HDG TRACK POS DEV DURING AN OVERWATER OP.

Narrative

UPON ARRIVING AT THE ACFT; THE CAPT WAS INFORMED BY THE MECH THAT THE INBOUND CREW HAD VERBALLY TOLD HIM TO TELL THE OUTBOUND CREW THE #3 INS WAS STARTING TO DEGRADE. THIS INFO WAS NOT DOCUMENTED IN THE ACFT MAINT LOG BY THE INBOUND CREW. PRIOR TO ENTERING THE N ATLANTIC TRACK SYS; THE CREW PERFORMED AN INS ACCURACY CHK USING STRUMBLE VOR. ALL INS'S WERE CHKED WITH TOLERANCES. INS'S 1 AND 2 CHKED TO WITHIN 1 NM USING THE #2 DME; GIVEN THE #1 DME WAS INOP. INS'S #1 AND #2 BEARINGS CHKED TO THE DEG. DUE TO SUSPECT #3 INS; AS VERBALLY TOLD BY THE MECH; THE CAPT; USING HIS DISCRETION; ELECTED NOT TO TRIPLE MIX THE INS'S. AS THE FO AND THE PF; THE #2 INS WAS USED TO NAV THE AIRPLANE. AT APPROX 2 DEGS LONGITUDE PASSED EACH WAYPOINT OUR POS WAS PLOTTED ON OUR PLOTTING CHART. AT NO TIME WERE WE SHOWN TO BE OFF COURSE. AFTER MAKING OUR POS RPT AT 50 DEGS N 50 DEGS W AND BEING HANDED OFF TO GANDER ATC; WE WERE TOLD AT 50 DEGS N 50 DEGS W WE WERE 20 DEGS N OF COURSE. (60 NM.) BY THIS TIME WE WERE WITHIN VOR/DME RANGE OF GANDER VOR AND WE WERE N OF COURSE. WITHIN 10 MINS OF OUR NOTIFICATION BY GANDER ATC; THE #2 INS FAILED. AT THIS POINT NAV WAS TRANSFERRED TO THE #1 INS. WE WERE CLRED DIRECT TO BANGOR AND LANDED. AT BANGOR ARPT; THE #2 INS WAS WRITTEN UP AS BEING INOP. AFTER REVIEWING PREVIOUS INS ACCURACY LOGS; ON (PREVIOUS) ARR TO RAMSTEIN AIRBASE; OUR DEP POINT; THE #2 INS ACCURACY LOG WAS NOT COMPLETED. IT SHOULD BE NOTED THE CAPT WAS TOLD THE #3 WAS SUSPECT; IN REALITY IT WAS THE #2 INS. IN THE FUTURE; EVERY CREW SHOULD PUT ANY DISCREPANCY IN WRITING AND NOT JUST PASS ON VERBALLY.