WHILE EN ROUTE TO A FOREST SERVICE AIRSTRIP, THE AIRPLANE COLLIDED WITH OPEN ROLLING TERRAIN. A WITNESS REPORTED THAT THE AREA WAS COVERED WITH LOW FOG AND VISIBILITY WAS LIMITED THE ENTIRE MORNING. WHEN THE FOG DID LIFT, VISIBILITY WAS ABOUT ONE-AND-A-HALF MILES. THE WITNESS ALSO REPORTED THAT THE MOUNTAINS TO THE SOUTH, WHERE THE DESTINATION AIRSTRIP WAS LOCATED, WERE COMPLETELY OBSCURED BY CLOUDS AND FOG. DURING THE INVESTIGATION, NO EVIDENCE WAS FOUND TO INDICATE A MECHANICAL FAILURE OR MALFUNCTION.