What happened
On February 2, 2000, an AS350 BA helicopter, registration PT-HNS, operated by North Star Táxi Aéreo Ltda, was performing a power line inspection flight for CHESF near Itaitinga, Ceará. Approximately 15 minutes into the flight, while cruising at a low altitude, the pilot experienced a sudden yaw and a loss of engine power, accompanied by an oil pressure warning light and an audible alarm.
In an attempt to reach a selected landing area, the pilot initiated an autorotation by lowering the collective and turning right to approach the wind. However, the maneuver resulted in the aircraft tail-drifting into a tailwind configuration. In an attempt to reach the intended site, the pilot increased the collective, which caused the rotor RPM to drop below minimum limits, preventing a successful flare. The aircraft struck the ground with a 30-degree right bank and a 20-degree nose-down attitude, causing extensive damage to the airframe.
All four occupants sustained light injuries.
The investigation
CENIPA investigators examined the engine and found intergranular fractures on the second-stage gas generator disc serrations. Evidence suggested that excessive engine vibration, potentially caused by debris accumulation in the torque coupling due to axial compressor wear from saline/sandy environments, may have caused brake components to loosen. This led to the loss of three vanes and subsequent engine seizure.
The investigation also scrutinized maintenance and training records. The maintenance facility was found to be using outdated manuals and lacked specialized tools; specifically, a required vibration test from a previous 1,500-hour inspection had not been recorded. Furthermore, the investigation identified a lack of objective training oversight, as the pilot's proficiency was being evaluated by a subordinate within the same company, creating a conflict of interest and inconsistent training quality.
Findings
- Engine Failure: The primary mechanical failure was caused by the fracture of serrations on the gas generator disc, leading to engine seizure.
- Improper Emergency Management: The pilot failed to monitor critical airspeed and rotor RPM (NR) parameters during the autorotation attempt.
- Poor Flight Planning: The flight was conducted at a low altitude, which significantly reduced the safety margin for responding to an engine failure.
- Maintenance Deficiencies: Inadequate maintenance practices and a lack of documented vibration testing contributed to the engine's degraded state.
- Training and Supervision Issues: Ineffective emergency training and a lack of objective proficiency evaluations due to the organizational hierarchy between the instructor and the trainee.
- Human Factors: The pilot exhibited overconfidence and made poor judgment decisions regarding the landing site and the use of the collective during the flare phase.