MD80 CREW HAD A TRACK DEV DEPARTING PHX ON AN RNAV DEP. THE ACFT TURNED TOWARD A FIX NOT INTENDED BY THE CREW. THE ACFT IS EQUIPPED WITH AN AFTER MARKET FMS; AND HAS NO NAV DISPLAY.

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MISSED RNAV DEP. CLRNC WAS FOR CHEZZ2 RNAV DEP; WHICH NEITHER CREW MEMBER HAD FLOWN BEFORE. INITIALLY THE DEP FOR RWY 8 WAS LOADED WHICH WOULD HAVE HAD THE ACFT PROCEED TO PRIMY AND THEN CHEZZ. HOWEVER; THE ASSIGNED RWY WAS RWY 7L WHICH INCORPORATES A DEP TO ZILLA AND THEN CHEZZ. BOTH DEPS HAVE A TURN TO CLOSE (I THINK THAT'S THE NAME) ALTHOUGH NO HEADING IS DEPICTED FOR THIS TURN. THE CREW REALIZED WHILE TAXIING INTO POS THAT RWY 8 WAS STILL LOADED INTO THE GFMS. WE CHANGED THIS IN POS AND WERE SHORTLY AFTER CLRED FOR TKOF. NAV WAS SELECTED AT 400 FT AND THE AUTOPLT WAS ENGAGED AT 500 FT. THE ACFT TRACKED NAV BUT AFTER CHEZZ APPARENTLY STARTED HEADING FOR BAYTA INSTEAD OF MAKING THE R TURN FIRST TO CLOSE. ALL OF THIS OCCURRED DURING ACCELERATION AND CLEANUP AND ALSO VERY QUICKLY. I BELIEVE IT'S ONLY 2.2 MI FROM ZILLA TO CHEZZ. AS I SAID; NEITHER ONE OF US HAD FLOWN THIS DEP BEFORE SO; EVEN THOUGH WE HAD REVIEWED THE DEP AT THE GATE; THE POST TKOF ENVIRONMENT WAS TOO BUSY FOR US TO CATCH THE TRACKING ERROR. OUR INDICATION THAT THERE WAS A NAV PROB WAS A QUERY FROM ATC ASKING US IF WE HAD BEEN CLRED DIRECTLY TO BAYTA. IT TOOK A MIN OR SO OF REEXAMINING THE CHART TO FIGURE OUT WHY HE WAS ASKING AND SEE THAT WE HAD MISSED THE R TURN AFTER CHEZZ. HE SAID THAT THERE WAS NO REAL PROB (THERE WAS NO CONFLICTING DEP TFC FROM THE N RWY) AND CLRED US DIRECTLY TO BAYTA. TO MY MEMORY; THE CORRECT DEP WAS DISPLAYED ON THE NAV DISPLAY AND I'M STILL NOT SURE WHY THE CORRECT DEP PATH WASN'T FLOWN; ALTHOUGH IT SEEMS OBVIOUS THAT THERE WAS A PROGRAMMING ERROR THAT I DIDN'T CATCH. A FEW THINGS MIGHT HAVE PREVENTED ALL THIS FROM HAPPENING. FIRST OFF; ENTERING THE RWY 7L DEP OVER ZILLA FIRST WOULD HAVE HELPED. THE TPS GAVE RWY 8 AS THE FIRST RWY EXPECTED AND THAT WAS LOADED EVEN THOUGH I KNEW THAT WE USUALLY DEPART FROM RWY 7L. SECOND; IT WOULD HAVE BEEN BETTER TO CATCH THE CHANGE BEFORE BEING CLRED INTO POS. EVEN THOUGH THE DEP WAS REPROGRAMMED TO RWY 7L; WE MADE A DATA ENTRY ERROR SOMEWHERE. I SUPPOSE WE COULD HAVE TAXIED CLR OF THE RWY TO REPROGRAM AND NOT BEEN SOMEWHAT RUSHED; BUT WE THOUGHT WE HAD MADE THE CORRECT CHANGE IN THE FMS. ALSO; FOR A NON RNAV SID; HEADINGS FROM ALL OF THE FIXES ARE DEPICTED. ON THE CHEZZ2 DEP THERE IS NO ADVISORY HEADING LISTED ON THE CHART AFTER CHEZZ; ONLY A DIRECTION ARROW TO CLOSE AND THEN A CONTINUATION OF THE RTE TO BAYTA. I THINK THAT IF A HEADING HAD BEEN DEPICTED; IT WOULD HAVE BEEN AN ADDITIONAL AID IN THE PRE-TKOF BRIEF TO REMIND US TO LOOK FOR THE R TURN AFTER CHEZZ. AS I STATED EARLIER; NEITHER ONE OF US HAD FLOWN THIS DEP BEFORE (IT WAS ACTUALLY ONLY MY SECOND EVER RNAV DEP) AND THINGS OCCUR QUITE RAPIDLY IN THE IMMEDIATE AFTER TKOF FLOW. ATC INDICATED THAT THERE WAS NO PROB (I EXPLAINED THE PROBABLE PROGRAMMING ERROR WITH THE RWY CHANGE) AND HAPPILY WISHED US ON OUR WAY.