Narrative:

While on approach to runway 22 at ama, I started my descent prior to the final approach fix. The localizer at ama has DME to allow you to know where the final approach fix is located. It is 4.2 DME on the localizer. The ama VOR is abeam the final approach fix. I had the VOR tuned in navigation 2 and the localizer tuned in navigation 1. The DME (1 DME in aircraft) was inadvertently placed on navigation 2, so when I saw 4.2 DME I started my descent to minimum descent altitude of 3940 MSL, not realizing I was starting my descent 4 mi early. After leveling at minimum descent altitude, the tower informed me I was at the wrong altitude. I realized at this point I was looking at the wrong DME. A few seconds later the runway lights came into sight and I landed. This could have been avoided by using xchk radials and not depending on DME alone, proper identify of DME source and better awareness of the current situation. A factor that weighs heavily into this scenario is this approach occurred after flying for 7 hours and being on duty for 10 hours. I do not consider this an excuse, but something worth noting.

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Original NASA ASRS Text

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

Narrative: 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.

Data retrieved from NASA's ASRS site as of July 2007 and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.