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Inside the FMS

ATC Log Message Retention Logic on G450/G550

The ATC Log found in the Gulfstream G450 and G550 Planeview FANS system retains messages that are sent to – and received from – ATC. In response to recent questions from pilots on how the log works, Honeywell Engineering would like to share some information about ATC Log Management Logic.

Minimum ATC Log Message Retention

The ATC Log retains the following messages at a minimum:

  1. Five non-pending uplink messages, with only one occurrence of a message containing the “ROUTECLEARANCE” variable and the corresponding downlink response message.
  2. Five pending uplink messages, with only one occurrence of a message containing the “ROUTECLEARANCE” variable. A pending uplink is defined as an uplink in the NEW or OPEN state.
  3. Five pending downlink messages, with only one occurrence of a message containing the “ROUTECLEARANCE” and/or “POSITIONREPORT” variable. A pending downlink is defined as a downlink in the OPEN state.

(If messages are required to be removed due to memory limits, they are removed as described below in “Deletions From the ATC Log.”)

Additions to the ATC Log

Messages are added to the ATC Log sequentially. If the ATC Log is full, the existing log entries are scanned as defined below under “Deletions From the ATC Log.” When a sufficient number of messages have been deleted, the new entry is added to the ATC Log.

Figure 1: ATC Message Log Page

Deletions From the ATC Log

When there isn’t enough storage space available in the ATC Log to handle a new uplink message, the oldest logged uplink message not required to be retained (as described above) is deleted. If there still isn’t enough storage space after a deletion, additional uplink messages (that do not violate minimum ATC log message retention rules) are deleted until the necessary amount of storage space is achieved or all valid removals have been processed, whichever comes first. If the ATC center connection that sent the pending uplink has been terminated, the pending messages are not required to be retained and can be deleted using this deletion logic.

If there is insufficient storage for a new pending uplink message after all valid deletions have been processed, the RESPOND TO ATC UPLINKS scratchpad message is displayed. When there’s insufficient storage in the ATC Log to handle a new downlink message, then the oldest logged downlink message not required to be retained (as described above) is deleted.

If the log message to be deleted due to a manual or automatic action is currently displayed on any of the following pages, then the page displayed transitions to ATC LOG page 1:

  • XXXXZ ATC UPLINK
  • VERIFY RESPONSE
  • REJECT DUE TO
  • XXXXZ ATC REPORT
  • XXXXZ ATC REQUEST
  • XXXXZ ATC EMERGENCY

Please contact Honeywell Flight Technical Services with any questions or operational issues.


Candace Gunning is a Principal Systems Engineer with Honeywell Aerospace Electronic Systems Engineering & Applications. Please direct questions and comments about her article to FTS@Honeywell.com.