ZFW CTLR RPTS MD80 ON TRISS RNAV DEP FROM DFW FAILED TO FOLLOW TXK TRANSITION.

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THE MD80 WAS ASSIGNED TO THE TRISS RNAV DEP FROM DFW. AFTER TRISS INTXN; THE MD80 TURNED DIRECT TXK INSTEAD OF FOLLOWING THE PROPER DEP PROC. THIS CAUSED HIM TO DRIFT INTO THE B737 THAT WAS ON THE DALL8 DEP LIT TRANSITION. THE PLT QUESTIONED HIS FMS AND DID NOT THINK HE WAS SUPPOSED TO BE GOING DIRECT TXK ONCE I INFORMED HIM AND ASKED WHERE HE WAS GOING. THE SUPVR FILED THIS AS A PLT DEV AS PER FACILITY POLICY. I BELIEVE THAT THE FMS WAS EITHER PROGRAMMED INCORRECTLY; OR MALFUNCTIONED. THIS SIT OCCURS TOO FREQUENTLY ON THE RNAV DEPS TO CONTINUE. IF IT IS A PROGRAMMING ERROR ON THE PART OF THE PLTS; THEY NEED TO BE TRAINED PROPERLY; QUICKLY. IF IT IS A BUG IN THE BOXES; THE RNAV PROCS SHOULD BE SUSPENDED UNTIL IT IS CORRECTED. THESE TWO ACFT WERE SIDE BY SIDE AT THE SAME ALT WITH LESS THAN 5.5 NM LATERAL SEPARATION BECAUSE THE PLT AND THE FMS ON THE MD80 DID NOT FLY THE PROPER RTE. I AM AT LEAST AS CONCERNED BY THE FACT THAT THE PLT DID NOT KNOW EXACTLY WHERE THE FMS WAS FLYING HIS PLANE. THIS IS EXCEPTIONALLY DANGEROUS. CONFUSION IN THE COCKPIT IS BAD. WHEN IT LEADS TO ACFT IN SUCH CLOSE PROXIMITY; IT CAN BECOME TRAGIC. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: RPTR EMPHASIZED THAT TRACK DEVS ON THE RNAV DEPS ARE BOTH COMMON AND POTENTIALLY HAZARDOUS. THE USE OF MULTIPLE SIDS REQUIRES PRECISE HORIZONTAL AND VERT NAVIGATION AND DEVS OF ONLY A COUPLE OF MILES CAN RESULT IN POTENTIAL LOSS OF SEPARATION. THE FREQUENCY OF DEVS IS SUCH THAT THE TRACON HAS DEVELOPED A SCRIPTED STATEMENT TO BE READ TO FLT CREWS THAT DEVIATE. HE FEELS THE CREWS ARE EITHER NOT PROGRAMMING THE DEPS CORRECTLY OR THAT THE RNAV DATABASES ARE FLAWED.