THE CONSENSUS OF WITNESSES WAS THAT THE PLT & 2 PAX HAD BEEN AWAKE ALL OR MOST OF THE PREVIOUS NIGHT & HAD CONSUMED ALCOHOLIC BEVERAGES UNTIL AT LEAST 0100 PDT. ONE WITNESS STATED THAT THE PLT WAS 'CUT OFF' FROM ALCOHOLIC DRINKS AT 0100, BUT THE 3 MEN REMAINED AT THE BAR UNTIL HE WENT OFF DUTY AT 0400. LATER, HE SAW THEM AT ANOTHER BAR AT ABOUT 1100,BUT COULD NOT RECALL IF THEY WERE DRINKING. AT ABOUT 1200, THE 3 TOOK OFF IN A PIPER PA-20 (PREVIOUSLY CONVERTED FROM A PA-22). THE PLT REPORTED THAT AFTER TAKEOFF, HE CONTD CLIMBING SOUTHBOUND FOR APRX 12 MI, THEN TURNED EAST TO CROSS OVERA MTN RANGE. SUBSEQUENTLY, THE ACFT ENCOUNTERED DOWNDRAFTS & BEGAN LOSING ALT. THE PLT ATTEMPTED TO MAKE A 180 DEG LEFT TURN AWAY FROM RISING TERRAIN, BUT THE ACFT SETTLED INTO TREES & CRASHED TO THE GND. REPORTEDLY, THERE WERE NO PREIMPACTMECHANICAL PROBLEMS. ALTHO SERIOUSLY INJURED, THE PLT WALKED FROM THE CRASH SITE. HE OBTAINED ASSISTANCE AT APRX 1840 & WAS TAKEN TO A HOSPITAL. THE ACFT & OTHER OCCUPANTS WERE FND THE FOLLOWING MORNING DURING AN AIR SEARCH.