AN A320 CREW RPTS ERRATIC TRACK DEVS ON THE ORD BRADFORD 3 (BDF3) ARR. ATC ACKNOWLEDGED SEVERAL ACFT EXPERIENCED THE SAME ERRORS.

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DURING DSCNT ON THE BDF3 ARR; NAV DISPLAYS BEGAN INDICATING INCREASING DIFFERENCES IN OFF-TRACK ERROR AND HDG BEGAN SLOWLY DEVIATING BACK AND FORTH ACROSS THE TRACK HDG WITH INCREASING DEVS. IRS POS WERE SEEN TO BE VARYING BTWN 0.5 AND 5.0 NM DIFFERENCE FROM RADIO POS. AS DEVS BECAME LARGER; REVERTED TO RAW DATA NAV. MAX DEV ESTIMATED AT 5-6 NM BASED ON RAW DATA FROM ORD VORTAC. FO RPTED HE HAD SEEN THIS SAME BEHAVIOR IN THE SAME PLACE ON THE BDF ARR LAST WK. ATC RPTED TO US THAT THEY HAVE SEEN NUMEROUS ACFT DISPLAY SIMILAR BEHAVIOR IN THAT AREA RECENTLY AND REQUESTED THAT WE RPT IT TO THE COMPANY FOR INVESTIGATION. DURING APCH TO RWY 4R MONITORED CAPT'S NAV DISPLAY AND POS MONITOR ON FMGC. NO DISCREPANCIES NOTED. ENTIRE APCH WAS FLOWN BY FO IN RAW DATA UNDER VFR FLT CONDITIONS. SUPPLEMENTAL INFO FROM ACN 663657: AUTOPLT WAS ON AND THE ACFT TURNED TO 090 DEGS TO GET BACK ON COURSE. ATC THEN ADVISED US THAT WE WERE HEADING E AND GAVE US A HDG OF 010 DEGS AND DIRECT ORD. WE USED ONLY RADIO NAV THE REST OF THE WAY. ATC ADVISED THAT THEY'D SEEN NUMEROUS ACFT HAVE THE SAME PROB IN THE SAME SPOT AND ASKED US TO ADVISE THE COMPANY. HE DID NOT SPECIFY IF IT WAS JUST ACR ACFT OR JUST AIRBUSES HAVING THIS PROB. THE FMGC DID HAVE THE FMGC 1/2 DISAGREEMENT (I THINK THAT WAS THE MESSAGE; BUT AS THE PF I WAS BUSY CORRECTING THE PROB). I PERSONALLY HAD THE SAME PROB ON THE SAME FLT EARLIER IN THE MONTH -- FLT XXX ON JUN/TUE/05. IN THAT CASE WE ATTRIBUTED THE PROB TO THE AIRPLANE AND WROTE IT UP AT ORD. I DIDN'T FILE A REPORT AT THAT TIME. IN THAT CASE; WE ATTEMPTED TO USE THE GPS TO GO TO ORD AFTER BENKY AND WE HAD ANOTHER GROSS NAV ERROR (5 NM ERROR 30 NM FROM ORD) THAT APCH ADVISED US OF. AFTER THAT WE USED ONLY RADIO NAV. IN BOTH CASES; I WAS THE PF AND IT APPEARED THAT MY NAV SCREEN/ADIRU WAS THE ONE WITH THE GROSS ERROR. IN BOTH CASES TENDENCY IS FOR THE ACFT TO DRIFT OFF COURSE RATHER QUICKLY WITHIN ABOUT 30 NM OF A FIX YOU'RE PROCEEDING DIRECT TO (SEVERAL MI IN ABOUT 1 MIN OR SO). IT THEN TAKES DRASTIC ACTION TO RETURN TO COURSE. PERHAPS ONLY RADIO NAV SHOULD BE REQUIRED ON THE BDF 3 UNTIL THIS PROB IS RESOLVED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR STATED THAT THEY EXPERIENCED +/-30 DEG HDG CHANGES WITH THE AUTOPLT ENGAGED. BECAUSE THIS ACFT DID NOT HAVE GPS INSTALLED; THE NAV SYS WAS UTILIZING DME/DME FOR POS UPDATING. THE L AND R FMS'S WERE SWAPPING CTL DEPENDING ON WHICH SYS DISPLAYED THE LEAST TRACK ERROR. HE RECALLS THAT THE CLOSER THE ACFT WAS TO BDF THE GREATER THE ERROR BUT THAT ERRORS EXISTED OUT TO 30 DME NE. THIS PLT IS ORD BASED BUT HAD NOT FLOWN THIS ARR RECENTLY. HE STATED THAT HIS FO HAD FLOWN THIS ARR AND HAD EXPERIENCED SIMILAR ACTIVITY. FOLLOWING THE RPTR'S POSTFLT CALL TO HIS FLT MGR; THE MGR RETURNED HIS CALL APPROX 6 HRS LATER AND RPTED THAT THE FAA WAS EXAMINING ERRORS ORIGINATING WITHIN THE BDF NAV FACILITY.