Incident Overview
During a flight, the N2 and NR parameters of a helicopter suddenly accelerated to 110 percent. The pilot responded by shutting down the engine and executing an autorotation. An open field was selected as the landing site, but the terrain was muddy and contained standing water. Choosing to perform a full stop landing rather than a slide-on landing to avoid a potential nose-over, the pilot touched down. Upon touchdown, the main rotor blades flexed downward and severed the tail boom.
Investigation Findings
An investigation into the incident revealed corrosion and foreign material in the engine's systems. Specifically, sand and aluminum hydroxide corrosion were found in the turbine governor, part number 2524706-3. Additionally, corrosion and foreign material were discovered in the fuel control unit, part number 2524644-1. Both components had accumulated only 2.5 hours of operation since being installed on the aircraft.
Further inspection of the governor determined that the PG channel in the drive body was completely blocked. This blockage, according to the investigation, would have resulted in an engine overspeed of up to 112 percent. The governor had been salvaged from a helicopter that had been involved in an accident in Brazil. The corrosion present was indicative of long-term environmental exposure.
The investigation did not explicitly state a probable cause, but the findings point to the blockage in the governor as a key factor in the overspeed event.