THE PLT & PAX HAD FLOWN FROM HOBART TO OKLAHOMA CITY EARLIER IN THE DAY. AT APRX 2115 CDT, THE PLT TOOK OFF ON THE RETURN FLT WITHOUT FILING A FLT PLAN OR RCVG AN FAA WX BRIEFING. BY THAT TIME, WX HAD DIMINISHED ALONG THE ROUTE OF FLT, WHICH WAS TO THE WSW OF OKLAHOMA CITY. SUBSEQUENTLY, THE ACFT IMPACTED ON RISING TERRAIN, WHILE IN ABOUT A 30 DEG, RIGHT BANK ATTITUDE ON A HDG OF 130 DEG. THE ACFT WAS EXTENSIVELY DAMAGED FROM IMPACT & FIRE. NO PREIMPACT FAILURE OF THE FLIGHT CONTROLS OR ENG WAS FOUND. ACCORDING TO A LAW ENFORCEMENT OFFICIAL, THERE WAS THUNDERSTORM ACTIVITY WITH HEAVY RAIN & REDUCED VISIBILITY IN THE AREA OF THE ACDNT. THE PLT'S LAST MED CERT WAS DATED 3/30/84. A TOXICOLOGY CHECK OF THE PLT'S BLOOD SHOWED THE PRESENCE OF ETHANOL; HOWEVER, THE TOXICOLOGIST NOTED THAT ETHANOL PRODUCTION MAY HAVE OCCURRED FROM TISSUE DECOMPOSITION OR BACTERIAL CONTAMINATION. THE 2155 CDT WX AT LAWTON, OK, WAS IN PART: 300' OBSCURED, VIS 7 MI.