Accident Overview
On July 3, 2011, about 0630 Pacific daylight time, an Arrow Falcon Exporters, Inc., OH-58C helicopter, registration N6264D, was substantially damaged when it impacted terrain near Brentwood, California. The commercial pilot, the sole occupant, was seriously injured. Aerial Control operated the helicopter under 14 Code of Federal Regulations Part 137 for an aerial application flight. Visual meteorological conditions prevailed, and the flight had originated from Brentwood approximately 20 minutes before the accident. No flight plan was filed.
Pilot Account
The pilot reported that he was applying insecticide to a corn field and had just completed his first pass. While making a reversal turn for another pass, he heard a “clanging sound” from above and behind his head, immediately followed by a “huge loud noise that sounded like something broke.” The helicopter dropped abruptly about one foot, paused briefly, then continued dropping. The pilot pulled back on the cyclic to raise the nose, but the helicopter did not respond. The helicopter fell approximately 30 feet and impacted the ground. After impact, the engine was still running.
Examination Findings
Postaccident examination of the wreckage revealed that the main rotor mast had fractured near the point where the mast extends from the transmission. The fracture occurred above the mast seal retainer plate in the threaded section that retains the mast nut. Both ends of the fracture were sent to Bell Helicopter’s Engineering Laboratories and examined under Federal Aviation Administration (FAA) supervision. The fracture surface had a fatigue area extending about 26 percent of the mast’s circumference. The remaining portion was identified as low cycle fatigue and a final overload section. Multiple origins at corrosion pitting on the mast threads were found. The source of the corrosion was not determined. No inconsistencies were noted in the alloy steel microstructure, and the material conformed to hardness, composition, and dimensional specifications.
When the mast is installed on the helicopter, the fracture location is hidden. Full disassembly of the mast assembly is required to inspect that area.
Maintenance History
The helicopter was initially manufactured by Bell Helicopter for the U.S. Army in July 1972. Maintenance records show that in November 2006, the helicopter received a 2,400-hour inspection that included disassembly of the main rotor mast assembly and a magnaflux inspection of components. Reassembly included application of sealant to the mast threads for corrosion protection. On January 19, 2007, the helicopter was certificated as a civil aircraft in the restricted category, with Arrow Falcon Exporters listed as the manufacturer.
In November 2010, a 1,200-hour inspection was completed but did not include disassembly of the main rotor mast assembly, nor was it required. At the time of the accident, the helicopter had accumulated 1,468 flight hours since the 2,400-hour inspection.
Subsequent Actions
A second mast failure on an Arrow Falcon Exporters OH-58A occurred on January 20, 2012, in Brawley, California. On February 20, 2012, Arrow Falcon issued Alert Service Bulletin 2012-58-01, reducing the overhaul interval for main rotor mast assemblies on its OH-58 helicopters from 2,400 to 1,200 hours. On July 23, 2012, the FAA issued Airworthiness Directive (AD) 2012-14-11, applicable to Arrow Falcon Exporters, Rotorcraft Development Corporation, and San Joaquin Helicopters model OH-58A, OH-58A+, and OH-58C helicopters. The AD required, within 30 days, overhaul of the main rotor mast assemblies and specific inspections for cracks, pitting, or corrosion in the threaded area of the mast and associated parts.