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using a Garmin GRS 77/77H AHRS or GDC 74A/74B/74H Read This

The FAA issued a Safety Alert for Operators (SAFO) in August that alerts owners, operators, repair stations, and mechanics about the temporary loss of displayed heading and attitude in aircraft equipped with the Garmin GRS 77/77H AHRS (Attitude Heading Reference System). Two possible causes identified by Garmin are moving the aircraft on the ground during the first 10 seconds after powering up the AHRS, and magnetic interference introduced near the magnetometer(s) during AHRS initialization. Either of these can cause a noticeably drifting heading to be displayed on the primary flight display (PFD) prior to takeoff. If drifting is observed, it is recommended you power down and reinitialize the AHRS and observe a 10-second limitation before moving. Contact Garmin Aviation Product Support at avionics@garmin.com or 888-606-5482 if the conditions described in the SAFO are encountered.

You can read the SAFO here: http://www.faa.gov/other_visit/aviation_industry/airline_operators/airli....

www.faa.gov/documentLibrary/media/Advisory_Circular/AC_91-79A.pdfSAFO

also see:
http://garmin.blogs.com/aviationalerts/2013/11/service-advisory-1129-rev...