What happened
On the morning of 2 September 2004, a Bell 407 helicopter, registration HB-ZBA, was performing logging operations in the Bisistal valley near Muotathal, Switzerland. The aircraft was engaged in transporting tree trunks from steep slopes to an unloading area using a longline.
During one of the transport cycles, a portion of the suspended load became entangled in the branches of a beech tree. This entanglement created significant sudden shifts in load and torque. The resulting mechanical stress caused the main rotor to strike the tailboom, which in turn severed the tail rotor shaft. This failure led to an immediate loss of directional control, causing the helicopter to rotate violently around its vertical axis and crash into the steep, rocky terrain. The pilot was able to exit the cockpit following the impact, but the aircraft was destroyed.
The investigation
The investigation conducted by the Aircraft Accident Investigation Bureau (AAIB) focused on the mechanical failure of the tail rotor drive and the operational circumstances of the flight. Investigators examined the wreckage and found that the tail rotor drive shaft had separated at two distinct points: one caused by the impact of the main rotor and another caused by the fracture of the tailboom from the fuselage during the crash.
Technical analysis of the engine and the electronic control unit (ECU) revealed no mechanical anomalies or pre-existing technical faults. The investigation also noted that the pilot was utilizing a "vertical reference" technique, which involves leaning out of a bubble window to view the load directly. The investigators also reviewed the pilot's use of an unapproved audio warning system used to monitor engine torque.
Findings
- The primary cause of the accident was the entanglement of the external load with an obstacle, which induced extreme torque and load variations.
- The main rotor blade impact directly caused the destruction of the tail rotor shaft.
- The pilot's flight tactics prioritized operational efficiency over maintaining sufficient safety margins.
- There was a lack of a mentally prepared and readily recallable emergency recovery procedure for such an entanglement event.
Safety action
The investigation identified several safety deficiencies regarding cargo operations using vertical reference techniques and issued the following recommendations:
- The Federal Office of Civil Aviation (FOCA) should ensure that emergency cargo hook jettison mechanisms are easily accessible during vertical reference flights.
- Safety measures should be implemented to ensure pilots can wear helmets while using vertical reference techniques, as current space constraints often prevent this.
- The investigation emphasized the necessity for flight data recording to monitor performance parameters and suspended loads for quality assurance and safety monitoring.