Narrative:

Message #XXXX regarding possible map shift in the vicinity of spi instructed the crews to inhibit spi in the FMS. We complied with the operating manual instructions for inhibiting a VOR. Nevertheless; after passing bdf on the arrival into ord; the controller called and asked us if we showed on course. We replied that we showed on track. He replied that we were about 5 mi south of course. We checked raw data and saw that we were off course. He reclred us to go direct ord and said that 'a lot of the B757's and airbuses are having the same problem.' the operating manual procedure for inhibiting a VOR also has a note which states that this procedure only inhibits the VOR but does not inhibit the DME. Since the FMS updates with both radial and DME; I believe that the procedure recommended in the message is inadequate to prevent the FMS from using the DME for updating. Therefore; the map shift can still occur. On the next leg to stl we put the VOR receivers in manual and selected rbs. This prevented a map shift as we passed spi. This may be the only way to prevent the map shift problem on the B757. Callback conversation with reporter revealed the following information: the reporter stated that this was in fact a legacy B757-200 on an air carrier 'canned' flight plan. The next day; upon picking up an inbound aircraft; the arriving crew was writing up this navigation discrepancy believing that the deviation was an aircraft mechanical problem rather than an FMC navigation database problem.

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Original NASA ASRS Text

Title: A B757-200 CREW RPTS ERRATIC TRACK DEVS ON THE ORD BRADFORD 3 (BDF3) ARR. ATC STATED THAT SEVERAL ACFT HAVE EXPERIENCED THIS DEV.

Narrative: MESSAGE #XXXX REGARDING POSSIBLE MAP SHIFT IN THE VICINITY OF SPI INSTRUCTED THE CREWS TO INHIBIT SPI IN THE FMS. WE COMPLIED WITH THE OPERATING MANUAL INSTRUCTIONS FOR INHIBITING A VOR. NEVERTHELESS; AFTER PASSING BDF ON THE ARR INTO ORD; THE CTLR CALLED AND ASKED US IF WE SHOWED ON COURSE. WE REPLIED THAT WE SHOWED ON TRACK. HE REPLIED THAT WE WERE ABOUT 5 MI S OF COURSE. WE CHKED RAW DATA AND SAW THAT WE WERE OFF COURSE. HE RECLRED US TO GO DIRECT ORD AND SAID THAT 'A LOT OF THE B757'S AND AIRBUSES ARE HAVING THE SAME PROB.' THE OPERATING MANUAL PROC FOR INHIBITING A VOR ALSO HAS A NOTE WHICH STATES THAT THIS PROC ONLY INHIBITS THE VOR BUT DOES NOT INHIBIT THE DME. SINCE THE FMS UPDATES WITH BOTH RADIAL AND DME; I BELIEVE THAT THE PROC RECOMMENDED IN THE MESSAGE IS INADEQUATE TO PREVENT THE FMS FROM USING THE DME FOR UPDATING. THEREFORE; THE MAP SHIFT CAN STILL OCCUR. ON THE NEXT LEG TO STL WE PUT THE VOR RECEIVERS IN MANUAL AND SELECTED RBS. THIS PREVENTED A MAP SHIFT AS WE PASSED SPI. THIS MAY BE THE ONLY WAY TO PREVENT THE MAP SHIFT PROB ON THE B757. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR STATED THAT THIS WAS IN FACT A LEGACY B757-200 ON AN ACR 'CANNED' FLT PLAN. THE NEXT DAY; UPON PICKING UP AN INBOUND ACFT; THE ARRIVING CREW WAS WRITING UP THIS NAV DISCREPANCY BELIEVING THAT THE DEV WAS AN ACFT MECHANICAL PROB RATHER THAN AN FMC NAV DATABASE PROB.

Data retrieved from NASA's ASRS site as of January 2009 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.