Narrative:

Upon arriving at the aircraft, the captain was informed by the mechanic that the inbound crew had verbally told him to tell the outbound crew the #3 INS was starting to degrade. This information was not documented in the aircraft maintenance log by the inbound crew. Prior to entering the north atlantic track system, the crew performed an INS accuracy check using strumble VOR. All INS's were checked with tolerances. INS's 1 and 2 checked to within 1 NM using the #2 DME, given the #1 DME was inoperative. INS's #1 and #2 bearings checked to the degree. Due to suspect #3 INS, as verbally told by the mechanic, the captain, using his discretion, elected not to triple mix the INS's. As the first officer and the PF, the #2 INS was used to navigation the airplane. At approximately 2 degrees longitude passed each waypoint our position was plotted on our plotting chart. At no time were we shown to be off course. After making our position report at 50 degrees north 50 degrees west and being handed off to gander ATC, we were told at 50 degrees north 50 degrees west we were 20 degrees north of course. (60 NM.) by this time we were within VOR/DME range of gander VOR and we were north of course. Within 10 mins of our notification by gander ATC, the #2 INS failed. At this point navigation was transferred to the #1 INS. We were cleared direct to bangor and landed. At bangor airport, the #2 INS was written up as being inoperative. After reviewing previous INS accuracy logs, on (previous) arrival to ramstein airbase, our departure point, the #2 INS accuracy log was not completed. It should be noted the captain 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.

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Original NASA ASRS Text

Title: 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.

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.