AN ACR MLG HAD AN ACFT EQUIP PROB RNAV FAILURE. THIS DROVE THE ACFT OFF COURSE UNTIL THE FLC WAS ABLE TO CORRECT BACK TO THE STAR.

Narrative

WHILE DSNDING FROM FL180 TO 15000 FT AND IN THE PROCESS OF GIVING AN APCH BRIEFING INTO STL; I LOOKED UP FROM MY APCH PLATE AND NOTICED THE AUTOPLT HAD ROLLED THE ACFT INTO A L TURN FROM ITS PRESENT ON-COURSE TRACK. THIS TURN WAS UNCOMMANDED AND NOT AT ALL PROGRAMMED. WE WERE NOW AT LEAST 20 DEGS OFF THE INTENDED HDG. A QUICK GLANCE AT THE ATTITUDE INDICATOR AND OUTSIDE CONFIRMED THE L TURN. HOWEVER; THE EHSI COMPASS HDG WAS TURNING INCORRECTLY IN THE OPPOSITE DIRECTION. IN THE NEXT 2 SECONDS; THE EHSI COMPASS ROSE BEGAN TO SPIN LIKE A ROULETTE WHEEL. THE ACFT IS NOW ABOUT 40 DEGS OFF THE INTENDED HDG AS CONFIRMED BY THE STANDBY COMPASS. THE AUTOPLT HDG MODE WAS SELECTED; BUT THE ACFT DIDN'T RESPOND. THE AUTOPLT WAS NOW IMMEDIATELY DISCONNECTED AND THE AIRPLANE TURNED BACK TO THE R TO RE-ESTABLISH COURSE. I DECLARED AN RNAV FAILURE AND A COMPASS FAILURE WITH STL APCH AND THEY VECTORED US BACK ON COURSE WHICH WAS CONFIRMED WITH CONVENTIONAL NAVAIDS. UPON ARR AT STL;; THE SYS FAILURE WAS ENTERED INTO THE ACFT LOGBOOK.