ACR WDB TRACK HDG DEV WHEN IRS UNITS MALFUNCTIONED.

Narrative

DEPARTED SAN JUAN; DEST BARBADOS. SHORTLY AFTER TKOF; WE WERE CLRED DIRECT TO ILURI INTXN; ABOUT 150 MI AWAY. SHORTLY BEFORE OR AFTER REACHING CRUISING ALT; SAN JUAN CENTER CTLR INFORMED US THAT WE APPEARED TO BE DRIFTING L OF A DIRECT COURSE TO ILURI. AT THIS TIME THE FMC/AUTOPLT WAS ENGAGED IN 'NAV' AND TRACK DEV WAS ANNUNCIATED AS 0.1 MI L OF COURSE. AN IMMEDIATE CORRECTION WAS INITIATED INTO THE WIND (TO THE R). A XWIND EXISTED FROM 90 DEGS TO THE R AT ABOUT 75 KTS. BY MANUALLY TUNING IN COY AND PPR AND FOF VORS; NAV TO ILURI AND THENCE TO DEST WAS ACCOMPLISHED AS THESE NAVAIDS BECAME AVAILABLE; WITHOUT INCIDENT. ETA AT ILURI WAS MET WITHIN 1 MIN. IT QUICKLY BECAME APPARENT THAT FMC NAV SYS WAS PRODUCING SUBSTANTIAL POS ERRORS EVEN AFTER MANUAL UPDATES OF POS WERE ACCOMPLISHED USING VOR/DME INFO. AFTER LNDG; IRU PERFORMANCE CHK INDICATED DRIFT RATES OF 99.9 FOR ALL 3 IRU UNITS; WHICH IS RADIAL POS ERROR AND FOR THIS APPROX 1 1/2 HR FLT A READING OF '3' WOULD BE MAX ALLOWED. APPROPRIATE E-6 LOGBOOK ENTRIES WERE MADE.