Bell UH1B N9970F

The purpose of the flight was to longline equipment to firefighters working on the ground. After a normal liftoff, the helicopter was hovering with the longline fully extended as it maneuvered to hook up to the external load. A loud “snap” or “bang” was heard, and one witness described that the helicopter “bucked.” The helicopter subsequently rolled to the right, entered a nose-low descent, and impacted the ground before bursting into flames. A video camera captured the helicopter’s final seconds before impact with the ground. In the video recording, the helicopter is seen hovering about 140 ft above the ground as the pilot maneuvered the helicopter to hook up to the external load. During the maneuver, a loud mechanical noise occurred, and a shudder could be seen radiating through the helicopter’s fuselage, followed by an extreme roll to the right past 90°, and a nose-low descent. As the helicopter continued its descent, the transmission, with the rotor head and blades attached, and the tail boom separated from the helicopter before ground impact. The loud noise and shuddering of the helicopter suggest that a mechanical problem likely preceded the loss of control. The sudden drop in altitude, combined with the forward pitch and right roll, enabled a mast bump condition that bent the rotor mast and caused an imbalance in the rotor system. This imbalance likely overloaded the transmission pylon structure, separating the transmission and main rotor from the rest of the airframe. The following out-of-plane divergence of the main rotor precipitated the tail boom separation. The circumstances of the accident and motion of the helicopter during its descent are similar to the findings of a flight test in which a similar helicopter was subjected to a loss of hydraulic pressure to the main rotor flight control system. Because the factual information available was limited, in part due to the post-crash fire and a lack of available recorded data, it could not be determined whether a loss of hydraulic pressure occurred during the accident sequence. In addition, there was no evidence of a malfunction in the hydraulic flight control servos or hydraulic pump; therefore, the reason for the loss of control could not be determined. The pilot’s medical records showed a history of cardiovascular conditions that placed him at an increased risk for a sudden cardiac event, including a heart attack or arrhythmia that could cause acute symptoms such as chest pain, shortness of breath, palpitations, or fainting; however, there were no autopsy findings to suggest that any of those occurred, and his most recent cardiologist follow-up showed him to be doing very well, asymptomatic, and tolerating his medications. His recent exercise stress test showed he had above average exercise tolerance for someone his age. Further, a witness reported that the pilot was alert and attempting to retain control of the helicopter as it descended. Therefore, it is unlikely that the pilot’s cardiovascular conditions were a factor in this accident.

Quoted verbatim from the NTSB record.