PLT OF PA28 REQUESTS PRACTICE STRAIGHT IN NDB APCH TO LXL UTILIZING GPS NAV TO DETERMINE POSITION VICE FLYING THE PRESCRIBED PROC TURN. DSNDS PREMATURELY DUE TO DETERMINING DISTANCE FROM ARPT BASED ON FMS WAYPOINT SHORT OF THE APCH WHICH IS NOT SHOWN ON THE APCH PLATE.

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I WAS GOING TO FLY THE NDB 31 APCH AT LXL FOR PRACTICE. THE CTR CTLR CLRED ME FOR THE APCH. I WAS ALMOST EXACTLY ALIGNED WITH THE FINAL APCH COURSE AT 20 MILES OUT FROM LXL. I COULD USE THE GPS TO MEASURE MY DISTANCE FROM THE NDB; SO I ASKED IF I COULD DO A STRAIGHT-IN. THE CTLR APPROVED. I LOADED THE APCH IN THE GARMIN 430 AND SELECTED 'VECTORS.' 10 MILES FROM THE NDB; I BEGAN A DSCNT. THE PROC TURN HAS TO BE DONE WITHIN 10 MILES OF THE NDB; AND CONDITIONS WERE DAY VMC; SO I FELT COMFORTABLE DOING A STRAIGHT-IN. THIS APCH HAS NO FINAL APCH FIX. I WAS INSTRUCTED TO MAINTAIN MY ASSIGNED ALT UNTIL ESTABLISHED. AFTER BEGINNING MY DSCNT; I REALIZED THAT I WAS NOT WITHIN 10 MILES OF THE NDB. THE APCH IN THE GPS HAS AN EXTRA WAYPOINT BEFORE THE NDB; AND I WAS MISTAKENLY MEASURING MY DISTANCE FROM THAT WAYPOINT INSTEAD OF THE NDB. THIS COULD HAVE BEEN PREVENTED BY FLYING THE FULL APCH RATHER THAN TRYING TO TAKE A SHORTCUT. ALSO; A BETTER UNDERSTANDING OF HOW TO USE A GPS AS AN NDB SUBSTITUTE OR FOR SITUATIONAL AWARENESS IS NEEDED.