FLC OF HS125 DSNDED BELOW PROTECTED AIRSPACE DURING NDB APCH TO BKV; FL.

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WE WERE ATTEMPTING AN IFR APCH TO BKV. WX FROM THE ASOS WAS RPTED AT 4 MI VISIBILITY AND 100 FT OVCST. ON THE INITIAL APCH TO THE ARPT (FOR AN ILS); WE NOTED THAT THERE WAS NO LOC SIGNAL BEING BROADCAST. WE INFORMED APCH AND THEN DISCUSSED OUR OPTIONS. OF COURSE A BIG CONCERN WAS FOR OUR PAX. AFTER DISCUSSING THIS; MY COPLT AND I DECIDED TO ATTEMPT THE NDB 9 APCH; AND USE THE FMS/GPS AS AN OVERLAY FOR COURSE GUIDANCE. I PROGRAMMED THE FMS AND THE NDB FREQ HAD ALREADY BEEN TUNED ON THE ADF AS IT WAS PART OF THE ILS. WE WERE GIVEN A QUICK VECTOR FOR THE FINAL APCH COURSE; AND I FELT VERY RUSHED TO ACCOMPLISH EVERYTHING. APCH CTL THEN INDICATED THAT WE WERE 9 MI FROM WHAT I THOUGHT WAS THE NDB (WHICH WAS ON THE FIELD). THIS APCH HAS NO FINAL APCH FIX; BUT RATHER THE BEACON ON THE FIELD AND A PROC TURN THAT MUST BE COMPLETED WITHIN 10 NM; AT THE END OF WHICH DSCNT MAY BE MADE TO MDA. I INDICATED TO MY COPLT (WHO WAS FLYING) THAT WE WERE WITHIN 10 MI ON THE FINAL APCH COURSE AND; AS SUCH; COULD DSND TO MDA. LOOKING AT THE HSI; I NOTICED THE MI COUNTDOWN FROM THE FMS; BUT JUST AS IT COUNTED DOWN TO 0; REALIZED THAT THE COUNTDOWN WAS NOT TO THE BEACON; BUT RATHER TO THE FMS GENERATED WAYPOINT FOR THE PROC TURN. AS SUCH; WE HAD DSNDED OUTSIDE OF THE PROTECTED 10 MI OF AIRSPACE. AFTER COMPLETING A MISSED APCH; WE DECIDED TO DIVERT TO OUR ALTERNATE. SEVERAL FACTORS CONSPIRED TO OUR POSSIBLY SERIOUS MISTAKE. 1) ALLOWING OURSELVES TO BE RUSHED. 2) NOT USING THE FMS FOR AN APCH OF THIS TYPE RECENTLY; WE DID NOT CAREFULLY REVIEW THE INFO BEING PRESENTED. 3) LETTING PAX CONCERNS CLOUD OUR JUDGEMENT. I WOULD URGE EVERY FMS CREW MEMBER TO USE EXTREME CAUTION WHEN UTILIZING THESE DEVICES FOR LOW ALT/HIGH WORKLOAD SITS; WHERE INTERP NEEDS TO BE VERY; VERY ACCURATE.