THE ACFT DEPARTED CRESWELL, OR, SOMETIME AFTER 1800 PST ON 3/19/83. AT 2110 PST, THE PLT CONTACTED PORTLAND FLT WATCH BY RADIO & STATED THAT HE WAS OVER ROSEBURG, OR AT 9500 FT. HE REQUESTED PIREPS OF WX OVER THE SISKIYOU MOUNTAINS & INTO THE SAN FRANCISCO BAY AREA. FLT WATCH ADVISED THAT THE MEDFORD, OR WX WAS ESTIMATED 8000 FT OVERCAST, VISIBILITY 10 MI & THAT A PLT HAD REPORTED THAT CONDITIONS FROM REDMOND, OR TO RED BLUFF, CA, AT 9500 FT, WERE CLEAR & SMOOTH. THE PLT REPLIED THAT HE WAS HAVING A SMOOTH RIDE, ON TOP, & THAT THERE WAS 'A LITTLE BIT OF BUILDUPS TO THE WEST.' THERE WERE NO FURTHER TRANSMISSIONS FROM THE ACFT. WHEN THE ACFT DID NOT ARRIVE AT ITS DESTINATION, A SEARCH WAS INITIATED. IT WAS FOUND BY HIKERS ON 8/15/83 WHERE IT HAD CRASHED IN MOUNTAINOUS TERRAIN WHILE IN A STEEP DIVE. AT THE TIME OF THE ACCIDENT, SEARCH PERSONNEL COULD NOT GET INTO THE AREA DUE TO LOW CLOUDS, RAIN & SNOW. AN EXAM OF THE WRECKAGE REVEALED NO EVIDENCE OF A PREIMPACT/MECHANICAL MALFUNCTION OR FAILURE.