On March 17, 2001, about 2134 pacific standard time, an American Eurocopter AS350-B2 helicopter, N996PD, sustained substantial damage following a loss of engine power, and a forced landing in a residential area in Hayward, California. The helicopter collided with a set of small gauge power lines during the emergency descent. The helicopter was being operated as a visual flight rules (VFR) public use law enforcement flight under Title 14, CFR Part 91 when the accident occurred. The East Bay Regional Park District police department operated the helicopter. The airline transport certificated pilot and the two passengers were not injured. Visual meteorological conditions prevailed at the time of the accident, and company flight following procedures were in effect. The accident occurred during dark night conditions. The flight originated at the Hayward Executive Airport, Hayward, about 1955. According to the operator's written report, the purpose of the flight was to conduct nighttime aerial surveillance in support of law enforcement activities over the city of Hayward. The operator reported that as the pilot orbited the helicopter about 600 feet above a residential area while in search of a suspect, all engine power was lost. The pilot entered an autorotation and attempted an emergency landing on a residential lawn, located within the dimly lit residential area. During the emergency descent, just before touchdown, the helicopter struck a small gauge residential power supply line that was stretched across the glide path. After striking the power line the pilot attempted to cushion the landing, but the helicopter landed hard, and nosed down. The helicopter came to rest upright, in the front lawn of a residence. It sustained substantial damage to the main rotor hub assembly (starflex), tail boom, and fuselage. The accident helicopter was subsequently recovered from the accident site, and transported to the operator's maintenance facility. The engine was removed and shipped to the engine manufacturer. The accident helicopter was equipped with a Turbomeca Arriel 1D1 turbo shaft engine, which is comprised of several modules/components. Module 1 includes the accessory gearbox section, the fuel control unit, which is a hydro-mechanical design, and the freewheeling unit shaft and power shaft housed in a liaison tube that runs longitudinally under the engine between the accessory case and the reduction gearbox. Module 2 is comprised of the axial compressor wheel. Module 3 includes the centrifugal compressor, combustion chamber, and two gas generator turbines. Module 4 is the power (free) turbine wheel and shaft. Module 5 is the reduction gearbox. TEST AND RESEARCH On March 27, 2001, a postaccident disassembly and examination of the accident engine was conducted at Turbomeca's analytical facility in Grand Prairie, Texas. In attendance was a Federal Aviation Administration inspector from the Rotorcraft Directorate, Ft. Worth, Texas, and air safety investigators from American Eurocopter Corporation and Turbomeca. The inspection revealed a fractured bevel gear, part number 0292107960, located in the engine's accessory gearbox, in Module 1. According to an air safety investigator from Turbomeca, the fracture of the bevel gear will disable all mechanically driven accessories, including the fuel control unit and the fuel pump, resulting in a loss of engine power. The operator reported that the fractured bevel gear had accumulated about 1,025 hours in operation. During the course of the investigation, Turbomeca disclosed that they had experienced three separate events of bevel gear fractures in Arriel series engines. At the direction of the National Transportation Safety Board (NTSB) investigator-in-charge (IIC) the fractured bevel gear was removed from the accident engine and sent to Turbomeca's materials laboratory in France for examination. Metallurgical Examination A Turbomeca metallurgist stated in a written report dated May 29, 2001, that the material used to construct the bevel gear met the specification requirements for hardness and microstructure. Additionally, the metallurgist reported that the fractured bevel gear displayed evidence of radial cracks between the teeth of the gear, which was consistent with high cycle fatigue (HCF) phenomenon. A complete copy of Turbomeca's metallurgical examination report is included in the public docket for this accident. Additional Information On June 28, 2001, Turbomeca issued a service letter to all operators of Arriel engines. The service letter reported on the four previous bevel gear failures, and outlined an inspection method for engines currently in service, suggesting: At each return to a Turbomeca approved service repair center, the bevel gear will be checked systematically. The service letter reported, in part: "The fracture was the result of a fatigue propagation. Metallurgical and dimensional analyses have not revealed any defect." The helicopter was released to the operator on March 17, 2001, and the engine was released to the operator on July 9, 2001.