ACARSAircraft Communications Addressing and Reporting

System

ACARSAircraft communications are for Air Traffic Control (ATC) and Airline Operational Control (AOC), sometimes called “Company Communications”.

ACARS is operated by ARINC in North American and by SITA (Société Internationale de Télécommunications Aéronautiques) in Europe.

On the VHF bands, SITA is referred to as AIRCOM.Both ARINC and SITA provide a full-range of services via satellite, known as GLOBALink® for ARINC.

Full coverage is at altitudes > 20,000 feet (FL200) with additional stations at about 300 airports for on-the-ground communications.

ACARS System

Textbook page 42

OUT-OFF-ON-IN(OOOI)

ACARS first job was to report when flights were leaving and arriving.

OUT Flight leaving, triggered by release of parking brake.

OFF Takeoff, triggered by the air-ground sense system.ON Landing, triggered by the air-ground sense system.IN Parking, triggered by setting of the parking brake.

Old ACARS control unit, late 1970s

OOOI

Textbook page 43

ACARS Message Format

An ACARS message has 3 building blocks: Preamble, Text and Block Check.

Characters are composed from digital bits transmitted as 2 audible tones (1200 and 2400HZ), through the VHF transceiver.

The tones are decoded back to digital form.

ACARS Message Format

Textbook page 43

ACARS “OFF” Message

Each aircraft has its’ own unique identifier, so the crew only receives ACARS messages intended for that particular aircraft.A message can be sent in milliseconds by data which would take several minutes by voice.Some messages only require a push of a button to send, saving crew workload.

Textbook page 44

The ARINC 429 Specification

The ARINC 429 Specification establishes how avionics equipment and systemscommunicate on commercial aircraft. The specification defines electrical characteristics,word structures and protocol necessary to establish bus communication. ARINC 429utilizes the simplex, twisted shielded pair data bus standard Mark 33 Digital InformationTransfer System bus.

ARINC 429 data words are 32 bit words made up of five primary fields:

• Parity – 1 bit• Sign/Status Matrix (SSM) – 2 bits• Data – 19 bits• Source/Destination Identifier (SDI) – 2 bits• Label – 8 bits

ARINC Pulse

PM Pulse Modulation

Carrier Wave

Transmission

Modulation of Carrier Wave

Modulation Signal

Addressing and Protocols

Open Systems Interconnection (OSI)

OSI

ACARS Control PanelsThe upper panel is the newer version on aircraft equipped with “glass” or CRT Displays.

The lower panel is the older version on aircraft with “round dial” or analog displays.

Textbook page 41

ACARS CDU Control and Display Unit

ACARS Bands and Frequencies

Textbook page 45

Review Questions Chapter 6 ACARS6.1 What is the meaning of the abbreviation “ACARS”?Answer: Aircraft Communication Addressing and Reporting System.6.2 What type of communications occur on ACARS?Answer: Air Traffic Control (ATC) and Airline Operational Control (AOC).6.3 What is the meaning of the ACARS message “OOOI”?Answer: Out-Off-On-In6.5 How is an ACARS message received only by the aircraft it’s intended for?Answer: By it’s own unique identifier.6.6 What 2 bands carry ACARS services?Answer: VHF and HF.6.7 What satellite-based system carries ACARS services worldwide?Answer: ARINC GLOBALink®