On July 3, 2011, about 0630 Pacific daylight time, an Arrow Falcon Exporters, Inc., OH-58C helicopter, N6264D, sustained substantial damage when it impacted terrain near Brentwood, California. The commercial pilot, the sole occupant, was seriously injured. Aerial Control was operating the helicopter under the provisions of 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed for the aerial application flight, which had originated from Brentwood approximately 20 minutes before the accident. No flight plan had been filed. The pilot stated that he was applying an insecticide to a corn field and had just completed his first pass. He was making a reversal turn for another pass when he heard a “clanging sound” coming from above and behind his head. This was immediately followed by a “huge loud noise that sounded like something broke,” and the helicopter dropped abruptly about a foot. There was a brief pause, and then the helicopter’s drop continued. The pilot reacted by pulling back on the cyclic to bring the nose up, but the helicopter did not respond to his control inputs. The helicopter fell approximately 30 feet and impacted terrain. The pilot said that, after impact, the engine was still running. Postaccident examination of the wreckage revealed that the main rotor’s mast had fractured near the point where the mast extends from the transmission. The fracture occurred above the mast seal retainer plate in the section of the mast that was threaded to retain the mast nut. Both ends of the fracture were sent to Bell Helicopter’s Engineering Laboratories, where they were examined under the supervision of a Federal Aviation Administration (FAA) inspector. The fracture surface had a fatigue area that extended for about 26 percent of the mast’s circumference. The remaining portion of the fracture was identified as low cycle fatigue and a final section of overload. The fracture had multiple origins at corrosion pitting on the treads of the mast. The source of the corrosion was not determined. There were no inconsistencies noted in the microstructure of the alloy steel main rotor mast. The alloy steel material, which made up the mast, conformed to hardness and composition specifications, and all dimensional measurements met engineering drawing specifications. When the mast is installed on the helicopter, the location where the corrosion and fracture occured is hidden. To fully inspect the area, the mast assembly must be removed from the helicopter and completely disassembled. The helicopter was initially manufactured by Bell Helicopter for the U.S. Army in July 1972. Maintenance records indicated that, in November 2006, the helicopter received a 2,400-hour inspection, which included disassembly of the main rotor mast assembly and a magnaflux inspection of the components. Reassembly of the mast included the application of a 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 aircraft manufacturer. In November 2010, a 1,200-hour inspection was completed on the helicopter. This inspection did not include, nor was it required to include, disassembly of the main rotor mast assembly. At the time of the accident, the helicopter had accumulated 1,468 hours of flight time since the 2,400-hour inspection. A second mast failure on an Arrow Falcon Exporters OH-58A occurred on January 20, 2012, in Brawley, California (WPR12LA096). On February 20, 2012, Arrow Falcon issued Alert Service Bulletin 2012-58-01, which reduced the overhaul interval for the main rotor mast assemblies on its OH-58s from 2,400 hours 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 of these helicopters. The AD also required performance of specific inspections for a crack, pitting, or corrosion in the threaded area of the mast and associated parts.