THE PLT REPORTED THAT HE BEGAN AN ILS APPROACH TO THE MERCER COUNTY ARPT. HE STATED THAT HE PLANNED TO DESCEND TO 3200 FT MSL, THEN EXECUTE A MISSED APPROACH & RETURN TO ABINGTON, VA. THE ARPT ELEVATION WAS 2857 FT MSL. HE STATED THAT HE REMEMBERED GLANCING AT THE ALTIMETER AT 3200 FT, THAT HE LOOKED FOR THE RWY ENVIRONMENT, AND THAT HE WAS PREPARING TO EXECUTE THE MISSED APPROACH, WHEN THE ACFT SUDDENLY HIT TREES & CRASHED. THE IMPACT OCCURRED APRX 1/2 MI FROM THE APPROACH END OF RWY 23. THE WEATHER WAS: 100 FT OBSCURED, VISIBILITY 1/2 MI WITH FOG, TEMP 42 DEG, WIND FROM 280 DEG AT 5 KTS. THE ILS & VORTAC FACILITIES WERE FOUND TO BE OPERATIONAL. THE ACFT NAVCOMM UNIT, GLIDE SLOPE RECEIVER & COURSE INDICATOR HEAD WERE BENCH CHECKED & FOUND TO BE WITHIN TOLERANCES. THE ALTIMETER WAS TESTED & SHOWED A CONSIDERABLE ERROR AT EACH TEST POINT; BUT WAS NOT DISASSEMBLED TO DETERMINE IF INTERIM DAMAGE HAD OCCURRED.