SEA85FA203

At 1445 MDT, the 60 yr old plt departed on a routine, VFR, repositioning flt which he had frequently flown before. After takeoff, he leveled the acft at 9500 ft on a direct course toward the destination. While en route at about 1515, the acft began a shallow oscillatory descent when it was about 28 mi from the black mtn range. Subsequently, the acft collided with a ridge about 1 mi from black mtn peak while in a wings level attitude at an elevation of about 8500 ft. the wx at that time was VFR with only scattered clouds & 15 mi visibility. An exam of the wreckage revealed no evidence of a preimpact part failure or malfunction. The acft was equipped with an arnav-20 loran unit & a century iib autopilot. The autopilot had no alt hold function, but its mode selector was found positioned to the 'hdg' position. Also, the elevator trim was found near the neutral position. Pathological & toxicological exams revealed no indication of a physiological problem. The possibility of the plt falling asleep or becoming incapacitated could not be determined.