What happened
The FAA’s narrative, in sentence case. The FAA writes it in capitals and removes private details.
Lost tail rotor gear box and tail rotor enroute. Crash landed in autorotation. Found fatigue cracking problem. (.4) during an air ambulance flight in the public-use helicopter, the tail rotor and gearbox separated from the helicopter. The pilot autorotated to a forced landing. During the descent over mountainous terrain, the helicopter collided with trees and impacted hard terrain on its left side which crushed inward. The operator's policy required all crewmembers to wear helmets during flight. Helmets were not provided for the two paramedics. During the crash sequence, the passenger seat stanchions and tubing buckled, which resulted in multiple lap belt anchor point separations and the catapulting of crewmembers into the overhead cockpit panel. Safety board survival factors documentation in conjunction with helmet crashworthiness analysis revealed helicopter impact forces were within human tolerance. The lack of and/or inadequate strength helmets and the lap belt anchor point failures allowed crewmembers' excursions resulting in head trauma. The tail rotor component separations in flight resulted from a fatique crack originating in the surface of the yoke onto which the tail rotor blades had been attached. In 1996, Bell issued an alert service bulletin (asb) number 205-96-68, which was designed to measure yoke deformationresulting from adverse in-flight or ground handling operations which imposed excessive bending loads. The test protocol was found problematic in its accuracy due to technical errors in the bulletin and a lack of clarity. City mechanics failed to adhere to all of the asb's requirements. The bent yoke fractured at a total time in service of approximately 4,113 hours, about 117 hours after its inspection for evidence of deformation. The yoke's stainless steel composition and requisite metallurgical properties were confirmed by the safety board. An x-ray diffraction examination of the yoke revealed reduced compressive residual stress in the fracture origin region which allowd operational loads to initiate and propagate the fatigue crack. This significant reduction of the residual stress was likely due to excessive flexure (bending) of the yoke. The initiating event which overstressed and bent the yoke was not identified.
As the FAA filed it
LOST TAIL ROTOR GEAR BOX AND TAIL ROTOR ENROUTE. CRASH LANDED IN AUTOROTATION. FOUND FATIGUE CRACKING PROBLEM. (.4) DURING AN AIR AMBULANCE FLIGHT IN THE PUBLIC-USE HELICOPTER, THE TAIL ROTOR AND GEARBOX SEPARATED FROM THE HELICOPTER. THE PILOT AUTOROTATED TO A FORCED LANDING. DURING THE DESCENT OVER MOUNTAINOUS TERRAIN, THE HELICOPTER COLLIDED WITH TREES AND IMPACTED HARD TERRAIN ON ITS LEFT SIDE WHICH CRUSHED INWARD. THE OPERATOR'S POLICY REQUIRED ALL CREWMEMBERS TO WEAR HELMETS DURING FLIGHT. HELMETS WERE NOT PROVIDED FOR THE TWO PARAMEDICS. DURING THE CRASH SEQUENCE, THE PASSENGER SEAT STANCHIONS AND TUBING BUCKLED, WHICH RESULTED IN MULTIPLE LAP BELT ANCHOR POINT SEPARATIONS AND THE CATAPULTING OF CREWMEMBERS INTO THE OVERHEAD COCKPIT PANEL. SAFETY BOARD SURVIVAL FACTORS DOCUMENTATION IN CONJUNCTION WITH HELMET CRASHWORTHINESS ANALYSIS REVEALED HELICOPTER IMPACT FORCES WERE WITHIN HUMAN TOLERANCE. THE LACK OF AND/OR INADEQUATE STRENGTH HELMETS AND THE LAP BELT ANCHOR POINT FAILURES ALLOWED CREWMEMBERS' EXCURSIONS RESULTING IN HEAD TRAUMA. THE TAIL ROTOR COMPONENT SEPARATIONS IN FLIGHT RESULTED FROM A FATIQUE CRACK ORIGINATING IN THE SURFACE OF THE YOKE ONTO WHICH THE TAIL ROTOR BLADES HAD BEEN ATTACHED. IN 1996, BELL ISSUED AN ALERT SERVICE BULLETIN (ASB) NUMBER 205-96-68, WHICH WAS DESIGNED TO MEASURE YOKE DEFORMATIONRESULTING FROM ADVERSE IN-FLIGHT OR GROUND HANDLING OPERATIONS WHICH IMPOSED EXCESSIVE BENDING LOADS. THE TEST PROTOCOL WAS FOUND PROBLEMATIC IN ITS ACCURACY DUE TO TECHNICAL ERRORS IN THE BULLETIN AND A LACK OF CLARITY. CITY MECHANICS FAILED TO ADHERE TO ALL OF THE ASB'S REQUIREMENTS. THE BENT YOKE FRACTURED AT A TOTAL TIME IN SERVICE OF APPROXIMATELY 4,113 HOURS, ABOUT 117 HOURS AFTER ITS INSPECTION FOR EVIDENCE OF DEFORMATION. THE YOKE'S STAINLESS STEEL COMPOSITION AND REQUISITE METALLURGICAL PROPERTIES WERE CONFIRMED BY THE SAFETY BOARD. AN X-RAY DIFFRACTION EXAMINATION OF THE YOKE REVEALED REDUCED COMPRESSIVE RESIDUAL STRESS IN THE FRACTURE ORIGIN REGION WHICH ALLOWD OPERATIONAL LOADS TO INITIATE AND PROPAGATE THE FATIGUE CRACK. THIS SIGNIFICANT REDUCTION OF THE RESIDUAL STRESS WAS LIKELY DUE TO EXCESSIVE FLEXURE (BENDING) OF THE YOKE. THE INITIATING EVENT WHICH OVERSTRESSED AND BENT THE YOKE WAS NOT IDENTIFIED.
NTSB investigation
The NTSB’s investigation of this event, with its probable cause and findings. Matched to this report by N-number and date.