Amateur-built LEGACY N707MM incident near Rochester, Minnesota

Probable cause

Mechanically distressed reduction gears causing an engine overspeed and turbine blade tensile overload and subsequent uncontainment. Contributing to the uncontainment was the operator’s use of the engine after knowingly purchasing the engine in an unserviceable condition and only conducting a periodic inspection before flight.

NTSB analysis

The teeth of the 1st stage sun gear in the reduction gearbox were ground away in the plane of the mating gear teeth of the 1st stage planet gears. The mating gear teeth of the three planet gears were battered and fractured.

Distress of the 1st stage sun gear teeth released the torque normally transferred to the propeller, causing the power turbines to overspeed and the power turbine blades to release in tension and become uncontained.

It was unknown when the last inspection of the reduction gearbox was performed.

During the post incident examination of the engine at the P&WC facility, the engine was found to be in poor overall condition with missing surface protective paint and surface corrosion noted throughout the external and internal surfaces of the engine.

A review of the purchase documentation revealed that the operator knowingly purchased the engine in unserviceable condition and only performed a periodic inspection before flight.

Text from the NTSB. Open the NTSB docket for ENG23LA034 for the full record.