FLC OF B737-300 ERRONEOUSLY DEVIATES FROM PUBLISHED TRACK ON ARR TO PHX.

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I WAS THE PNF ON THE LAST LEG OF A 2-DAY PAIRING. THE CAPT WAS FLYING. WE WERE CLRED TO CROSS ARLIN AT OR BELOW 13000 FT TO DSND TO 11000 FT AND CONTINUE ON THE ARLIN ONE ARR TO PHX. SO WE'VE FLOWN THIS ARR HUNDREDS OF TIMES. WE WERE IN A B737-300 AND I HAD VERIFIED OUR RTE IN THE FMC BEFORE WE HAD LEFT SAN. AS WE CROSSED ARLIN I WRONGLY ASSUMED THAT THE 256 DEG RADIAL OFF OF IWA VOR WAS PROGRAMMED IN THE FMC TO CONTINUE THE ARR. AT ABOUT YC56 LCL TIME THE PHX APCH CTLR ASKED US WHAT OUR HEADING WAS. I LOOKED AT THE DIRECTIONAL GYROSCOPE AND READ BACK 'HDG 060 DEGS.' I THEN REALIZED THAT THIS WAS NOT THE HEADING WE WERE SUPPOSED TO BE ON. THE PHX APCH CTLR THEN ASKED US WHY WE WERE ON A 060 DEG HDG. THE CAPT TOLD HIM THE ARR WAS NOT PLUGGED INTO THE BOX. THE CTLR THEN GAVE US A NEW HEADING AND WE WERE ON RADAR VECTORS UNTIL BEING CLRED FOR A VISUAL APCH. I WAS WRONG FOR NOT VERIFYING OUR POS AT ALL TIMES. ALSO; I SHOULD HAVE HAD MY INSTS ON RAW DATA WITH THE 256 DEG RADIAL OFF IWA VOR TUNED IN INSTEAD OF 'NAV' OR RNAV DATA. WITH GREATER VIGILANCE I COULD HAVE AVOIDED THIS DEV.