DURING ARRIVAL, THE PLT WAS CLEARED FOR AN ILS APCH. THE REPORTED WX AT THAT TIME WAS: 600 FT OVERCAST, VISIBILITY 7 MI, WIND 010 DEG AT 6 KTS. THE PLT STATED THAT DURING THE APCH, HE FELT A DOWNDRAFT THAT CAUSED THE ACFT TO BANK TO THE LEFT & PITCH DOWN. THE LAST ALT THAT HE RECALLED READING BEFORE THE 'DOWNDRAFT' WAS 750 FT. SUBSEQUENTLY, THE ACFT COLLIDED WITH TREES & CRASHED. A PASSENGER, WHO WAS OCCUPYING A REAR SEAT, SAID THAT THE ACFT WAS IN THE CLOUDS, THEN CAME OUT INTO THE CLEAR WITH JUST ENOUGH TIME TO SEE THE TREES BEFORE IMPACT. AN INVESTIGATION DID NOT REVEAL ANY PREIMPACT MALFUNCTION OR FAILURE OF THE AIRFRAME OR POWERPLANT. THE 0057 CST WX (APRX 33 MIN AFTER THE ACCIDENT) WAS, IN PART: 100 FT OBSCURED, VISIBILITY 3/4 MI WITH FOG, WIND CALM. THE ELEVATION OF THE CRASH SITE WAS ABOUT 200 FT MSL. THE ARPT ELEVATION WAS 215 FT.