AN SMA PVT PLT FLEW AN NDB APCH IMPROPERLY.

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ON VECTORS FOR NDB RWY 17 APCH TO AURDRA STATE ARPT I RECEIVED VECTOR 'TURN L TO 280 DEGS; VECTOR TO TUALA; CROSS TUALA AT OR ABOVE 2500 FT; CLRED FOR THE NDB APCH' (TUALA IS A RADAR FIX FROM WHICH THE APCH BEGINS). I ACKNOWLEDGED THESE INSTRUCTIONS; TURNED AND INTERCEPTED THE 157 DEG (INBOUND) HDG TO PRAHL. I MISTAKENLY VIEWED THIS INTERCEPT AS MEANING I WAS OVER TUALA (A PREVIOUS APCH SOME YRS EARLIER HAD A RESULT OF BEING VECTORED DIRECTLY TO TUALA). I THEREFORE STARTED MY DSCNT IMMEDIATELY. A COUPLE OF MINS LATER; APCH CALLED MY POS AS 'OVER TUALA.' NEEDLESS TO SAY; I HAD ALREADY DSNDED BELOW THE LIMIT OF 2500 FT. I WAS IN MARGINAL VFR AND WAS IN NO DANGER OF COLLIDING WITH TERRAIN; BUT REALIZED I HAD BUSTED THE LIMIT. I HAD BEEN VECTORED TO INTERCEPT THE 157 DEG INBOUND SEVERAL MI N OF TUALA AND; DUE TO MY UNDERSTANDING OF THE VECTOR INSTRUCTION AND MY PRIOR EXPERIENCE WITH VECTORS FOR THIS APCH; I MISTAKENLY UNDERSTOOD MY INTERCEPT TO BE OVER THE FIX. IDENT OF THE RADAR FIX USING NEWBERG VOR 70 DEG RADIAL IS NOT VERY PRECISE AND IS NOT A GOOD SUBSTITUTE FOR RADAR IDENT.