What happened
At approximately 0755 EST, an Aerospatle AS 350 B3 helicopter, registration C-FRPQ, was performing a flight from London, Ontario, to Windsor, Ontario. While cruising at 2000 feet, the aircraft experienced a sudden rightward yaw accompanied by a drop in rotor RPM. The pilot observed the engine chip and governor lights illuminate and heard the warning horn.
The engine, a Turbomeca Arriel 2B, had suffered a failure. The pilot immediately initiated an autorotation procedure and landed the aircraft in a farm field. During the descent, the pilot issued a mayday call and activated the emergency locator transmitter. The landing resulted in no injuries and no damage to the aircraft. The pilot successfully completed the shutdown procedures following the landing.
The investigation
Investigators examined the engine and identified that a 41-tooth bevel gear within the accessory gearbox module 1 had fractured. Metallurgical analysis revealed that the gear had succumbed to high-cycle fatigue cracking, with cracks visible at the roots of the teeth and along the rim. No manufacturing defects were identified in the gear itself.
The investigation also looked into the starter-generator system. It was discovered that the torque dampening system on the aircraft's starter-generator was over-tightened. Testing demonstrated that such an over-tightened setting can induce torsional oscillations. This pattern of failure had been observed in several other instances involving similar engine configurations.
Findings
- The 41-tooth bevel gear failed due to high-cycle fatigue.
- An over-tightened torque dampening system in the starter-generator caused torsional oscillations, which likely contributed to the fatigue of the gear.
- The failure of this specific gear causes the engine to decouple from its accessories, leading to an uncommanded shutdown that cannot be restarted in flight.