A B727-200 IN CRUISE AT FL240 WITH AUTOPLT COUPLED TO THE GPS SYS HAD THE ACFT START A SUBTLE HDG DRIFT OFF COURSE. ALERTED BY ATC OF COURSE DEV.

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DURING ENRTE PHASE OF FLT AT FL240 IN THE VICINITY OF THE LAJUG INTXN (FO FLYING) WE EXPERIENCED A GFMS ANOMALY. THE GFMS; WHILE STILL CONNECTED TO THE AUTOPLT; STARTED A SUBTLE HDG DRIFT; WHICH DISPLACED US OFF COURSE. TO OUR EMBARRASSMENT; ATC BROUGHT THE DRIFT TO OUR ATTN. THE AUTOPLT WAS STILL IN AUX MODE; CONNECTED TO THE GFMS WHICH APPEARED TO JUST STOP GIVING COURSE GUIDANCE. THERE WAS NO DISCONTINUITIES OR DR LIGHT. IT IS THE STRANGEST GFMS ANOMALY I HAVE SEEN IN 4 YRS WITH THE GFMS ON THE B727. I SURMISE THAT SOME TYPE OF OPERATOR ERROR MUST HAVE BEEN INVOLVED ON OUR PART; BUT I DON'T KNOW WHAT. IT CERTAINLY WAS A PWRFUL LESSON IN AUTOMATION COMPLACENCY. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THE GFMS SYS IS A GLOBAL POSITIONING SYS THAT CAN BE COUPLED TO THE AUTOPLT IN THE AUX MODE. THE RPTR SAID NEITHER THE WARNING SYS ON THE AUTOPLT NOR THE GPS WARNING SYS ALERTED THE CREW TO THE HDG DRIFT. THE RPTR SAID THE SYS WORKED OK PRIOR TO THIS EVENT AND AFTER THE EVENT. THE RPTR SAID THE ITEM WAS WRITTEN UP FOR MAINT BUT THE CORRECTIVE ACTION IS UNKNOWN.