THE ACFT COLLIDED WITH RISING TERRAIN DURING A NIGHT X-COUNTRY. NO FLT PLAN WAS FILED. RADIO COMMUNICATIONS WERE NORMAL DURING THE SHORT (40 MILE) FLT, UNTIL THE PLT WAS ASKED TO CHANGE TRANSPONDER CODES. THE TRANSPONDER SIGNAL CEASED INSTEAD. THE CRASH SITE WAS LOCATED ON THE WEST SLOPES OF A VALLEY RUNNING NORTH/SOUTH BETWEEN MT RANGES IN AN AREA CHARACTERIZED BY GENTLY RISING TERRAIN. AN ON-SCENE INVESTIGATION WAS CONDUCTED ON 3/1/85, ABOUT 7 MONTHS AFTER THE ACCIDENT. THE DELAY WAS DUE TO THE FAILURE IN LOCATING THE ACFT, WHICH BECAME OVERDUE (MISSING) ON 8/6/84. SKELETAL REMAINS OF THE PLT, FOUND 2 MILES FROM THE ACCIDENT SITE, MADE PATHOLOGICAL TESTS VERY DIFFICULT. HOWEVER, EVIDENCE OF ASKULL FRACTURE WAS PRESENT. THERE WAS NO EVIDENCE OF FIRE ON THE REMAINS OR CLOTHING OF THE PLT. THE PAX REMAINS WERE FOUND NEAR THE REAR COCKPIT AREA OF THE WRECKAGE AND THERE WAS SOME EVIDENCE OF FIRE INVOLVEMENT ON THE CLOTHING COVERING ON LOWER EXTREMITY.