Acars
Aircraft Communication Addressing and Reporting
System
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. 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superseded by the Aeronautical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 and
others more sophisticated.
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
1980s. (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.
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
realtime 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 onboard 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 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, 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
an 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.
Since few years ago, some manufactures
developed an integrate CMU and ACU (Aircraft Communication Unit) bringing to
market a new concept of ACARS over satellite.
One of these manufactures is Wingspeed corp
that had developed a very nice system with graphical interface trough EFB
(Electronic Flight Bag) and MCDU.
This new concept allow the airlines operates
the system with more flexibility as well include several others applications.
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 or manually.
SATCOM and subnetworks
SATCOM provides worldwide coverage with no “black
holes”
Datalink message types
ACARS messages may be of three types:
Air Traffic Control (ATC)
Aeronautical 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 either defined by the
users, but must then meet at least the
guidelines of ARINC Standard 618, or they are
standardized according ARINC
Standard 633. Various types of messages are
possible and these include fuel
consumption, engine performance data, and
aircraft position as well as free text data.
Example transmissions
Departure delay downlink
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.
Again, Wingspeed Corp, increased a very nice
solution for this technical restriction of message size. Using own protocol called
“XLLINK” it is possible to send more than 1.500 characters with the same
header.
At first glance, it seams to be a small
differential, but in fact, this reduces the transmission cost by half or more.
Weather report uplink
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).
By. Tiago Senna
Airline Transport Pilot
Software and telecom engineer
Autor: Tiago Senna
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