Narrative:

During descent on the bonneville profile STAR, PDT transition, we were notified that we were off the transition route (north of course). We then discovered an error in our navigation system (FMC's and irc's which continuously updates its position). We re-entered the STAR and approach to runway 28R. The new position showed that we were 15 NM east of 'rainr' intersection. We were cleared to cross 'rainr' at 13000 ft MSL to maintain 10000 ft MSL. We descended to 13000 ft until 'rainr' and we had another anomaly in the navigation system (including raw data) -- these interruptions are also known as 'map shifts!' we again reacquired our navigation data and it indicated we were west of 'rainr' intersection. We started our descent to 10000 ft MSL. We then discovered that we were east of 'rainr' intersection. No warning was given by ZSE or pdx approach control as to our elevation (altitude) deviation. I think that with highly automated cockpits, 2 pilots and bad WX, that 1 pilot should keep his/her navigation data in raw data, rather than in the map/FMC presentation. I called pdx approach and they indicated that no problem, i.e., deviation of altitude or close proximity to another aircraft occurred. Callback conversation with reporter revealed the following information: the reporting captain stated that the map presentation can slowly shift as the aircraft flies along and there is no good way to monitor this except by going over VOR stations and very carefully watching station passage. The IRS units will continuously update themselves, so that when the aircraft is on the ground, any error usually has corrected itself. The reporter's air carrier immediately takes the aircraft out of CAT ii or III when a navigation error is written up. Maintenance then tests the equipment on the ground before returning the aircraft to a higher status. The reporter has had 2 other map shifts. The presentation moves very slowly, and in his case, moved up to 10 mi. This particular instance, ZSE asked the reporter 'where the hell are you going?' or some such words.

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Original NASA ASRS Text

Title: A 'GLASS COCKPIT' ACR CAPT RPTS A MAP SHIFT RECOGNIZED BY ZSE.

Narrative: DURING DSCNT ON THE BONNEVILLE PROFILE STAR, PDT TRANSITION, WE WERE NOTIFIED THAT WE WERE OFF THE TRANSITION RTE (N OF COURSE). WE THEN DISCOVERED AN ERROR IN OUR NAV SYS (FMC'S AND IRC'S WHICH CONTINUOUSLY UPDATES ITS POS). WE RE-ENTERED THE STAR AND APCH TO RWY 28R. THE NEW POS SHOWED THAT WE WERE 15 NM E OF 'RAINR' INTXN. WE WERE CLRED TO CROSS 'RAINR' AT 13000 FT MSL TO MAINTAIN 10000 FT MSL. WE DSNDED TO 13000 FT UNTIL 'RAINR' AND WE HAD ANOTHER ANOMALY IN THE NAV SYS (INCLUDING RAW DATA) -- THESE INTERRUPTIONS ARE ALSO KNOWN AS 'MAP SHIFTS!' WE AGAIN REACQUIRED OUR NAV DATA AND IT INDICATED WE WERE W OF 'RAINR' INTXN. WE STARTED OUR DSCNT TO 10000 FT MSL. WE THEN DISCOVERED THAT WE WERE E OF 'RAINR' INTXN. NO WARNING WAS GIVEN BY ZSE OR PDX APCH CTL AS TO OUR ELEVATION (ALT) DEV. I THINK THAT WITH HIGHLY AUTOMATED COCKPITS, 2 PLTS AND BAD WX, THAT 1 PLT SHOULD KEEP HIS/HER NAV DATA IN RAW DATA, RATHER THAN IN THE MAP/FMC PRESENTATION. I CALLED PDX APCH AND THEY INDICATED THAT NO PROB, I.E., DEV OF ALT OR CLOSE PROX TO ANOTHER ACFT OCCURRED. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTING CAPT STATED THAT THE MAP PRESENTATION CAN SLOWLY SHIFT AS THE ACFT FLIES ALONG AND THERE IS NO GOOD WAY TO MONITOR THIS EXCEPT BY GOING OVER VOR STATIONS AND VERY CAREFULLY WATCHING STATION PASSAGE. THE IRS UNITS WILL CONTINUOUSLY UPDATE THEMSELVES, SO THAT WHEN THE ACFT IS ON THE GND, ANY ERROR USUALLY HAS CORRECTED ITSELF. THE RPTR'S ACR IMMEDIATELY TAKES THE ACFT OUT OF CAT II OR III WHEN A NAV ERROR IS WRITTEN UP. MAINT THEN TESTS THE EQUIP ON THE GND BEFORE RETURNING THE ACFT TO A HIGHER STATUS. THE RPTR HAS HAD 2 OTHER MAP SHIFTS. THE PRESENTATION MOVES VERY SLOWLY, AND IN HIS CASE, MOVED UP TO 10 MI. THIS PARTICULAR INSTANCE, ZSE ASKED THE RPTR 'WHERE THE HELL ARE YOU GOING?' OR SOME SUCH WORDS.

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.