SAS Datalink System

ACARS avionics

By Björn Syrén, SAS


The ACARS avionics architecture in a typical airplane

The drawing above illustrates a typical architecture for ACARS-related avionics in an airplane. The ACARS Management Unit (MU) is the heart of the Datalink System. The latest generation of datalink avionics is called a Communication Management Unit (CMU) and offers additional capabilities over an MU, particularly for support for future new-generation air/ground datalinks expected to become operational during 2001-2005.

The MU/CMU receives and sends messages through a VHF radio. The VHF radio provides communication with ACARS Remote Ground Stations (RGSs) within line-of-sight, ie up to approximately 200-250 nautical miles from the airplane at cruising altitude. The 3rd VHF radio is used by ACARS on SAS' airplanes.

On long-range airplanes e.g. SAS' Boeing 767 airplanes the MU is also connected to a SATCOM system which provides communication when flying over remote areas where there are no VHF RGSs.

An alternative or complement to Satcom for long-range communications is High Frequency Data Link (HFDL). However, HFDL is not currently used by SAS. On advantage of HFDL is that it offers coverage in extreme polar regions which is not possible with current geostationary Satcom satellite systems.

The pilot's main interfaces with ACARS are the Multi-Function Control and Display Unit (MCDU) and the Cockpit Printer. The MCDU displays ACARS menus and pages. A keyboard allows the pilot to navigate through the ACARS system, select applications and enter data.

Downlink messages are sent manually by the pilot or automatically by the MU/CMU. They can also originate from another end-system eg the Aircraft Condition Monitoring System (ACMS), the Flight Management Computer (FMC), the Airshow system or a Cabin Datalink Terminal (not shown above). In either case the MU/CMU will automatically select the most appropriate air/ground link.

Most uplink messages are meant for the pilot and will be forwarded by the MU/CMU to the cockpit printer. Some uplink messages are sent only for display on the MCDU if a printout is not required. Depending on type of message and the phase of the flight, a message alert in the form of a chime and a light may be activated to bring the pilot's attention to the message.

Uplink messages may also be sent other end-systems eg the ACMS, the FMC, the Airshow or a Cabin Terminal/Printer (not shown above). In such cases the MU/CMU will only route the message to the applicable end system where it is processed.

SAS Boeing 767-383ER cockpit. The left and right MCDUs are located on the forward part of the center pedestal.
The printer is located in the aft right corner of the pedestal (just out of view in the picture).
Photo by Ottmar Raeymaeckers 1999.


Latest update: 25-Jan-2010
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