Hiller UH12E N99579 accident near Hanford, California

Probable cause

The separation of the tail rotor drive gear shaft due to torsional fatigue of its threaded portion, which resulted in a loss of tail rotor control.

NTSB analysis

The pilot reported that during a preflight inspection he observed a small oil leak originating where the tail rotor drive shaft attaches to the transmission. He said that, following a discussion with his boss, they decided to reposition the helicopter from a vehicle-mounted platform onto the ground so they could perform a more detailed examination of the oil leak. The pilot stated that following a normal engine start, he began to lift the helicopter off the platform and heard a "growling" sound originating from the transmission followed by an immediate loss of tail rotor control. The pilot said that the helicopter immediately began to rotate to the right as he rolled the throttle off and lowered the collective. Subsequently, the helicopter landed hard onto the ground adjacent to the truck and platform. Postaccident examination of the helicopter revealed that the tail rotor gear shaft was separated from the tail rotor drive shaft coupling.

Metallurgical analysis of the fracture surface of the tail rotor gear shaft revealed that the fracture was consistent with fatigue cracking that emanated from multiple origins at the root portion of a thread on the threaded portion of the shaft. The fatigue crack origin exhibited signatures consistent with being open for an extended period of time compared to the other regions of the fracture face; the amount of time could not be determined. The fatigue crack eventually propagated around the gear shaft as torsional fatigue. No maintenance records for the helicopter were recovered during the course of the investigation, and therefore the amount of flight time on the tail rotor gear shaft was undetermined.

Text from the NTSB. Open the NTSB docket for WPR11LA282 for the full record.