Garlick Helicopters Inc UH 1H N462CC accident near Cove, Oregon

Probable cause

The failure of the engine-to-transmission drive shaft coupling assembly due to a fatigue fracture of one of its attachment bolts, which resulted in a loss of power to the main rotor.

NTSB analysis

The commercial pilot reported that, while maneuvering the helicopter at low altitude during logging operations, he felt a vibration and heard a howling sound coming from the transmission area. He immediately initiated a precautionary landing; however, as the helicopter settled to the ground, the main rotor rpm dropped, and the pilot attempted to cushion the landing with aggressive collective input. The helicopter subsequently landed hard.

Postaccident examination revealed multiple fractures of the Kaflex engine-to-transmission drive shaft coupling assembly. According to the flight manual, a failure of the engine-to-transmission drive shaft would lead to a complete loss of power to the main rotor. The coupling assembly failure was initiated by the failure of an attachment bolt due to a fatigue crack on the flange section that attached the coupling assembly to the transmission shaft flange. No evidence of any material anomalies were found in the fractured bolt, and the fatigue crack was likely caused by the severe stress imposed by logging operations. The manufacturer did not specify a replacement interval for the bolt. The investigation could not determine when the fatigue crack developed and propagated; therefore, it could not be determined whether the crack would have been identified during an annual inspection.

Text from the NTSB. Open the NTSB docket for WPR15LA178 for the full record.