RELATIVES EXPECTED THE PLT TO RETURN HOME ON THE EVENING OF 10/4/83. WHEN HE DID NOT RETURN, RELATIVES REPORT THE ACFT WAS OVERDUE. THE ACFT WAS FOUND WHERE IT COLLIDED WITH A MOUNTAIN AT AN ELEVATION OF ABOUT 3580 FT, 10 MI NORTH OF THE DESTINATION ARPT. THERE WERE NO KNOWN WITNEESES TO THE ACCIDENT. THERE WAS EVIDENCE THE ACFT HAD DEPARTED BIG BEAR, CA AT ABOUT 1715 PDT. ABOUT 15 MI WEST OF THE ACCIDENT SITE, A PERSON SAW AN ACFT, MATCHING THE DESCRIPTION OF N8146T. ACCORDING TO HER, THE ACFT WAS OVER INTERSTATE 5, HEADED TOWARD BURBANK, CA. IN HER OPINION, THE PLT WAS TRYING TO GET OVER NEWHALL PASS, CIRCLED 3 OR 4 TIMES. THEN DEPARTED TO THE EAST. SHE REPORTED INTERMITTENT RAIN & ABOUT 1000 FT CEILING. AT ABOUT 1830, AN ATP PLT REPORTED THUNDERSTORM ACTIVITY OVER THE MOUNTAINS. AT 1845, THE WX AT BURBANK WAS 1300 FT OVERCAST, 4 MI VISIBILITY WITH HAZE. THE ELEVATION OF THE BURBANK ARPT WAS ABOUT 3000 FT LOWER THAN THE ACCIDENT SITE. NO RECORD OF A PREFLT WX BRIEFING WAS FOUND. IMPACT OCCURRED ON 50 DEG RISING TERRAIN.