Narrative:

While cruising over the north atlantic, we experienced an unannunciated autoplt failure which was not immediately detected by the crew, resulting in departure from our desired flight track. The first indication the crew noticed was a change in the sun's position in the windscreen. Upon checking raw data indicators (we were off course) we attempted to return to course using the autoplt, but it would not respond to inputs. The second autoplt was selected and the aircraft returned to proper track. The malfunctioning autoplt maintained a barely perceptible right bank resulting in a turn rate that was not immediately perceptible. The flight management indicators erroneously portrayed on course/on track. We could have minimized the impact of this malfunction by x-chking the raw data more frequently and not relying solely on our flight management displays.

Google
 

Original NASA ASRS Text

Title: TRACK DEV OVER THE NORTH ATLANTIC WHEN UNANNUNCIATED AUTOPLT FAILURE WAS NOT CAUGHT IN A TIMELY MANNER.

Narrative: WHILE CRUISING OVER THE NORTH ATLANTIC, WE EXPERIENCED AN UNANNUNCIATED AUTOPLT FAILURE WHICH WAS NOT IMMEDIATELY DETECTED BY THE CREW, RESULTING IN DEP FROM OUR DESIRED FLT TRACK. THE FIRST INDICATION THE CREW NOTICED WAS A CHANGE IN THE SUN'S POS IN THE WINDSCREEN. UPON CHKING RAW DATA INDICATORS (WE WERE OFF COURSE) WE ATTEMPTED TO RETURN TO COURSE USING THE AUTOPLT, BUT IT WOULD NOT RESPOND TO INPUTS. THE SECOND AUTOPLT WAS SELECTED AND THE ACFT RETURNED TO PROPER TRACK. THE MALFUNCTIONING AUTOPLT MAINTAINED A BARELY PERCEPTIBLE R BANK RESULTING IN A TURN RATE THAT WAS NOT IMMEDIATELY PERCEPTIBLE. THE FLT MGMNT INDICATORS ERRONEOUSLY PORTRAYED ON COURSE/ON TRACK. WE COULD HAVE MINIMIZED THE IMPACT OF THIS MALFUNCTION BY X-CHKING THE RAW DATA MORE FREQUENTLY AND NOT RELYING SOLELY ON OUR FLT MGMNT DISPLAYS.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.