Incident Overview
A helicopter conducting nighttime aerial surveillance for law enforcement activities experienced a complete loss of engine power while orbiting approximately 600 feet above a densely populated residential area. The pilot initiated an autorotation and attempted an emergency landing on a residential lawn within the dimly lit area. During the descent, the helicopter struck a small-gauge residential power supply line that crossed the emergency glide path. After contacting the power line, the helicopter landed hard, pitched nose-down, and came to rest on the front lawn of a residence.
Aircraft Damage
Post-accident inspection revealed substantial damage to the main rotor hub assembly (starflex), tail boom, and fuselage.
Engine Examination
Examination of the Turbomeca Arriel 1D1 turboshaft engine identified a fractured bevel gear (part number 0292107960) located in the engine accessory gearbox. Metallurgical analysis of the failed gear disclosed radial cracks between the teeth, consistent with high cycle fatigue (HCF). Turbomeca subsequently issued a service letter to all operators of Arriel engines, outlining an inspection method for in-service engines. The service letter stated: "The fracture was the result of a fatigue propagation. Metallurgical and dimensional analyses have not revealed any defect."
Operational Consequences
The failure of the accessory bevel gear caused the failure of the fuel pump and fuel control unit, leading to the loss of engine power.