FLC OF A CPR JET DSNDED TOO EARLY WHILE MAKING A BACK COURSE LOC APCH RESULTING IN ERRONEOUS PENETRATION OF A PROHIBITED AREA.

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WHILE ON APCH TO RWY 22 AT AMA; I STARTED MY DSCNT PRIOR TO THE FINAL APCH FIX. THE LOC AT AMA HAS DME TO ALLOW YOU TO KNOW WHERE THE FINAL APCH FIX IS LOCATED. IT IS 4.2 DME ON THE LOC. THE AMA VOR IS ABEAM THE FINAL APCH FIX. I HAD THE VOR TUNED IN NAV 2 AND THE LOC TUNED IN NAV 1. THE DME (1 DME IN ACFT) WAS INADVERTENTLY PLACED ON NAV 2; SO WHEN I SAW 4.2 DME I STARTED MY DSCNT TO MINIMUM DSCNT ALT OF 3940 MSL; NOT REALIZING I WAS STARTING MY DSCNT 4 MI EARLY. AFTER LEVELING AT MINIMUM DSCNT ALT; THE TWR INFORMED ME I WAS AT THE WRONG ALT. I REALIZED AT THIS POINT I WAS LOOKING AT THE WRONG DME. A FEW SECONDS LATER THE RWY LIGHTS CAME INTO SIGHT AND I LANDED. THIS COULD HAVE BEEN AVOIDED BY USING XCHK RADIALS AND NOT DEPENDING ON DME ALONE; PROPER IDENT OF DME SOURCE AND BETTER AWARENESS OF THE CURRENT SIT. A FACTOR THAT WEIGHS HEAVILY INTO THIS SCENARIO IS THIS APCH OCCURRED AFTER FLYING FOR 7 HRS AND BEING ON DUTY FOR 10 HRS. I DO NOT CONSIDER THIS AN EXCUSE; BUT SOMETHING WORTH NOTING.