HALL LESTER R LEGACY near Rochester, MN — 2023-07-20

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.

Contributing factors

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 narrative

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.

Analysis

Primary failure mode

Mechanical failure

First missed decision gate

Engine condition should have been assessed before purchase and flight.

NTSB coding

• Uncontained engine failure · Enroute

Evidence available

• 1 docket document

Docket documents1

• POWERPLANTS FACTUAL REPORTreport

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