What happened
On August 5, 2007, a Eurocopter AS3/350 BA, registration OO-HCW, was conducting local passenger flights as part of a fly-in event at Spa Airport. While cruising at approximately 500 feet over the Aywaille/Remouchamps area, the aircraft experienced a sudden, violent, and uncontrolled yaw to the left, accompanied by intense vibrations.
In response to the instability, the pilot immediately reduced engine power and transitioned the aircraft into autorotation. The pilot managed to steer the helicopter into the wind and executed a sliding landing on a grassy meadow. While the landing was smooth enough that the 6 occupants (the pilot and 5 passengers) sustained no injuries, the aircraft sustained significant damage. Upon inspection after the passengers had disembarked, it was discovered that the aft rotor transmission shaft had been severed.
The investigation
The AAIU(Be) investigation focused on the mechanical integrity of the transmission system and the cause of the initial yaw. Physical examination of the wreckage revealed that the tail rotor drive shaft had ruptured at three distinct points. The first rupture, located between the rear flector and the aft transmission gearbox, showed signs of axial overload. The second rupture was caused by a collision between the shaft and a main rotor blade, evidenced by yellow and blue paint transfers on the blade. The final rupture was a bending failure where the shaft was bent over 90 degrees.
Technical inspections of the engine and flight controls were conducted at specialized facilities in France. While the hydraulic, electrical, and flight control systems were found to be functioning correctly, investigators noted anomalies in the engine's free wheel assembly, including a slightly bent shaft and circular grooves on the inner casing. However, it could not be definitively proven if these defects were pre-existing or a result of the accident itself.
Findings
- The primary damage to the aft rotor shaft was caused by the main rotor blade flapping during the autorotative landing phase. As the helicopter slid across the uneven terrain, the increased flapping movement led to the yellow rotor blade striking the aft rotor shaft.
- The investigation could not definitively confirm the cause of the initial violent leftward yaw. A working theory suggests a brief decoupling of the engine from the rotor transmission occurred; when the free wheel subsequently re-engaged, the sudden acceleration of the drive rotor triggered the uncontrolled movement.
Safety action
- The investigation recommended that EASA consider encouraging the development and use of simplified Flight Data Recorders, such as on-board video cameras, for General Aviation aircraft to improve accident data collection.