Accident Details
On June 23, 2011, at about 0400 Pacific daylight time, a Hiller UH-12E helicopter (registration N99579) sustained substantial damage during a hard landing near Hanford, California. The helicopter was operated by Central Valley Aviation Inc. under 14 Code of Federal Regulations Part 91. The commercial pilot, who was the sole occupant, was not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the positioning flight.
Sequence of Events
According to the pilot, after completing a nighttime aerial application on a nearby field, he landed the helicopter on a platform mounted on a truck to refuel and refill the chemical hopper. Following refueling, the pilot conducted a quick pre-flight inspection and observed a small oil leak where the tail rotor drive shaft attaches to the transmission. After consulting with his boss, they decided to reposition the helicopter from the platform onto the ground for a more detailed examination. During a normal engine start, as the pilot began lifting off the platform, he heard a "growling" sound from the transmission, followed by an immediate loss of tail rotor control. The helicopter began to rotate to the right, and the pilot rolled off the throttle and lowered the collective. The helicopter then landed hard on the ground adjacent to the truck.
Postaccident Examination
A Federal Aviation Administration inspector and a Hiller Aircraft representative examined the wreckage. 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 fractured into two pieces at the threaded portion. The gear shaft and coupling were sent to the NTSB Materials Laboratory for further examination.
Metallurgical Analysis
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. The gear shaft was fractured at the threaded portion, with a nut and cotter pin still attached to the fractured area. Bench binocular microscope examination revealed a twist fracture pattern consistent with torsion separation. A light brown region was found on one side of the fracture, extending between the thread root and an area about 0.08 inch below the thread. Areas outside this region had a metallic luster. The fracture displayed ratchet marks typical of fatigue cracking emanating from multiple origins at the thread root. Scanning electron microscope examination showed striation-like features within the ratchet marks and in the center twist fracture pattern, consistent with torsional-fatigue.
Maintenance Information
The pilot reported that the last annual inspection was conducted on June 14, 2011, at a total airframe time of 10,485 hours. No maintenance records were recovered. The amount of flight time on the tail rotor gear shaft was undetermined. A Hiller Aircraft representative stated that the tail rotor gear shaft is subject to a 1,200-hour overhaul that includes magnetic particle inspection.