AT 0053 MDT, THE PLT RADIOED GREAT FALLS FSS & REQUESTED WX INFO. HE WAS ADVISED OF IFR CONDITIONS & WAS PROVIDED THE LATEST OBSERVATION. THE PLT REQUESTED & WAS PROVIDED A SPECIAL VFR CLEARANCE TO DEPART TO THE EAST. HE TOOK OFF AT 0100 MDT & REPORTED OVER MALMSTROM AFB (OUTSIDE THE CONTROLLED AIRSPACE) AT 0106. NO FURTHER RADIO TRANSMISSIONS WERE RECEIVED FROM THE ACFT. SUBSEQUENTLY, THE ACFT IMPACTED RISING TERRAIN AT AN ELEVATION OF ABOUT 3500 FT & APRX 8 FT BELOW THE EDGE OF A PLATEAU. THE CRASH SITE WAS LOCATED ABOUT 400 FT ABOVE THE VALLEY FLOOR. AN EXAM REVEALED EVIDENCE THAT THE ACFT WAS IN A SLIGHT LEFT WING LOW, NOSE LEVEL ATTITUDE & WAS TRAVELING AT HIGH SPEED. NO PREIMPACT PART FAILURE OR MALFUNCTION WAS FOUND. A TOXICOLOGICAL TEST OF THE PLT'S BLOOD SHOWED AN ALCOHOL LEVEL OF 0.083%. ABOUT 15 MI WEST AT MALMSTROM AFB, THE 0125 MDT WX WAS IN PART: 900 FT BROKEN, 2000 FT OVERCAST, 7 MI VISIBILITY WITH FOG & LIGHT RAIN. THE ELEVATION AT MALMSTROM WAS 3000 FT MSL.