FLC OF AN LGT ACR ACFT DEVIATED FROM ASSIGNED SID TRACK DUE TO A MALFUNCTION IN THE ACFT FMC.

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NASA; THIS IS THE INFO I HAVE PERTAINING TO FLT HARTFORD TO PHILADELPHIA; JUN/XX/93. THE ACFT WAS XXX. THE FMC WAS A PROGRAM NUMBER -- 160 AND THE CURRENT INSTALLED REVISION WAS JUN/XX/93 TO JUL/XX/93. THE FMC WAS LOADED ACCORDING TO THE FLT PLAN. THE ROUTING WAS VIA RWY 6; WITH A COASTAL 1 DEP AND SHERL TRANSITION. THIS INITIAL ROUTING WAS SELECTED FROM THE ARR/DEP PAGE. THE AIRWAYS ROUTING CONTINUED VIA SHERL; J121; TO BRIGS WITH A CEDAR LAKE ARR INTO PHILADELPHIA. I FEEL CONFIDENT THAT THE ROUTING WAS ENTERED CORRECTLY BECAUSE IT WAS READ BACK AND ACKNOWLEDGED BY THE FO. I ALSO CHKED EACH POINT ON THE LEGS PAGE AGAINST THE COASTAL 1 DEP IN THE MANUAL. TKOF WAS MADE ON RWY 6 WITH A TURN TO HDG 075 DEGS AFTER 400 FT. WE WERE THEN GIVEN A CLRNC DIRECT TO HARTFORD VOR; THEN ON COURSE. THE ONLY PLT INPUT TO THE FMC OCCURRED AT THIS TIME. THE AIRPLANE THEN FLEW A CORRECT PATH UNTIL REACHING YODER INTXN; AT WHICH TIME IT TURNED DIRECT TO SHERL INTXN; IN ERROR. THE LEAD CTLR FOR NY APCH ASKED ME TO CHANGE TO A DISCRETE FREQ AND HE ASKED ME TO VERIFY WHAT HAD HAPPENED. I RECHKED THE COASTAL 1 DEP AND FOUND THE CORRECT ROUTING INSTALLED IN THE FMC. THE CTLR INFORMED US THAT THIS HAD BEEN THE THIRD FLT IN THE LAST 3 DAYS TO MAKE THIS ERROR. I DO NOT KNOW HOW THIS COULD HAPPEN; BUT IN SOME WAY THE CALVERTON VOR IS DROPPING OUT OF THE FMC RTE ABOUT THE TIME OF THE TURN FROM YODER TO CALVERTON. WE FLEW THE ACFT FROM PIT TO BDL TO PHL AND IT WAS NORMAL IN ALL OTHER RESPECTS. THE COMPANY HAS DONE A GOOD JOB OF TRYING TO RUN DOWN THE PROB. THE FMC MANUFACTURER IS RUNNING THE PROGRAM THROUGH A SIMULATOR AT THIS TIME. MY ONE COMPLAINT WOULD BE THAT AFTER HAVING HAD THIS PROB TWICE ON THIS TRIP; THE NEXT FLC WAS NOT NOTIFIED OF THE PROB. WE MADE SURE ON OUR ARR THAT SYS CTL; AIR TFC FACTORS DEPT AND FLT MGMNT PEOPLE WERE NOTIFIED AND THE APPROPRIATE WARNINGS FOR THE NEXT FLC WERE ISSUED.