NTSB Probable Cause

THE PILOT'S IMPROPER USE OF EQUIPMENT WHICH AFFECTED THE OPERATION OF THE STANDBY VACUUM PUMP, THE PILOT'S LOSS OF AIRCRAFT CONTROL AFTER EXPERIENCING SPATIAL DISORIENTATION, AND HIS EXCEEDING THE DESIGN STRESS LIMITS OF THE AIRCRAFT. FACTORS RELATED TO THE ACCIDENT WERE: THE MECHANIC'S IMPROPER USE OF THREAD COATING MATERIAL, THE PILOT'S OPERATION OF THE AIRCRAFT WITH A KNOWN MALFUNCTION OF THE PRIMARY VACUUM PUMP, AND THE ADVERSE WEATHER CONDITIONS.