NTSB Factual Report

On June 23, 2011, about 0400 Pacific daylight time, a Hiller UH-12E, N99579, sustained substantial damage during a hard landing following a loss of tail rotor control near Hanford, California. The helicopter was registered to and operated by Central Valley Aviation Inc., Selma, California, under the provisions of 14 Code of Federal Regulations Part 91. The commercial pilot, sole occupant of the helicopter, was not injured. Visual meteorological conditions prevailed and no flight plan was filed for the positioning flight that was originating at the time of the accident.

During a telephone conversation, the pilot reported that upon completing a night time aerial application on a nearby field, he landed the helicopter on a platform that was mounted on a truck to refuel the helicopter and refill the chemical hopper. Following refueling operations, the pilot conducted a quick pre-flight inspection to look over the helicopter. Upon inspection of the transmission, the pilot observed a small oil leak originating where the tail rotor drive shaft attaches to the transmission. The pilot said that following a discussion with his boss, they decided to reposition the helicopter from the 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.

Postaccident examination of the helicopter by a Federal Aviation Administration (FAA) inspector and a representative from Hiller Aircraft revealed that the tail boom exhibited structural damage. One of the two tail rotor blades was separated at the blade root. The tail rotor gear shaft was found separated from the tail rotor drive shaft coupling. The gear shaft was found separated into two pieces at the threaded portion of the shaft. The gear shaft and tail rotor drive shaft coupling were removed from the wreckage and sent to the NTSB Materials Laboratory, Washington, D.C. for further examination.

An NTSB senior metallurgist reported that the external gears on the gear shaft and mating internal gears on the coupler showed no evidence of deformation damage. The gear shaft was fractured at the threaded portion. A nut and cotter pin remained attached to the fractured portion of the threaded area of the gear shaft. Bench binocular microscope examination of the thread fragment revealed the fracture face contained a twist fracture pattern consistent with torsion separation. A light brown region was found on one side of the fracture that extended between the root portion of the thread and an area located approximately 0.08 inch below the thread. The areas outside of the dark brown region exhibited a metallic luster. The fracture also showed ratchet marks typical of fatigue cracking that emanated from multiple origins at the root portion of the thread. The ratchet marks extended all around the thread. Scanning electron microscope examination of the thread fragment revealed the fracture face in the areas within the ratchet marks showed striation-like features and the twist fracture pattern at the center of the fracture showed striation-like features consistent with torsional-fatigue. For further details of the metallurgical examination, see the Materials Laboratory Factual Report within the public docket.

The pilot reported that the last annual inspection was conducted on the helicopter on June 14, 2011, at a total airframe time of 10,485-hours. No maintenance records for the helicopter were recovered during the course of the investigation. The amount of flight time on the tail rotor gear shaft was undetermined. A representative from Hiller Aircraft reported that the tail rotor gear shaft is subject to a 1,200 hour overhaul which includes magnetic particle inspection of the part.

NTSB 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.