What happened
On 18 October 2011, an AS355N helicopter, registration PR-HBC, was undergoing engine start-up procedures at the Sepé Tiarajú Estate in Itacoati/Amazonas. The aircraft was being prepared for a test flight with a pilot and two passengers on board.
Following the start-up of the second engine, the aircraft began to vibrate, and the pilot perceived the onset of ground resonance. In an attempt to mitigate the vibration, the pilot decided to attempt a takeoff. However, the aircraft lacked the performance capability for a single-engine takeoff. During the maneuver, the helicopter yawed to the right and tilted heavily to the left, causing the left skid to strike the concrete pavement. This movement caused the main rotor blades to collide with the ground, resulting in the severance of the tail boom and significant structural damage to the airframe and engine components. All three occupants escaped the wreckage uninjured.
The investigation
CENIPA investigators examined the wreckage and analyzed data from the aircraft's Monitair Recorder. The investigation focused on the stability of engine parameters and the mechanical condition of the rotor assembly.
Technical analysis of the flight data revealed that the engine parameters failed to stabilize after the start-up of engine number 2; instead, torque and rotor speed (NR) increased rapidly. Investigators found that the collective control was in the unlocked position, which likely contributed to the instability. The investigation ruled out mechanical failures in the landing gear, hydraulic dampers, or the main rotor head, as well as external factors like uneven ground or improper tire pressure. The investigation also found no evidence of actual ground resonance occurring due to mechanical imbalance or blade tracking issues.