Narrative:

Prior to our departure from ord, our aircraft had arrived with some INS problems. 2 out of 3 units had been deferred maintenance items. The 2 units were replaced by maintenance at ord. All INS units were checked and operable for the north atlantic flight. Our flight was uneventful except for a navigation error discovered after crossing 59N/302 for 56N/20W. The aircraft appeared to track correctly. The waypoints had been carefully checked per our company procedures. The first officer confirmed that a position check after leaving 30W was plotted correctly. I felt that our course was more south than required. To my shock, I was correct. We were indeed tracking more south than required. We were south of our intended track. A check of the waypoint 'to' indicated an incorrect 'north' longitude position, but the 'west' latitude of 20W was correct. Somehow it had changed after it was verified. The only probable cause was the first officer had inputted additional waypoints after 30W, and somehow made a mistake. Additional checking also verified that the 'check plot' after 30W was incorrect. Somehow all the safeguards had been overlooked. We made a correcting course towards north to be back on course by 20W. My first officer and I discussed the event in detail after the trip to prevent a recurrence in the future. In the future, know your rvsm, mnps procedures. Doublechk, doublechk, always! Do not let your guard down. We were lucky this time.

Related reports:

Google
 

Original NASA ASRS Text

Title: A B747-200 FLC EXPERIENCES A COURSE DEV FROM AN IMPROPERLY MONITORED INS AT MID-ATLANTIC ON FREQ WITH EGGX, FO.

Narrative: PRIOR TO OUR DEP FROM ORD, OUR ACFT HAD ARRIVED WITH SOME INS PROBS. 2 OUT OF 3 UNITS HAD BEEN DEFERRED MAINT ITEMS. THE 2 UNITS WERE REPLACED BY MAINT AT ORD. ALL INS UNITS WERE CHKED AND OPERABLE FOR THE NORTH ATLANTIC FLT. OUR FLT WAS UNEVENTFUL EXCEPT FOR A NAV ERROR DISCOVERED AFTER XING 59N/302 FOR 56N/20W. THE ACFT APPEARED TO TRACK CORRECTLY. THE WAYPOINTS HAD BEEN CAREFULLY CHKED PER OUR COMPANY PROCS. THE FO CONFIRMED THAT A POS CHK AFTER LEAVING 30W WAS PLOTTED CORRECTLY. I FELT THAT OUR COURSE WAS MORE S THAN REQUIRED. TO MY SHOCK, I WAS CORRECT. WE WERE INDEED TRACKING MORE S THAN REQUIRED. WE WERE S OF OUR INTENDED TRACK. A CHK OF THE WAYPOINT 'TO' INDICATED AN INCORRECT 'N' LONGITUDE POS, BUT THE 'W' LATITUDE OF 20W WAS CORRECT. SOMEHOW IT HAD CHANGED AFTER IT WAS VERIFIED. THE ONLY PROBABLE CAUSE WAS THE FO HAD INPUTTED ADDITIONAL WAYPOINTS AFTER 30W, AND SOMEHOW MADE A MISTAKE. ADDITIONAL CHKING ALSO VERIFIED THAT THE 'CHK PLOT' AFTER 30W WAS INCORRECT. SOMEHOW ALL THE SAFEGUARDS HAD BEEN OVERLOOKED. WE MADE A CORRECTING COURSE TOWARDS N TO BE BACK ON COURSE BY 20W. MY FO AND I DISCUSSED THE EVENT IN DETAIL AFTER THE TRIP TO PREVENT A RECURRENCE IN THE FUTURE. IN THE FUTURE, KNOW YOUR RVSM, MNPS PROCS. DOUBLECHK, DOUBLECHK, ALWAYS! DO NOT LET YOUR GUARD DOWN. WE WERE LUCKY THIS TIME.

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.