network in North America, and have also recently started operating a network in Europe. ARINC has also assisted the CAAC in China, 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 US 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.

Ground end system

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, or airline operations headquarters. CAA end systems provide air traffic services such as clearances. Airline operations provides information necessary for operating the airline efficiently, such as gate assignments, maintenance, passenger needs, etc. Airlines have more recently started to use a Rockwell Collins supplied product, HERMES, for the collation, parsing and reformatting of ACARS messages for delivery into back end airline systems and back via the ACARS networks to the originating or other aircraft in the fleet. see HERMES

This product is now been extended into the eFlight concept for integrated airlines operations see eFlight

ARINC Specifications

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 607 – Design Guidance for Avionics Equipment. Includes definition of Aircraft Personality Module (APM), required for ARINC 758 CMU installation.

ARINC 429 – specification for receiving and broadcasting ARINC 429 broadcast data (data received from other avionics LRUs).

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, Cabin Terminal, Maintenance Computers, SATCOM systems and HF Voice Data Radios.

ARINC 620 – Defines ground-to-ground communication protocols. This includes the message 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 724B – specification for an ACARS MU for ARINC 724B wiring.

ARINC 739 – Specification for interfacing with Multi-purpose Cockpit Display Units

ARINC 740 and 744 – Specification for interfacing to cockpit Printers.

ARINC 758 – Specification for a CMU relative to ARINC 758 wiring. This specification actually identifies various levels of functionality, these defining future growth phases for the CMU. Initial CMU systems which perform today’s ACARS functions are classified as Level OA.

Acronyms and Glossary

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.

ACARS : Aircraft Communications Addressing and Reporting System
ACMS : Aircraft Condition Monitoring System
AOA : ACARS Over AVLC. With the introduction of VDL Mode 2, the ACARS protocols were modified to take advantage of the higher data rate made possible by Mode 2. AOA is an interim step in replacing the ACARS protocols with ATN protocols.
ATN : Aeronautical Telecommunications Network. As air traffic increases, ACARS will no longer have the capacity or flexibility to handle the large amount of datalink communications. ATN is planned to replace ACARS in the future and will provide services such as authentication, security, and a true internetworking architecture. Europe is leading the US in the implementation of ATN.
AVLC : Aviation VHF Link Control. A particular protocol used for aeronautical datalink communications.
CMF : Communications Management Function. The software that runs in a CMU, and sometimes as a software partition in an integrated avionics computer.
CMU : Communications Management Unit. Successor to the MU, the CMU performs similar datalink routing functions, but has additional capacity to support more functions. CMU standards are defined in ARINC Characteristic 758.
FDAMS : Flight Data Acquisition and Management System
FMS : Flight Management System. FMS standards are defined in ARINC Characteristic 702 and 702A.
HF : High Frequency. A portion of the RF spectrum.
LRU : Line Replaceable Unit. An avionics "black box" that can be replaced on the flight line, without downing the aircraft for maintenance.
MCDU : Multifunction Control Display Unit. A text-only device that displays messages to the aircrew and accepts crew input on an integrted keyboard. MCDU standards are defined in ARINC Characteristic 739. MCDUs have seven input ports and can be used with seven different systems, such as CMU or FMS. Each system connected to an MCDU generates its own display pages and accepts keyboard input, when it is selected as the system controlling the MCDU.
MU : Management Unit. Often referred to as the ACARS MU, this is an avionics LRU that routes datalink messages to and from the ground.
OOOI : Shorthand for the basic flight phases -- Out of the gate, Off the ground, On the ground, In the gate.
POA : Plain Old ACARS. Refers to the set of ACARS communications protocols in effect before the introduction of VDL Mode 2. The term is derived from POTS (Plain old telephone service) that refers to the wired analog telephone network.
SATCOM : Satellite Communications. Airborne SATCOM equipment includes a satellite data unit, high power amplifier, and an antenna with a steerable beam. A typical SATCOM installation can support a datalink channel as well as several voice channels.
VDL : VHF Digital Link
VHF : Very High Frequency. A portion of the RF spectrum.

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