ERA21FA195 · Bell Helicopter Textron Canada 429

The helicopter departed and flew cross-country to begin a powerline patrol, which included flying adjacent to powerlines at an altitude of about 100 ft above ground level and with the helicopter gaining altitude during turns and when flying between line inspections. During the final minutes of flight, the helicopter was traveling at an airspeed between 50 and 60 kts along powerlines at an altitude between 700 and 775 ft above mean sea level (msl). As the helicopter flew south, near the end of the north-south powerline’s right of way, the helicopter began a climbing right turn towards the west. A utility lineman in the helicopter’s front left seat recalled that the pilot was reversing course and the helicopter was in a banking turn to the right when he heard a very loud noise, “almost cannon-like, very deep, within a second or two we were heading into the trees.” Onboard data recovered from the helicopter showed rotor speed suddenly increased to 107% about 1.5 seconds before the end of recorded data. Examination of the wreckage showed evidence the main rotor blades impacted the tail rotor drive system. Therefore, it is likely the loud noise heard by the onboard lineman was caused by the main rotor blades impacting the tail rotor drive system, resulting in a severance of the tail rotor drive shaft and subsequently a loss of directional control. Successful recovery of the helicopter after the loss directional control at a low altitude was unlikely and resulted in impact with terrain. Based on analysis of flight data and a scan of the local terrain, it is unlikely the helicopter impacted powerlines or nearby trees at the time the main rotor blades impacted the tail rotor drive system. The loss of clearance between the main rotor and the tail of the helicopter can be caused by sudden aft cyclic control inputs, usually in conjunction with a reduction in collective pitch that would reduce main rotor thrust and increase downward flapping of the blade. The lack of flight control parametric data precluded analysis of flight control inputs at the time the main rotor blade contacted the tail boom. However, it is likely the pilot was not maneuvering to avoid trees based on the increasing distance between the accident helicopter and treetops near the end of the accident flight. Examinations of the helicopter wreckage found no evidence of any preimpact anomalies or malfunctions that would have precluded normal operation of the helicopter.

Verbatim NTSB analysis and probable cause from the NTSB dataset