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The ACARS MU/CMU transmits messages from the FMC prior to those created from direct interface with the
ACARS MU/CMU, and other peripherals.
B777:
On the B777, the AFN and CPDLC functions reside in the Flight Deck Communication Function (FDCF) and
the ADS function resides in the Flight Management Computing Function (FMCF). The Data Communications
Management Function (DCMF) manages transmission of messages to and from the airplane. Downlink
message prioritization occurs at the FDCF, FMCF and DCMF.
The FDCF prioritizes downlink message in the following order of priority (highest to lowest):
• ATC Datalink Messages
• AFN Messages
• AOC Messages
The FMCF prioritizes downlink message in the following order of priority (highest to lowest):
• ADS Messages
• AOC Messages
If multiple messages are queued in the FDCF and FMCF, higher priority messages will be inserted ahead of
lower priority messages.
19
FANS-1/A Datalink Communications Environment
DCMF will prioritize messages received from FDCF and FMC in the following order of priority (highest to
lowest):
• ATC Datalink Messages
• ADS Messages
• AFN Messages
• AOC Messages
The DCMF is typically connected to multiple functions and peripheral devices. The DCMF is capable of
receiving a downlink from the FDCF and FMCF while a message from another peripheral is being
transmitted.
The DCMF transmits messages from the FDCF and FMCF prior to those from other peripherals.
5.3.4 Airbus ATS downlink Media Selection and ATS/AOC Prioritization and Differences, if any, Among
Airframe Type
4.3.4.1 Media selection in stable communication conditions
Downlinks
The Airbus avionics routing policy for down linked ATC messages via different media is factory set,
and is in line with that implemented by Datalink Service providers:
- In FANS A Avionics the priority is VDL Mode A, then SATCOM
- In FANS A+ Avionics the priority is VDL Mode2, then VDL Mode A, then SATCOM, then HFDL.
FANS A avionics exist on A330/A340 aircraft and FANS A+ on A330/A340, A320 Family and A380.
When a media connection is established but is busy transmitting a higher priority message (AFN,
ADS or CPDLC) the next lower priority media per the list above is used immediately. This is called
the next-on-busy mechanism.
AOC messages routing policy is customizable and has no impact on ATC messages routing.
Uplinks
Uplinks are acknowledged over the media on which they are received, and the MAS are
reported using this media. The media used for the uplink has no impact on the media
used for any associated downlink. Downlink routing is only determined by the routing
policy described above.
4.3.4.2 VHF coverage limit, VHF/SATCOM/HFDL transition:
If VHF link is considered available
The avionics makes up to 6 transmission attempts over VHF at 12s intervals for ATC (AFN, ADS &
CPDLC) messages. If unsuccessful on VHF the avionics will attempt delivery via SATCOM (if
logged on) after 72 seconds.
If VHF link is NOT available
The message is immediately transmitted over SATCOM if logged on.
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FANS-1/A Datalink Communications Environment
If transmission over SATCOM is unsuccessful (despite being logged on) after 3 attempts at 3 minute
intervals, the SATCOM is put into sleep mode for 10 minutes. At the expiry of the 10 minute sleep
mode another transmission will be attempted over SATCOM if still logged on. If still unsuccessful
and HFDL if installed then HFDL is used by FANS A+.
This processing will apply to ADS messages whose lifetime in the downlink queue is unlimited with
FANS A, and limited to 30 minutes with FANS A+, after which they are flushed from the queue.
CPDLC messages have a downlink queue lifetime of 2 minutes with FANS A and 3min 30 seconds
with FANS A+.
If VHF is not available and SATCOM is not logged on
Messages are transmitted over HFDL with FANS A+, else they are purged at timer expiry
according to the lifetime described above.
5.4 FANS-1/A Ground End Systems and Message Assurance
All FANS-1/A Ground End systems should include the ARINC 620 Message Assurance (MA) Text
Element Identifier (TEI) with request delivery indication only (e.g. MA 123A, where the MA is the TEI, the
123 is the serial number of the uplink, and A designates request delivery indication only. Uplinks sent
with message assurance will get an indication of whether or not the uplink was successful or not.
Successful uplinks will be those in which the CSP that successfully delivered the uplink to the aircraft
receive a technical acknowledgment from the aircraft. Upon receipt of the technical acknowledgment, the
CSP that delivered the uplink sends a ground/ground message to the originator that uses the MAS
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