THE PILOT'S FAILURE TO MAINTAIN CONTROL OF THE AIRPLANE WHICH RESULTED IN AN INADVERTENT SPIRAL AND SUBSEQUENT RAPID DESCENT. THE DESIGN STRESS LIMITS OF THE AIRFRAME WERE EXCEEDED RESULTING IN AN OVERLOAD FAILURE OF THE EMPENNAGE. FACTORS RELATED TO THE ACCIDENT WERE: TURBULENCE AND THE PILOT'S PHYSICAL IMPAIRMENT DUE TO DRUGS.