THE FLT WAS RETURNING TO SAN JOSE,CA AFTER AN EXTENDED X-COUNTRY FLT TO OSHKOSH,WI WHICH BEGAN IN THE LATTER PART OF JULY. EARLIER ON THE DAY OF THE ACCIDENT, AT 0645, THE PLT RECEIVED A WX BRIEFING AT MILES CITY,MT. HE REQUESTED THE GENERAL OUTLOOK TO THE GRAND CANYON & ARIZONA AREA & WAS TOLD OF LARGE BUILDUPS IN WESTERN AZ & NV, FORECAST TO MOVE NE.THE PLT DEPARTED AT 0712 & MADE ENROUTE STOPS AT RIVERTON,WY & ROCK SPRINGS,WY. AT 1845, OVER VERNAL,UT, THE PLT REQUESTED A WX UPDATE. THE ACFT SUBSEQUENTLY IMPACTED A MOUNTAIN AT THE 8,700 FT LEVEL. THERE WAS CONSIDERABLE THUNDERSTORM ACTIVITY THROUGHOUT THE STATE AT THE TIME OF THE ACCIDENT. THE PLT'S BLOOD ALCOHOL LEVEL WAS 0.093%. THE PLT WAS NOT A CFI BUT HAD GIVEN THE STUDENT PLT 29 HRS OF DUAL. HIS LOGBOOK SHOWED THAT HE HAD ALSO GIVEN FLT INSTRUCTION TO OTHER STUDENT PLTS. ON 1/5/83 THE PLT'S DRIVERS LICENSE WAS SUSPENDED INDEFINITELY FOR DRIVING UNDER THE INFLUENCE OF ALCOHOL. HIS BAC WAS REPORTEDLY 0.093%.