E135 DEVIATES FROM DFW RNAV SID TRACK. CREW CITES AUTOFLT SYSTEM COMPUTATION OF TURN POINT AND RATE OF TURN AS THE CAUSE.

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WHILE DEPARTING DFW USING AN FMS SID; THE FMS INITIATED AN EARLY TURN THAT MAY HAVE RESULTED IN AN AIRSPACE DEV. THE PRE TKOF PROCS WENT NORMALLY. THE CLRNC INCLUDED THE FMS CLARE 2 DEP; WITH A DEP FROM RWY 17R. THE SID WAS RECALLED FROM THE FMS DATABASE AND CROSS CHKED WITH THE PAPER CHART. THE DEP PROC WAS BRIEFED AND ALL COMPANY FMS SID PROCS WERE FOLLOWED. THE TKOF AND INITIAL CLB WERE UNEVENTFUL. WHEN APCHING THE FIRST FIX (A 90 DEG TURN TO THE E FOR OUR TRANSITION); THE FMS BEGAN A TURN APPROX 8 MILES PRIOR TO THE FIX. IT PERFORMED A GENTLE TURN TO INTERCEPT THE OUTBOUND COURSE FOR THAT FIX. THAT TURN (WHILE COMMANDED BY THE FMS; ON AN FMS DEP); WAS MOST LIKELY OUTSIDE THE ONCOURSE AIRSPACE. NOTHING WAS MENTIONED BY ATC AT THE TIME OF THE DEV. I FEEL THE FMS IN THE 135/145 SERIES ACFT SHOULD BE EVALUATED FOR 90 DEG TURNS IN FMS DEP PROCS. I HAVE WITNESSED WHAT I CONSIDER TO BE SLOW TURNS TO COURSE; AND THAT IS UNACCEPTABLE IN THE TERMINAL ENVIRONMENT. SUPPLEMENTAL INFO FROM ACN 681514: WE DIDN'T THINK MUCH OF IT BECAUSE THE RNAV DEPS MUST BE FLOWN BY THE AUTOPLT; AND THAT'S HOW THE HONEYWELL ALWAYS OPERATES; SO WE THOUGHT IT WAS NORMAL BECAUSE OF PAST EXPERIENCE WITH THE HONEYWELL FMS. I THINK THE FMS NEEDS TO BE RE-SOFTWARED OR FIXED TO TURN CORRECTLY. EVERYBODY KNOWS IT TRIES TO TURN EARLY FOR ANY SIGNIFICANT TURN; AND IT CAN BARELY COMMAND A FULL STANDARD RATE TURN IN NORMAL FLT.