AIRCRAFT COMMUNICATION,

ADRESSING AND REPORTING

SYSTEM (ACARS)
ACARS
• The aircraft communications addressing and
reporting system (ACARS) is a data link
communication system.
• It lets you transmit messages and reports
between an airplane and an airline ground
base.
• They can be printed, and can replace listening
to an ATIS via radio.
• An ATIS received via data-link system is referred
to as D-ATIS.
• A message or report from the airplane to the
airline ground base is called a downlink.
• A message or report from the airline ground
base to the airplane is called an uplink.
• ACARS automatically sends reports at
scheduled times in the flight to reduce crew
workload.
• It also sends additional reports when
necessary.
These are typical
ACARS reports:
✓ - Crew identification
✓ - Out, off, on, in
(OOOI) times
✓ - Engine
performance
✓ - Flight status
✓ - Maintenance
items.
Data link Message Types
• They can be subdivided into two categories:-
A. Air Traffic Communications (ATC)
• To make a Connection between the aeroplane and the air
traffic control
• It mainly serves for the delivery of clearance (like clearance
for engine start, the oceanic clearance)
• Air Traffic Service Unit (ATSU) is only for datalink connections
to the appropriate Air Traffic Controller (not the airline
representative).
• Some airspaces may only be entered by aircraft capable of
datalink transmission using the ATSU
• The ATCSU is able to provide the following
services to aircraft:
• Flight Information Service
• Alerting Service
• Air Traffic Advisory Service
• Air Traffic Control Service
• Area
• Approach
• Aerodrome
There are many different types of message that
can be sent by data link to or from an ATSU.
Examples of the more important messages are:
Digital ATIS
/HKGATYA.TI2/VHHH ARR ATIS Z
2205Z
HONG KONG ATIS
RWY IN USE 07R
EXPECT ILS DME APCH
WIND LIGHT AND VARIABLE
VIS 10 KM
CLOUD FEW 1600FT
TEMP 28 DP 22 QNH 1007
ACK Z ON FREQ 119.1 FOR ARR AND 129.9 FOR DEP
B. Airline Operation Communication (AOC)
• messages exchanged between an aircraft and
ground station of its airline
Examples:
• Loadsheets:- uplink message from the airline ground
station containing most recent aeroplane loading
data
• Weather reports:- uplink message concerning the
current weather conditions at the airports for which
they where requested via downlink (METAR and TAF).
• OOOI (out of the gate; off the ground, on the
ground, into the gate):-downlink messages
containing the actual off-block, take off, landing
and on-block times of the aircraft
• Maintenance Reports :-downlink message
about exceedence of performance or system
limits, such as the engines; is sent automatically
and without crew notification.
• Free text messages: uplink or downlink
messages to communicate circumstances for
which no predefined patterns exist ( such as the
further proceedings following an unusual
situation)
• Maintenance Reports
3E03 SNAG 0999/31 KJFK/EGLL .G-BOAC
/FAL 35310102
35. OXYGEN
310L OXYGEN BTL AT STN6
HAS BEEN USED FOR A PAX
IN NEED.
ACARS Components
• These are the components of the ACARS:
I. Multifunction control display unit (MCDU)
I. Multi purpose interactive display unit (MPIDU) to
interface with the ACARS system (optional to MCDU)
II.ACARS Management unit (AMU).
• The AMU processes only uplink messages that
have the airplane registration code.
• All downlink messages have the airplane
registration code attached to identify the
airplane.
connections
ACARS Management Unit (MU) connects to these other
systems components:
I. Printer - to print ACARS reports and messages
(example, the load sheet- has to be carried
along and later stored for documentation)
II. Proximity switch electronics unit (PSEU) - to send
discrete signals for out, off, on and in (OOOI) events
III. Cockpit Voice Recorder (CVR) - to send data-link
messages for the CVR to record
IV. GPS - updates the ACARS clock with the global
positioning system clock time and date (if installed).
• ACARS also connects to these systems to
upload data from the airline operations or
download data to the airline operations:
• - Flight management computers (FMC)
• - Digital flight data acquisition unit (DFDAU)
• The ARINC system uses VHF, HF and SATCOM for
data transmission and is capable of selecting
the most appropriate way of transmission by
itself.
• ACARS uses data link format to pass
messages between the aircraft and ATC or
aircraft operating companies using VHF
• In the early 1990s this service was extended
to SATCOM for flights outside VHF coverage
using geostationary satellites.
• The gap in polar regions was closed in 2001
by extending the service to HF.
ACARS INTERFACE
• You use the MCDU to
control the operation of
the ACARS and to show
ACARS messages.
• It shows ACARS
messages in the scratch
pad.
• The flight crew use the
air traffic services (ATS)
and airline operations
control (AOC) selections
to operate ACARS.
ACARS PREFLIGHT MENU
• to get access to other ACARS pages, you use the
ACARS PREFLT (preflight) menu as you push the
ACARS LSK on the MENU page.
• The flight crew uses these selections and the control
indexes to transmit and receive data from ACARS:
• - PREFLIGHT - to put preflight information into ACARS
• - FUEL INIT - to put in starting airplane fuel
• - CREW REPORT - to make refuel and ground service
reports
• - ATC - to send reports to air traffic control
• - TELEX - to make, address, and send telexes to the
ground station
• WEATHER REQUEST - to get
weather information
• VOICE/DATA CONTROL - to
change the ACARS radio from
voice to data or data to
voice
• VOICE CONTACT - to tell
operations, engineering, or
maintenance to contact the
airplane on a voice
frequency
• RECEIVED MESSAGES - to view
or to print received messages
from ground stations.
Data-link Service Providers
• Mainly handled by two service providers:-ARINC and SITA
I. Aeronautical Radio, Inc. (ARINC)
• It is a US company
• was the first to provide ACARS data-link communication
system.
• used for exchange of messages between an aircraft and
the airline ground station (AOC) and also by ATC to deliver
clearance
• uses HF, VHF and SATCOM
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II. SITA (Societe International de Telecommunication
Aeronautique):
• It is a France company
• Is For the Air Traffic above Europe,
• SITA does not use SATCOM for its datalink
transmission.
• The user does not have to decide between the
service providers for the conduct of datalink
communication but can use both networks to
ensure continuous contact.
• Data links may be established on any frequency, but will
require additional equipment on both the ground and the
aircraft.
• Example, ACARS operates at 131.55MHz (US, Canada,
Australia (primary)) 131.725MHz (Europe)
• One block of message contains maximum of 220
alphanumeric characters.
• If long message is required, more than one block can be
transmitted.

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• ACARS operates in two modes
• Demand mode

• Polled mode

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Demand Mode
• The flight crew can initialize communication.
• The MU determines whether the channel is free.
• If the channel is busy, the MU waits until the channel is
free
• The ground station replies to the message
transmitted.
• If error or no reply is received, from the ground
station, the MU will alert the flight crew after six
attempts.
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Polled mode
• only operates when interrogated from the ground
station.
• The MU formats flight data prior to transmission and
upon request, it transmits.
• The ground station relays/switches this data to the
ARINC control center.
• This data is organized and sent to Operators’ control
center.
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ACARS System

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ACARS system in the A/C

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FUTURE Air Navigation Systems (FANS)
• ICAO defined a standard for FANS systems that is
referred to as Communication, Navigation,
Surveillance/ Air Traffic Management (CNS ATM).
• For better exploitation of airspace capacity which
will make flying more economic and ecologically
compatible without degrading safety level
• Boeing system:- FANS 1, FANS 2 (Further
improvements of FANS 1)
• Airbus system:- FANS A, FANS B (Further
improvements of FANS A)

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• CNS/ATM is designed to use various levels of
automation, digital technology and satellite systems
to give a Seamless Global Air Traffic Management
System.
• Communications, Navigation and Surveillance (CNS)
Systems refer to the facilities offered by an ATSU.
• Air Traffic Management (ATM) refers to the facilities
offered by an Aircraft Operational Centre (AOC). This
is basically the airline, or company, operating the
aircraft.
• Before introduction of the FANS concept, position reports
on oceanic routes were, for example, given by the pilot
via:
• HF voice (hence delayed position reports to ATC).
• The inertial position from an INS or IRS- inaccurate with time
• The sum of both facts demanded a greater separation of
aircraft.
• FANS A specified that the aircraft position be determined
by GPS rather than inertial systems and sent to the ground
station by VHF or satellite (INMARSAT) ACARS connection.
• Employment of GPS improves the accuracy of aircraft
position, and therefore reduces the risk of dangerous
close encounters or even collisions of aircraft.
• FANS A is used over oceanic and remote
airspace, and is transmitted over the ACARS
network, operated by ARINC.
• Its improvement FANS B will also be used on the
continental routes with their dense traffic.
• Use of different additional sensors is meant to
further improve additional accuracy compared
to the GPS based position reports.
• All medium-range airliners certified from 2011
onwards have to be equipped with FANS B
• FANS B is very similar to FANS A but operates
within High Density airspace having good VHF
coverage.
• FANS B is operated over the Aeronautical
Telecommunications Network (ATN), which is
operated by SITA.
• The ATN is an internetwork architecture that
allows ground/ground, ground/air and avionic
data sub-networks to inter-operate.
FANS A Consists of three sub-functions:
1. ATS Facility Notification (AFN): which ensures that every
aircraft is always in contact with the appropriate ground
station
2. Automatic Dependent Surveillance (ADS): which is
responsible for aircraft position reports to the ground
stations
3. Controller Pilot Datalink Communications (CPDLC): which
replaces voice communication via VHF or HF between an
aircraft and a ground facility for example for the purpose
of delivering clearances.
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1. ATS Facility Notification (AFN)
• This function is also referred to as “log-on”,
• At the boundary of a sector, a handover to the next sector
become necessary; hence the ground station tells the aeroplane
to report to the next sector.
• The aeroplane transmits a log-on request to the ground facility
concerned.
• The ground station thereafter checks if the aeroplane is qualified
to log-on and send a positive or negative answer.
• For positive answer, the aeroplane stays in contact with the new
facility and transmits a message to the former station that a log-
on to the next sector has been accomplished.
• After successful log-on the air traffic controller can make use of
both the ADS and CPDLC functions, or only one of them if
desired.
2. Controller Pilot Data-link Communication(CPDLC)
The types of messages:
• Clearance requests
• The corresponding clearances from the ground facility to
the aircraft (example: “fly to reach <flight level> at or
befre <time>)
• Conditions for the passage of certain air-spaces
• “Mayday” messages from the aeroplane
• Request for voice communication from both ends
• Instructions to the pilot to check the status of certain
systems
• Free text messages in both directions
• The Controller Pilot Data-link Communication(CPDLC):-
✓ replaces voice communication between an aircraft
and ground facility
✓ It facilitates the exchange of messages between the
pilot and ATC, who is currently in control of the
aircraft.
✓ clarifies dialogue between the ATC and aircraft crew
who speak different languages (removes human
accents)
✓ It allows the crew to review ATC instructions.
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3. Automatic Dependent Surveillance (ADS)
• FANS A uses the so-called “ADS-Contract” or ADS-C.
• In ADS-C, aircraft transmit their determined GPS position
as well as desired additional information for air traffic
surveillance on demand .
• This is done only to the appropriate ground facility.
• The system thereby replaces the Secondary Surveillance
Radar (SSR) transponder function
• ADS-C is an automatic function that does not require any
crew interference.
• The contracts are “controlled” by the ground end
• Due to its higher accuracy in comparison to positions
determined by radar antennas, ADS enables a reduction
of separation values.
• The ground end can choose between three
types of contracts: periodic, on demand
and on event.
• A change between those contracts can be
completed without crew notification.
• But the emergency mode is triggered by the
crew. This is the CPDLC downlink “Mayday”
I. Periodic contract:
• the aircraft ACARS repetitively transmits the
requested information to the ground station via
downlink when a defined time has passed.
• Such a contract lasts until it is either terminated
or replaced by another periodic contract.
• The time span after which the information is
transmitted can be adapted to the current
situation (e.g. traffic density) by the ground
facility.
II. Event contract:
• A downlink message is sent whenever a certain
“event” occurs.
Examples:
• Vertical speed change
• Lateral displacement change from the FMS flight
plan
• Departure from the altitude band
• FMS switch over to the next waypoint in the flight
plan
III. Demand contract:
• It is used, for example, in climb or descent.
• It is a one-time report transmitting only the basic data
(flight number, latitude and longitude, altitude, time and
navigational accuracy).
• The indicated altitude with an ADS-C contract only
changes when a new report is received.
IV. The emergency mode:
• This can only be activated or cancelled by the crew.
• ADS-C automatically changes to emergency mode if a
“Mayday” message is sent via the CPDLC.