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Aviation Alerts and Advisories

Service Alert 1202 Rev A: GTN 650/GTN 750 Units and OBS Issue


Under certain conditions, affected GTN 650/750 units may incorrectly interpret the course selected by the pilot on some panel-mounted OBS (Omni Bearing Selector) units. This condition may cause the CDI (Course Deviation Indicator) associated with the OBS to display incorrect course deviation information for the selected course to or from a VOR.

This issue only affects VOR course deviation displayed in certain installation configurations. The following items are NOT affected: VHF Glideslope, ILS, Localizer, Localizer-Back Course or GPS Navigation.


GTN 650/750 Units with Nav Board software version 6.01.


Perform the following procedure to determine if this issue affects a particular installation:

  1. Power up all avionics. 
  2. On the GTN unit, navigate to the Instrument Panel Self-Test Page. The lateral CDI (LCDI) output from the GTN is set to “Half Left” while on this page.
  3. If the CDI indicated one-half full scale left deflection, the installation is not affected and this Service Alert does not apply. 
  4. If the CDI does NOT indicate one-half full scale deflection left (CDI needle remains parked) determine the Nav Board software version with the following steps: 

a.  Power up the GTN. 
b.  From the Home screen, touch the ‘System’ button. 
c.  From the System screen, touch the ‘System Status’ button. 
d.  For 650 Units only: Touch the ‘Version Information’ button. 
     The NAV Software Version is displayed on the ‘System – System Status’ screen.

5.  If Nav Board software version 6.01 is confirmed, do not use the CDI for VOR navigation. The CDI can continue to be used for Glideslope, ILS, Localizer, Localizer-Back Course, or GPS  Navigation.


This issue will be corrected in a future release of the GTN 6XX/7XX Nav Board software.

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