A B777 FLC STRAYS 20 MI L OF COURSE AFTER RECEIVING A DIFFERENT TRACK ASSIGNMENT FROM SHANWICK; FO.

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AT APPROX 30 MINS E OF 55/10; I CONTACTED SHANNON CLRNC TO GET OUR OCEANIC CLRNC. (BY THIS TIME I HAD RELEASED THE IRO TO HIS SCHEDULED BREAK FROM THE COCKPIT. THIS WAS A MISTAKE. I SHOULD HAVE USED HIM TO MONITOR THE CLRNC.) THE IRO USUALLY GETS THIS CLRNC. I DID IT MYSELF TO KEEP MY HAND IN THIS ATC PROC. UPON INITIAL CONTACT WITH CLRNC; I WAS ADVISED THAT THERE WERE NO ALTS AVAILABLE ON TRACK C AND STANDBY FOR A RERTE. AFTER A LENGTHY DELAY; WE RECEIVED THE CLRNC: TRACK B VIA N56W10; FL350; MACH .84. THE FO HAD ADVISED ME THAT SHE WAS MONITORING BOTH THE ATC FREQ AS WELL AS THE CLRNC FREQ. JUST AS I READ BACK THE CLRNC; A FLT ATTENDANT CALLED AND ASKED WHY WE WERE RETURNING TO PARIS. APPARENTLY; THE CABIN 'AIR SHOW' DISPLAY INDICATED THAT I NEEDED TO CONTACT ATC; SO I PUT THE FLT ATTENDANT ON HOLD. THE FLT ATTENDANT SUBSEQUENTLY CAME INTO THE COCKPIT AND STARTED TO DISCUSS THIS ISSUE WITH THE FO. WHEN I CONTACTED ATC; I ADVISED HIM OF OUR TRACK CHANGE. ATC BEGAN GIVING US ERRONEOUS RADAR VECTORS AND FINALLY CLRED US TO 56/10. AT THIS POINT THE FO ENTERED 56/10 INTO THE FMC AND WE WERE GOING DIRECT. I ASSUMED THAT SHE HEARD THE CLRNC AND SINCE THERE WAS A 'DISCONTINUITY' ON THE SECOND LINE OF THE FMC DISPLAY; I PUT LOADING THE NEW RTE ON HOLD WHILE WE TRIED TO CALM DOWN THE FLT ATTENDANT AND THE PAX. WHILE I WAS DEALING WITH THIS; THE FO; THINKING WE WERE GOING TO 56/10 THEN REJOIN TRACK C; REACHED OVER AND CLRED THE DISCONTINUITY FROM THE FMC SCREEN. (MY QUEUE TO INITIATE THE NEW RTE WAS GONE AND SOMEHOW MY MIND COMPLETED THIS TASK.) THE DISTRS CONTINUED WITH THE FLT ATTENDANT BRINGING THE CREW MEALS. AT THIS TIME; WE WERE OVER 56/10 AND A PROGRESS RPT WAS REQUIRED TO SHANNON. IN THE HEAT OF BATTLE; I READ THE RPT DIRECTLY FROM THE FMC PROGRESS/RPT PAGE. FOR SOME REASON; MY MIND NEVER PICKED UP ON THE FACT THAT THE COORDINATES ON THE SCREEN WERE HDG TO THE WRONG PLACE. TO COMPOUND THINGS; ATC; AFTER RECEIVING MY READBACK; DID NOT PICK UP ON THE FACT THAT WE WERE ON OUR WAY TO REJOIN TRACK C RATHER THAN TO CONTINUE ON TRACK B. AT 10 MINS PAST 56/10; WE WERE STARTING TO INITIATE A STANDARD 'POST POS' CHK WHEN WE RECEIVED A SELCAL MESSAGE. THE RADIO OPERATOR ADVISED THAT ATC HAS CLRED US DIRECTLY TO N57W20; N57W30; TRACK B. MY FIRST THOUGHT WAS 'WHY ARE THEY ISSUING US THE SAME CLRNC TO US AGAIN?' -- THEN THE 'LIGHT CAME ON.' I REALIZED WHERE THE ACFT WAS GOING. WE PROCEEDED DIRECTLY TO 57/20 AND REJOINED TRACK B. I AM CONFIDENT THAT WE WOULD HAVE CAUGHT THIS ERROR WHEN WE COMPLETED OUR POS CHK (OR AM?). AT THIS POINT; I DON'T KNOW IF THE CTLR EVENTUALLY PICKED UP ON THE ERROR OR THE B777 FANS SYS GENERATED AN AUTOMATIC POS RPT AND ALERTED ATC. AT THE TIME WE MADE OUR CORRECTION BACK TO TRACK B; WE WERE 20 NM OFF COURSE. THE CHAIN OF EVENTS IN THIS SIT ARE OBVIOUS AND CLASSIC. I HAVE ASKED MYSELF OVER AND OVER -- HOW DID THIS HAPPEN TO ME!? THE ANSWER IS ALSO OBVIOUS AND CLASSIC -- I ALLOWED MYSELF TO BECOME DISTR AND I DID NOT CHALLENGE MY ASSUMPTION. I AM VERY LUCKY; I RELEARNED THIS LESSON THE EASY WAY. SUPPLEMENTAL INFO FRO ACN 509415: THE CAPT HAD NOT TOLD ME THERE HAD BEEN A CHANGE IN OUR ROUTING FROM TRACK C TO TRACK B. THEREFORE; I ENTERED N56WOLO A THE TOP OF THE FMC (FLT MGT. COMPUTER) AND ASKED THE CAPT ABOUT BRINGING UP THE REMAINDER OF THE ROUTE RO INSERT BELOW 56WOLO; OTHERWISE THE ACFT WOULD TURN S OUT OF N56WOLO TO FLY TO N55WOLO. HE SAID GO AHEAD AND IN INSERTED THE REMAINDER OF THE ROUTE (TRACK C) UNDER N56WOLO; THERE BY DELETING THE N55WOLO POINT AND EXECUTED IT. WE RECEIVED A SEL CAL. HE CLRED UN ON TRACK B; I PUT IT IN TO THE COMPUTER; PLOTTED IT ON THE CHART. THERE CONTINUED TO BE NOTHING SAID ABOUT HAVING HAD A ROUTE CHANGE EARLIER. I WAS NOT AWARE OF HAVING BEEN ON AN INCORRECT HEADING TO THE WRONG TRACK UNTIL TWO HOURS LATER WHEN I RETURNED TO FLT DECK.