Narrative:

Our aircraft was crossing over the onl VOR at FL350. Autoplt was coupled and operating in navigation mode correctly tracking the inbound right onl 076 degree of J-94. Autoplt interlocks normally decouple the radio beam signal as the aircraft crosses over the VOR station and the aircraft continues to fly at the drift corrected heading. After passing the station, the system automatically recouples to the beam as it approachs capture range and tracks the outbnd right. However, in this instance the autoplt apparently began to chase the CDI needle swing and began a left turn that went unnoticed by the crew. Both the captain and first officer were referencing hi-altitude charts verifying the outbnd right of J-94 as onl 252 degree, as well as comparing time and fuel over chkpoint against flight plan. This had created some confusion as both time and fuel were well ahead of flight plan, apparently due to winds different than forecast. By the time change north heading had been noticed, the aircraft had turned approximately 60 degree left of course. The drift south was compounded by an unusually strong jetstream from the north (330 degree at 161 KTS reported by another aircraft) causing the aircraft to drift undetermined distance south of course. ATC inquired as to our heading. The captain responded we were turning nwbnd to reintercept the outbnd course. ATC then advised us to descend to FL330 for traffic. No other comments followed until several mins later of a traffic advisory crossing over us northbound a FL350. By this time we had been well established on the arwy (J-94) and therefore believed that no traffic conflict had indeed existed as inferred by ATC, and that ATC would have probably requested the descent to FL330 regardless of our acfts change in heading and subsequent drift. More precise monitoring of the autoplt, especially when course changes or corrections are selected would have prevented this situation from developing.

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Original NASA ASRS Text

Title: ACR LGT TRACK HEADING DEVIATION FLYING ARWY.

Narrative: OUR ACFT WAS XING OVER THE ONL VOR AT FL350. AUTOPLT WAS COUPLED AND OPERATING IN NAV MODE CORRECTLY TRACKING THE INBND R ONL 076 DEG OF J-94. AUTOPLT INTERLOCKS NORMALLY DECOUPLE THE RADIO BEAM SIGNAL AS THE ACFT CROSSES OVER THE VOR STATION AND THE ACFT CONTINUES TO FLY AT THE DRIFT CORRECTED HDG. AFTER PASSING THE STATION, THE SYS AUTOMATICALLY RECOUPLES TO THE BEAM AS IT APCHS CAPTURE RANGE AND TRACKS THE OUTBND R. HOWEVER, IN THIS INSTANCE THE AUTOPLT APPARENTLY BEGAN TO CHASE THE CDI NEEDLE SWING AND BEGAN A L TURN THAT WENT UNNOTICED BY THE CREW. BOTH THE CAPT AND F/O WERE REFERENCING HI-ALT CHARTS VERIFYING THE OUTBND R OF J-94 AS ONL 252 DEG, AS WELL AS COMPARING TIME AND FUEL OVER CHKPOINT AGAINST FLT PLAN. THIS HAD CREATED SOME CONFUSION AS BOTH TIME AND FUEL WERE WELL AHEAD OF FLT PLAN, APPARENTLY DUE TO WINDS DIFFERENT THAN FORECAST. BY THE TIME CHANGE N HDG HAD BEEN NOTICED, THE ACFT HAD TURNED APPROX 60 DEG L OF COURSE. THE DRIFT S WAS COMPOUNDED BY AN UNUSUALLY STRONG JETSTREAM FROM THE N (330 DEG AT 161 KTS RPTED BY ANOTHER ACFT) CAUSING THE ACFT TO DRIFT UNDETERMINED DISTANCE S OF COURSE. ATC INQUIRED AS TO OUR HDG. THE CAPT RESPONDED WE WERE TURNING NWBND TO REINTERCEPT THE OUTBND COURSE. ATC THEN ADVISED US TO DSND TO FL330 FOR TFC. NO OTHER COMMENTS FOLLOWED UNTIL SEVERAL MINS LATER OF A TFC ADVISORY XING OVER US NBND A FL350. BY THIS TIME WE HAD BEEN WELL ESTABLISHED ON THE ARWY (J-94) AND THEREFORE BELIEVED THAT NO TFC CONFLICT HAD INDEED EXISTED AS INFERRED BY ATC, AND THAT ATC WOULD HAVE PROBABLY REQUESTED THE DSNT TO FL330 REGARDLESS OF OUR ACFTS CHANGE IN HDG AND SUBSEQUENT DRIFT. MORE PRECISE MONITORING OF THE AUTOPLT, ESPECIALLY WHEN COURSE CHANGES OR CORRECTIONS ARE SELECTED WOULD HAVE PREVENTED THIS SITUATION FROM DEVELOPING.

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.