Introduction
An accident involving a Eurocopter AS 350 B2 helicopter (registration C-GNMJ, serial number 2829) occurred in the Kamarang area of Guyana. The helicopter was engaged in mining support operations, including vertical reference work with a 120-foot longline attached to the belly hook. At 1725 local time, the pilot had finished slinging operations and began coiling the longline on the ground while maintaining a stable, out-of-ground-effect hover.
Flight History and Accident Sequence
As the pilot descended gradually, the rotor downwash caused the longline to blow beneath the landing skids. To reposition the line, the pilot raised the collective briefly and allowed the nose to drift right. At about 10 feet above ground level (AGL), the pilot experienced a restriction in the anti-torque pedals. He then found it difficult to control the cyclic and collective sticks, and the helicopter began to gyrate in pitch, roll, and yaw. The collective became nearly immovable, and the helicopter climbed to about 20 feet AGL. The pilot quickly retarded the throttle lever, causing the main rotor RPM to decay rapidly. The helicopter descended and turned right. Just before impact, the pilot applied considerable force to raise the collective, reducing the rate of descent. The helicopter struck the ground firmly on the left side while in a right turn, bounced, and landed upright. There was no main rotor blade-to-fuselage contact. The pilot was not injured, and the impact forces did not activate the emergency locator transmitter. The helicopter sustained substantial damage to the skids, tail boom, and main rotor head.
Investigation Findings
The investigation focused on the potential causes of the flight control restriction and hydraulic system malfunction. The helicopter's hydraulically assisted flight controls are designed to revert to manual control in the event of hydraulic pressure loss, but previous AS 350 occurrences have shown that control can be unpredictable. The investigation identified anomalies in the servo actuators and accumulators.
Servo Actuator Anomalies
Bench tests revealed that the left and right lateral servos had inconsistent piston extension and retraction travel rates. The left lateral servo extended 3.5 times faster than it retracted, and the right lateral servo extended 2.5 times faster than it retracted. Additionally, the left and forward servos exhibited high internal fluid leakage. These differences could cause interference in the swashplate movement when powered. The tail rotor servo was unremarkable.
Accumulator Depletion
The left lateral servo accumulator depleted pressure 1.7 times faster than the right lateral accumulator and almost 2 times faster than the forward servo accumulator. Asymmetric accumulator exhaustion can lead to uncommanded servo movement and control difficulties, a known issue addressed by previous airworthiness directives.
No definitive root cause was established for the loss of control, but the combined anomalies may have contributed to the pilot's inability to maintain control in hover flight.
Conclusion
The accident highlights challenges in controlling the AS 350 B2 during hydraulic system anomalies. The pilot's actions in reducing throttle and applying collective force likely prevented a more severe outcome.