ATX PLT GETS CONFUSED AS TO HIS DSCNT POINT ON A VOR DME APCH AND DSNDS EARLY; IN THE VICINITY OF A TWR; BASED ON AN IMPROPER LORAN REFERENCE VS DME REFERENCE. THE APCH CTLR CALLED THIS TO THE PLT'S ATTENTION. THE PLT STOPPED HIS DSCNT AT THAT ALT IN VMC CONDITIONS AND SUCCESSFULLY COMPLETED THE APCH.

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I WAS RADAR VECTORED TO INTERCEPT THE PUBLISHED RADIAL FOR A VOR/DME 33 APCH AT SIOUX FALLS; SD. I HAD ESTABLISHED MYSELF ON THE PUBLISHED RADIAL AND BEEN CLRED FOR THE APCH. THE VOR AND DME WERE PROPERLY SET AND IDENTED. ALSO SET UP WAS THE LORAN TO SHOW A REFERENCE TO THE ARPT IN ADDITION TO THE PUBLISHED APCH. THIS IS VFR LORAN AND I HAD TUNED IT TO FSD FOR A REFERENCE ONLY. I THOUGHT THIS WOULD BE HELPFUL SINCE THE VOR WAS LOCATED OFF THE FIELD. THE PUBLISHED APCH CALLED FOR DSCNT TO MDA BEGINNING AT THE 10 DME FIX (FINAL APCH FIX). MY MISTAKE CAME WHEN I BEGAN A DSCNT TO MDA BEGINNING AT APPROX 12- 13 DME. THIS HAPPENED BECAUSE I REFERENCED THE LORAN DISTANCE TO THE ARPT RATHER THAN THE VOR/DME FIX. THE APCH CALLED FOR A DSCNT FROM 2900 FT MSL AT THE FINAL APCH FIX TO 1920 FT MSL; THE STRAIGHT IN MDA. AT APPROX 2200 FT MSL SIOUX FALLS APCH CAME OVER THE RADIO ADVISING THE MINIMUM ALT 'IN THAT AREA' IS 2900 FT. I ACKNOWLEDGED WITH 'ROGER.' SINCE I WAS NOW BELOW THE CLOUD LAYER AND HAD 3-5 MI FORWARD VISIBILITY; I REMAINED AT 2200 FT. I ALSO SAW THEN MY MISTAKE IN RANGE INFO AND REALIZED I WAS ABOUT 2 MI (11.5-12 DME) SHORT OF THE FAF. NO MORE WAS SAID BY THE APCH CTLRS RELATING TO THIS. I FEEL THERE ARE TWO MAIN FACTORS THAT LED TO THIS ERROR. THE FIRST BEING WORKLOAD. THE MAJORITY OF OUR TRIPS ARE 2 PLT CREW. NOT THAT CONDITIONS WERE SO DIFFICULT FOR THE DAY EXCEPT FOR CONSIDERABLE TURB IN THE APCH PHASE; BUT SIMPLY GOING BACK TO A SINGLE PLT COCKPIT FLOW IS SOMEWHAT DIFFICULT WHEN IT IS SELDOM DONE. THE SECOND FACTOR IS THE USE OF MULTIPLE NAV METHODS. IT WAS JUST TOO EASY TO REFERENCE THE LORAN DISTANCE TO THE ARPT AND FORGET THE VOR WAS OFFSET FROM THE FIELD AND THEREFORE THE DME AND LORAN WERE READING DISTANCES TO DIFFERENT REFS. I HOPE AND PRAY I; NOR ANYONE ELSE; MAKES THIS TYPE OF ERROR AGAIN.