ACARS - How does it work

Aircraft Communication Addressing
and Reporting SystemAircraft
Communications Addressing and
Reporting System (or ACARS) is
adigital 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It will be superseded by the
AeronauticalTelecommunications Network (ATN) protocol and others
more sophisticated. History of ACARSPrior to the
introduction of datalink, all
communication between the aircraft
(i.e., theflight 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 SystemsThe
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 EventsOne of the initial
applications for ACARS was to
automatically detect and
reportchanges 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
weredetermined 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 InterfaceIn
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 DownloadIt 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 InterfaceAll 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
tosend to the ground a request for
various weather information. Upon
entering in thedesired locations for the
weather information and the type of
weather informationdesired, 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 otherairlines 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 worksA
person or a system on board may
create a message and send it via ACARS to asystem or user on the
ground, and vice versa. Messages may
be sent eitherautomatically or
manually. SATCOM and subnetworks SATCOM provides worldwide coverage
with no "black holes" Datalink message typesACARS
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 byaircraft crew to request clearances, and by ground controllers
to provide thoseclearances.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 theguidelines of ARINC
Standard 618, or they are
standardized according
ARINCStandard 633. Various types of
messages are possible and these
include fuelconsumption, engine performance data, and aircraft
position as well as free text data. Example transmissionsDeparture
delay downlinkA pilot may want to
inform his flight operations
department that departure has
beendelayed by Air Traffic Control
(ATC). The pilot would first bring up a CMU MCDUscreen 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 otherdepartments,
such as flight operations,
maintenance, engineering, finance or
otherorganizations 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 themessage, 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 uplinkFor 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. Forexample, 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).