AN ACR L1011 FLC GETS OFF COURSE AND OVERSHOOTS THEIR HOLDING FIX WHILE ENTERING A HOLDING PATTERN NEAR MANCHESTER; UNITED KINGDOM. THE FLC WAS USING THEIR INS TO NAV WHEN IT HAD A KNOWN ERROR AND VOR WAS AVAILABLE.

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ON SWITCHING TO LONDON CTL; WE WERE TOLD TO EXPECT WAL; MIRSI FOR ARR INTO MANCHESTER; UNITED KINGDOM; AND GIVEN VECTORS AT ABOUT LIFFY THAT TOOK US N OF THE MIRIB STAR (WHICH WE HAD NOT BEEN CLRED FOR). MANCHESTER APCH CTL CONTINUED GIVING US VECTORS WHICH TOOK US NNE OF WAL. AT THIS POINT WE THOUGHT WE WERE BEING VECTORED TO THE ARPT FOR LNDG. A FEW MI PRIOR TO REACHING MIRSI; STILL ON VECTORS (WE HAD NOT BEEN TOLD THE VECTORS WE WERE BEING GIVEN WERE FOR MIRSI) WE WERE TOLD TO EXPECT TO HOLD AT MIRSI FOR ABOUT 1 TURN. VERY SHORTLY THEREAFTER WE WERE TOLD TO TRACK 100 DEGS TO MIRSI FOR THE HOLD. WE STARTED THE TURN TOWARD 100 DEGS AND COULD SEE THAT THAT HDG ACTUALLY APPEARED TO TAKE US AWAY TO THE E FROM MIRSI; SO WE TURNED BACK TO THE NE TO REACQUIRE THE WAL 065 DEG RADIAL. WE COULD NOT RNAV TO MIRSI AS THE INS PLATFORMS HAD DRIFTED APPROX 2 MI IN THE 6.5 HR FLT. THE INITIAL CALL TO 'TRACK 100 DEGS TO MIRSI' THEREFORE CAUSED A MOMENT OF CONFUSION IN THE COCKPIT AS TO OUR EXACT POS. WHEN WE DECIDED THAT WE KNEW WHERE WE WERE AND THAT TRACK 100 DEGS WASN'T GOING TO WORK; WE WERE 2 MI PAST MIRSI AND WERE CALLED BY APCH TELLING US THAT. THE FLT WAS MET BY AN ATC STAFF MEMBER WHO TOLD US THAT NO CONFLICT WAS CREATED AND HE WANTED TO KNOW WHAT HAD HAPPENED. SUPPLEMENTAL INFO FROM ACN 362501: AFTER OUR 7.5 HR FLT; THUS FAR THE INS HAD DRIFTED ABOUT 1-2 NM WHEN WE CHKED IT AS WE OVERFLEW DUBLIN VOR. THEREFORE; WHEN XCHKING THE INS TO THE W/P MIRSI IT APPEARED TO BE SLIGHTLY OFF. THE CAPT SELECTED RADIO NAV INSTEAD AS HE WAS PASSING THROUGH THE 65 DEG OUTBOUND RADIAL FOR WAL VOR. HE THEN SELECTED HDG AND TURNED TO INTERCEPT THE HOLDING RADIAL (65 DEGS OFF OF WAL).