A B767 FLT CREW INBOUND TO ATL FLIGHT DEVIATED FROM CLEARED TRACK BECAUSE OF A MAP SHIFT.

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I WAS THE FO AND PF ON THE ABOVE REFED FLT. THE FLT WAS UNEVENTFUL UP TO THE INCIDENT IN QUESTION IN THE ATLANTA TFC PATTERN. I BRIEFED AND SELECTED THE ILS PRM RWY 27L APCH. AS WE GOT CLOSER TO THE FINAL APCH COURSE; IT WAS BECOMING CLEAR THAT IF WE DIDN'T GET A TURN TO INTERCEPT THE FINAL APCH COURSE SOON; WE WOULD OVERSHOOT THE COURSE. I MADE THE COMMENT TO THE PLTS THAT SHE BETTER TURN US QUICK OR WE'RE GOING TO OVERSHOOT. BEFORE THE CAPT KEYED THE MIKE TO QUERY THE CTLR; SHE QUICKLY TOLD US TO TURN R TO A 240 DEG HDG; DSCNT FROM 7000 FT TO 5000 FT; CLRED TO INTERCEPT RWY 27L LOC. HER INSTRUCTIONS BEGAN WHEN WE WERE JUST INSIDE 1 MI TO INTERCEPT AND SHE WAS STILL GIVING US THESE INSTRUCTIONS WHEN WE WERE APPROX 1/2-1 MI OVERSHOT OF THE FINAL. WITH OUR SPD AT 220 KIAS AND ON OUR TURN TO FINAL; IT APPEARED WE ROLLED OUT ABOUT 1 MI L OF COURSE. SHE GAVE THE INSTRUCTIONS VERY QUICKLY AND IN SUCH A WAY THAT BASED ON OUR APPARENT FMS POS; IT SEEMED CLEAR TO US THAT SHE REALIZED THAT HER DELAY IN GIVING INSTRUCTIONS HAD CAUSED US TO OVERSHOOT. AT THIS POINT; I WAS WORRIED ABOUT CONFLICTS WITH RWY 28 TFC; SO I FOLLOWED HER INSTRUCTIONS BY TURNING R AS QUICKLY AS POSSIBLE TO CORRECT BACK TOWARDS THE COURSE; BEGAN THE DSCNT TO 5000 FT; ARMED THE LOC AT THE COMPLETION OF HER INSTRUCTIONS; AND ROLLED OUT ON THE HDG SHE GAVE US. THE FMS COURSE WAS ABOUT 1 MI BACK TO OUR R. I COMMUNICATED TO THE CAPT TO TELL THE CTLR WE NEEDED TO COME FURTHER R TO INTERCEPT; BECAUSE WE WERE MOVING EVEN FURTHER L AWAY FROM THE COURSE. AS HE BEGAN TO ASK FOR THE CORRECTION; SHE INDICATED IN AN OBVIOUS VOICE OF CONCERN THAT WE WERE MOVING TOO FAR R. THIS WAS TOTALLY OPPOSITE TO OUR FMS INDICATIONS (FMS SHOWED US L OF TRACK). SHE FURTHER STATED THAT WE WERE ATTEMPTING TO LINE UP ON RWY 26R OR THAT WE MUST HAVE LOADED RWY 26R INTO OUR COMPUTER; OR SOMETHING TO THAT EFFECT. ALL OF THE PLTS QUICKLY RECONFIRMED THAT WE HAD THE CORRECT RWY 27L APCH LOADED IN THE FMS. THE CAPT STATED TO HER THAT THE CORRECT APCH WAS LOADED; WHEREBY SHE SAID WE WERE STILL MOVING FURTHER R. SHE ALSO SAID TO STOP OUR DSCNT AT 6000 FT VERSUS 5000 FT. THAT INSTRUCTION CAME WHEN WE WERE APCHING 6000 FT. I QUICKLY ATTEMPTED TO STOP OUR DSCNT BUT THE ACFT CONTINUED A DSCNT BELOW 6000 FT TO APPROX 5500 FT. I THEN BEGAN A CORRECTION UP TO THE ASSIGNED ALT. SIMULTANEOUSLY; THE CAPT TOLD THE CTLR TO 'GIVE US A VECTOR OUT OF THIS APCH; AS THERE IS AN OBVIOUS CONFLICT OF INFO' OR WORDS TO THAT EFFECT. AS WE WERE DEPARTING; ON A 290 DEG DIRECTED HDG; THE RELIEF OFFICER STATED THAT THE LOC NEEDLE INDICATED THE LOC COURSE WAS L; YET THE FMS COURSE WAS STILL INDICATING FAR R. IT BECAME CLR AT THAT POINT THAT WE HAD EXPERIENCED A MAP SHIFT ON OUR FMS. AS WE EXITED THE TFC PATTERN; WE AGREED THAT WE WOULD DISREGARD FMS DATA AND ONLY USE RAW DATA ON A SUBSEQUENT APCH DUE TO THE APPARENT FMS SYS MALFUNCTION. WE WERE RESEQUENCED INTO THE PATTERN AND TOLD TO SET UP FOR AN ILS RWY 26R APCH. WE FLEW THE ILS PRM RWY 26 APCH AND NOTED THAT THE MAP SHIFT WAS STILL IN EFFECT FOR THAT RWY AS WELL. LNDG WAS UNEVENTFUL. DURING THIS ENTIRE SEQUENCE OF EVENTS; WE NEVER RECEIVED ANY TCAS ADVISORIES OR WARNINGS. SUPPLEMENTAL INFO FROM ACN 750090: THE PROB AROSE BECAUSE OF A MALFUNCTION (MAP SHIFT) IN COMBINATION WITH A SLOW RECOGNITION OF THE MALFUNCTION AND FINALLY LESS THAN OPTIMAL ACFT CTL. WE WERE ALL THINKING SLOWLY (FATIGUE) BUT THE PF GOT VERY DISTR AND IT WAS DIFFICULT TO HELP HIM UNDERSTAND OUR PROB. IN MY JUDGEMENT THE GRAVEST MISTAKE WAS THE PF NOT FOCUSING ON FLYING THE ACFT WHILE WE WERE CONFUSED AND ATTEMPTING TO SORT OUT OUR PROB.