B350 CREW HAD A TRACK DEV IN STL CLASS B AFTER THE FMS TURNED THE WRONG WAY; AND THE CREW BECAME SPATIALLY DISORIENTED. THE CREW DID NOT HAVE RAW DATA DISPLAYED.

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OUR FLT WAS ON THE TRAKE EIGHT ARR INTO ST LOUIS LAMBERT FIELD (STL). WE WERE CLRED DIRECT FORISTELL (FTZ) AND HAD PREVIOUSLY BEEN ASSIGNED A TURN TO 130 DEGS AT SNYDR FOR THE VISUAL TO RWY 30R. THE WX WAS CLR AND THE VISIBILITY WAS UNRESTR. THE ACFT; A KING AIR 350; WAS NAVING THE ARR IN FMS MODE AND THE HEADING BUG WAS SET FOR THE TURN AT SNYDR. XING FTZ THE PF (CAPT) PROGRAMMED THE ILS RWY 30R INTO THE FMS AS THE COPLT WAS HAVING DIFFICULTIES RAISING THE FBO. AS WE CROSSED AUGST THE FMS UNEXPECTEDLY SEQUENCED TO AN UNDETERMINED LEG AND THE ACFT BEGAN A TURN TO THE L. THE SYMBOLOGY FOR THE ARR DISAPPEARED AND WE WERE MOMENTARILY UNSURE OF OUR LOCATION IN RELATION TO SNYDR. BASED ON THE LAST KNOWN POS AND OUR RELATIVELY HIGH SPD ON THE ARR (240 KTS ATC REQUESTED) I SWITCHED THE NAV MODE TO HEADING AND BEGAN THE TURN TO 130 DEGS. ATC CALLED US BEFORE WE REACHED THE HEADING AND REDIRECTED US TO AVOID POTENTIAL TFC THAT WE HAD VISUALLY. WE ALWAYS PROGRAM THE FMS FOR GUIDANCE ON APCHS. IT IS LIKELY THAT THIS APCH WAS PROGRAMMED TOO CLOSE TO THE FIELD AND TOO CLOSE TO AN ARRIVING WAYPOINT. WE DID NOT ASCERTAIN EXACTLY WHAT CAUSED THE IMPROPER SEQUENCING. THIS EVENT REINFORCED THE NECESSITY OF BACKING UP FMS NAV WITH RADIAL/DME AWARENESS WHICH WAS NOT PROPERLY PERFORMED. COMPLICATING THE SIT AND INCREASING OUR WORKLOAD WERE A HIGHER THAN NORMAL SPD ON ARR; DIFFICULTIES IN CONTACTING THE FBO; A JUMBLED ITINERARY ON THE PART OF OUR PAX AND DIVIDED DUTIES ON THE FLT DECK. APCH CTL WAS CONTACTED AFTER WE LANDED AND THE SUPVR SAID OUR MISTAKE WAS CAUGHT BY HIS CTLRS IN TIME TO KEEP US OUT OF HARM'S WAY. HE AND HIS STAFF WERE VERY PROFESSIONAL AND WE THANKED THEM FOR THEIR ASSISTANCE. SUPPLEMENTAL INFO FROM ACN 596863: WE WERE FLYING THE TRAKE 8 ARR INTO ST LOUIS INTL ARPT (STL). NEAR COMPLETION OF THE ARR ATC INSTRUCTED OUR ACFT TO TURN R TO A HEADING OF 130 DEGS AT SNYDR INTXN FOR RADAR VECTORS INTO STL. THE CLRNC WAS READ BACK AS STATED. OUR ACFT WAS NAVING ON THE FMS WITH THE AUTOPLT ON. UPON ARRIVING AT THE INTXN RIGHT BEFORE THE SNYDR INTXN THE ACFT BEGAN TO TURN L; THE OPPOSITE DIRECTION; AND THE FMS DELETED SNYDR INTXN; AND BEGAN DIRECTING THE ACFT TO THE INITIAL APCH FIX. REALIZING THAT THE ACFT WAS TURNING THE WRONG DIRECTION; THE CAPT IMMEDIATELY CORRECTED THE PROB AND BEGAN TO USE THE HEADING MODE ON THE AUTOPLT. A PROB STILL REMAINED THAT THE FMS DID NOT DISPLAY THE SNYDR INTXN. UNABLE TO EXACTLY DETERMINE THE LOCATION OF THE SNYDR INTXN THE ASSUMPTION WAS MADE THAT THE ACFT HAD PASSED SNYDR; AND WE BEGAN A R TURN TO 130 DEGS. ATC SAID THE ACFT HAD TURNED BEFORE SNYDR INTXN AND PROCEEDED TO GIVE US RADAR VECTORS INTO STL. WE WERE ALSO INSTRUCTED TO CALL APCH CTL UPON LNDG; WHICH WE DID. TO PREVENT THIS FROM HAPPENING AGAIN EVERY ACFT USING FMS OR GPS NAV SHOULD BACK IT UP WITH VOR AND DME NAV.