DRG ARRIVAL, THE PLT WAS CLRD FOR AN ILS RWY 7 APCH. ALSO, HE WAS ADZD OF A DC-9 THAT WAS 4 MI AHEAD & WAS TOLD TO USE CAUTION FOR WAKE TURBULENCE. AS THE ACFT WAS ON FINAL APCH, IT DESCENDED BELOW THE ILS GLIDE SLOPE & SUBSEQUENTLY HIT TREES & CRASHED ABOUT 1.8 MI SHORT OF THE RWY. NO PREIMPACT PART FAILURE OR MALFUNCTION OF THE ACFT OR ENGS WAS FOUND THAT WOULD HAVE RESULTED IN AN ACDNT. ALSO, THERE WERE NO REPORTED PROBLEMS WITH THE ILS SYSTEM & IT TESTED NORMAL AFTER THE ACDNT. THE PLT HELD A COMMERCIAL PLT CERTIFICATE WHICH WAS GOOD FOR SINGLE ENGINE LAND ACFT; HIS MULTI-ENGINE PRIVILEGES WERE AUTHORIZED AS A PRIVATE PLT, ONLY. AN NTSB PERFORMANCE STUDY SHOWED THE ACFT WAS 2 MIN & 57 SEC BEHIND THE DC-9. RADAR DATA INDCD THE ACFT DID NOT EXCEED A BANK ANGLE OF 32 DEG & NO EXCESSIVE G-VALUES WERE EVIDENT DRG THE APCH.