THE PRIVATE AIRPORT WAS LOCATED IN A VALLEY SURROUNDED BY MOUNTAINOUS TERRAIN; THE HIGHEST MTN PEAKS WERE LOCATED TO THE EAST & WEST OF THE NORTH/SOUTH (36/18) RUNWAY. THE AIRPLANE APPROACHED THE ARPT FROM THE SOUTH AND THE PILOT MADE A LOW PASS OVER RUNWAY 36 AS WAS CUSTOMARY TO INSURE THAT NO LIVESTOCK WERE ON THE RUNWAY. AFTER COMPLETING THE LOW PASS, THE PLT CLIMBED STRAIGHT AHEAD BEYOND THE RISING TERRAIN, THEN ENTERED LEFT TRAFFIC. GROUND WITNESSES REPORTEDTHAT THE AIRPLANE MADE ANOTHER LOW PASS OVER THE RUNWAY. AT THE NORTH END OF THE RUNWAY, THE AIRPLANE ENTERED A SHARP CLIMBING TURN, BUT AFTER ABOUT 90 DEG OF TURN, IT CRASHED ON RISING TERRAIN, ABOUT 1/4 MI NORTHWEST FROM THE CENTER OF THE AIRPORT. AN INVESTIGATION REVEALED THE ACFT IMPACTED THE GROUND IN A STEEP NOSEDOWN ATTITUDE. NO PREEXISTING MECHANICAL MALFUNCTION OR FAILURE WAS FOUND.