NAVIGATION AND
COMMUNICATION
AIRPLANE
OPERATIONS
MANUAL
2-18-24
Page
4
Code
01
MARCH 30, 2001
ACARS OPERATION
The Aircraft Communications Addressing and Reporting System
(ACARS) is controlled through the FMS Control and Display Units
(CDU) located on the control pedestal aft panel. The CDU provides
control functions and operating modes management for proper ACARS
operation. The FMS provides default data to the ACARS. Many
messages and requests have fields that are filled with FMS flight
progress data, but the flight crew can modify data by overriding that
value with a manual entry.
There are two types of ACARS data communications: ground to air via
uplink messages and air to ground via downlink messages. Once a
message is generated, it goes into transmission queue and when a
clear channel is obtained, the message is transmitted.
Uplink messages are generated by Airline Operations Centers (AOC),
ATC facilities or Data Link Service Providers (DSP). The message is
processed by the ACARS or, when the message is addressed for a
peripheral unit, is passed on. When an error free message has been
received, the ACARS generates a positive transmission
acknowledgment to the ground station, otherwise, a negative
acknowledgment is transmitted and the ground system should
retransmit the uplink message. The ACARS is capable of storing 25
blocks of uplink messages. When the message log is full the ACARS
will delete the oldest read message. If the message log contains 24
unread messages, the ACARS will announce a full message log to the
flight crew.
Downlink messages are generated by an occurred event or by the
flight crew. The messages are sent to a DSP, which then addresses
the messages to a designated AOC or ATC. ACARS transmits the
message having the highest priority. The ground system also
generates a positive acknowledgment for the message and transmits a
confirmation to the airplane. Upon receiving positive acknowledgment,
the ACARS deletes the message from the downlink queue. The
ACARS is capable of queuing 70 blocks of downlink messages.
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