A CL65 CREW HAS SOME PROBS WITH THEIR FMS MAKING A DSNDING TURN OVER SPA; SC. THE PIC HAD TO REVERT TO HEADING MODE TO TURN BACK TO THE DESIRED RADIAL. ATC STEPS IN AND GIVES AN ALT RESTR AND A TURN.

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WE WERE APCHING SPARTANBURG VOR FROM THE W AT FL310. GND SPD WAS +500 KTS AND WIND FROM NW AT +100 KTS. NAV WAS FMS IN NAV MODE. WE WERE GIVEN A XING RESTR ALSO THAT REQUIRED A RATE OF DSCNT OF +3000 FPM. TURN AND DSCNT WERE STARTED ABOUT THE SAME TIME. OUTBOUND COURSE WAS 035 DEGS (L TURN) WITH NW WINDS AT +100 KTS. SO I EXPECTED A LARGE HEADING CHANGE TO STAY ON TRACK. I WATCHED THE FMS GO THROUGH OUTBOUND COURSE AND TURN TO WHAT I THOUGHT WAS TOO MUCH CORRECTION. THE AIRPLANE WAS IN A 30 DEG BANK TO THE L. I WENT TO HEADING MODE AND SET HEADING TO REINTERCEPT OUTBOUND COURSE (ABOUT 85 DEGS). BY THIS TIME ATC SAID WE HAD A TFC CONFLICT AND MAINTAIN FL280. WE WERE ABOUT 27700 FT. I CANCELED THE AUTOPLT AND FLEW THE AIRPLANE TO FL280 AND BACK ON COURSE. I EXPECTED A LARGE CHANGE IN HEADING TO ACCOMMODATE WINDS AND COURSE CHANGE; AND HAD NO INDICATION OF A MALFUNCTION. THERE ALSO WAS VERY LITTLE TIME TO CHK AND VERIFY FMS INDICATIONS. SUPPLEMENTAL INFO FROM ACN 396189: ATC CALLED AND STOPPED OUR DSCNT AT FL280 AND GAVE US A 085 DEG HDG TO RETURN TO COURSE. WE WERE THEN CLRED TO TRAKS; ON THE FMS NAV MODE; WE WENT TO TRAKS BUT THE FMS DIDN'T MAKE THE TURN AT TRAKS. WE WENT TO HEADING MODE AND MADE THE CORRECTION. THE FMS WORKED PROPERLY THE REST OF THE DAY.