What happened
On January 22, 2023, a Bell 206B helicopter, registration PT-YDC, was performing a private flight from Itabuna to an unregistered landing area in Barra Grande, Bahia. During the flight, the aircraft experienced a sudden loss of main rotor RPM. In response to the loss of torque, the pilot performed an emergency landing in a wooded area at Fazenda Santa Bárbara, in the municipality of Itacaré.
The impact with the ground and several fruit trees caused substantial damage to the aircraft, including a severed tail boom, collapsed skids, and structural wrinkling. Despite the severity of the impact, the pilot and the passenger were uninjured.
The investigation
CENIPA investigators examined the aircraft' and the engine components. Laboratory testing of the fuel showed no contamination, and bench testing of the engine ruled out engine malfunction as a primary cause, as the engine was still producing power at the time of the incident. Instead, investigators focused on the power train transfer system.
Upon inspection, the inner coupling of the main driveshaft assembly was found detached from the engine-side power shaft. Examination of the bolts securing the inner coupling to the flange revealed crush-like damage to the thread ends. The investigation traced the failure to the loosening of the four nuts that secure the inner gear to the engine-side power shaft flange. This loosening caused misalignment and vibration, eventually leading to the separation of the components and the inability to transmit torque to the main rotor.
Findings
- The pilot's HMNT class rating, required for the 206B model, had been expired since February 2021.
- The aircraft's main driveshaft assembly failed due to the loosening of the four nuts securing the inner gear to the engine-side power shaft flange.
- The failure was likely linked to maintenance inadequacies, such as the potential reuse of locknuts or improper torque application during a previous overhaul.
- The engine was producing power, but the mechanical failure of the driveshaft prevented that power from reaching the main rotor.