What happened
On April 27, 2006, at approximately 13:40 UTC, a Bell 206B helicopter, registration PT-HOQ, departed from Campo de Marte (SBMT) in São Paulo, Brazil. The aircraft was performing a visual flight mission to inspect an electrical power grid, carrying a pilot and two passengers.
During the flight, the aircraft experienced an engine failure. The pilot attempted an emergency landing in a densely populated urban area; however, due to the lack of suitable clearings for an autorotation, the helicopter struck the roof of a building in the Lapa neighborhood. The impact was high-energy, causing the aircraft to explode and catch fire. The pilot and both passengers sustained fatal injuries and died at the scene. The aircraft was a total loss.
The investigation
CENIPA investigators examined the wreckage and the engine, an Allison 250C2/20B. Analysis of the engine by Rolls-Royce revealed that the Spur Adapter Gearshaft (SAG) had fractured due to fatigue. The investigation found that during a previous overhaul performed by a maintenance facility, the required "Slab Head Bolts" were not installed on the flanges between the gas turbine, power turbine, and exhaust collector.
This omission compromised the concentricity of the bearings, creating significant vibrations. Records showed that while these vibrations were detected during a dynamometer test on April 3, 2006, the engine was released for service without a documented corrective action or proper quality control signatures. The engine had operated for only 34 hours and 40 minutes following the overhaul before the fatigue-induced failure occurred.