AT APRX 1540 EST, THE ACFT WAS DISPATCHED TO PHILADELPHIA, PA ON A PAX FLT. AFTER ARRIVING, THE PLT REMAINED THERE TO PICK UP 4 OTHER PAX THAT WERE ARRIVING AT APPROX 2100 EST. AT 2007, THE PLT RECEIVED A PARTIAL WX BRIEF & FILED AN IFR FLT PLAN TO RTN TO POTTSTOWN. THE ACFT DEPARTED WITH AN IFR CLNC AT 2145 & PROCEEDED AT 3000 FT MSL, JUST ABV THE CLD TOPS. DRG ARRIVAL, THE PLT WAS CLRD FOR A VOR-B APCH TO THE POTTSTOWN MUNI ARPT. HE RPRTD THAT HE COULD SEE THE ARPT BEACON FROM OVER THE VOR, BUT NOTED PATCHY GND FOG. THE APCH WAS CONTD TO THE MISSED APCH POINT WHERE HE LOST SITE OF THE BCN. HE BGN MAKING A MISSED APCH; HOWEVER, WHEN THE THROTTLE WAS ADVANCED, NO DIFFERENCE IN ENG SOUND WAS NOTED, EXCEPT HE RECALLED THERE MAY HAVE BEEN SOME "FLUTTER IN THE ENGINE." RPRTDLY, THE ACFT CONTD DSNDG & SLOWING TIL IT HIT TREES AT 440 FT MSL, APRX 1 MI NW OF THE ARPT (ARPT ELEV 256 FT). NO PREIMPACT FAILURE WAS FOUND. TEMP & DEW PT WERE 48 DEG, ICG PROB CHART INDICATED SERIOUS CARB ICG CONDS WERE PRESENT. HOWEVER, PSN OF CARB HEAT CTL COULD NOT BE VERIFIED.