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Aviation Alerts and Advisories

Service Advisory 1370 Rev A: GTN/GTR/GNC COM Frequency Monitoring

AFFECTED PRODUCTS

GTN 750, GTN 635/GTN 650, GTR 225, and GNC 255 series units with COM Software Versions 2.10 through Version 2.12.

ISSUE

In limited circumstances, a monitored COM frequency may prevent the pilot from receiving communication on the active COM frequency. If this condition occurs, it will self-correct after a 2-second lull in the reception of transmissions on the monitored channel.

Details of the Affected Circumstance: During normal operation when the GTN/GTR/GNC COM
Frequency Monitoring feature is activated, the unit listens to the Standby frequency while checking the
Active frequency for activity. After the COM Push-to-Talk is used while there is a continuous signal being
received on the Standby frequency (such as ATIS), the unit will erroneously annunciate RX by the Active
frequency when it is actually receiving transmissions on the Standby frequency. Further, the unit will stop
checking for activity on the Active frequency until the Standby frequency does not receive transmissions
for approximately 2 seconds. When the Standby frequency does not receive transmissions for
approximately 2 seconds, the unit will resume normal COM frequency monitor operation.

The COM Frequency Monitor feature is different than the monitor feature of the audio panel. This issue
does not affect the monitor feature of audio panels.

PILOT ACTION

Garmin recommends not transmitting when using the GTN/GTR/GNC COM Frequency Monitoring feature until the issue is resolved in a future COM software release.

Flip-flopping the Standby and Active frequencies will also allow the unit to resume normal COM frequency monitor operation.

RESOLUTION

This issue will be corrected in a future COM software release.

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