What happened
During a nighttime aerial surveillance mission in support of law enforcement, a pilot was orbiting over a densely populated residential area at approximately 600 feet. During the maneuver, the Turbomeca Arriel 1D1 engine suffered a total loss of power.
In response to the engine failure, the pilot entered an autorotation and attempted an emergency landing on a lawn within the dimly lit residential neighborhood. During the descent, the helicopter struck a small-gauge residential power supply line that was positioned across the aircraft's glide path. Following the impact with the power line, the helicopter experienced a hard landing with a nose-down attitude, eventually coming to rest in the front yard of a residence. The aircraft sustained substantial damage to the fuselage, tail boom, and the main rotor hub assembly (starflex). There were no injuries reported.
The investigation
A post-accident examination of the engine revealed a fractured bevel gear (part number 0292107960) within the engine accessory gearbox. Metallurgical analysis of the component identified radial cracks between the gear teeth, which were consistent with a high cycle fatigue (HCF) phenomenon.
Investigation into the failure determined that the fracture was caused by fatigue propagation, though metallurgical and dimensional analyses found no specific manufacturing defects. The failure of this accessory bevel gear is known to cause the failure of both the fuel pump and the fuel control unit, resulting in a complete loss of engine power.