Bell Helicopter Textron 206L-1 N571CJ accident near Walland, Tennessee

Probable cause

The improper installation of the No. 8 bearing spanner nut, which resulted in the No. 8 bearing’s failure, and the subsequent total loss of engine power.

NTSB analysis

About 35 minutes into the local sightseeing tour flight the pilot observed the engine chip light illuminate on the instrument panel. The pilot immediately began maneuvering toward a landing area and, within a few seconds, he heard an abnormal noise from the engine and saw the engine’s oil temperature and pressure indications decrease to zero. The pilot entered an autorotation and, shortly after, the engine stopped producing power. During the autorotation landing to a field, the helicopter touched down hard, skidded forward, and the main rotor blades contacted one of the vertical stabilizers, nearly severing the tailboom.

Subsequent examination of the helicopter’s engine revealed that no locking crimp had been made on the No. 8 bearing spanner nut during the last assembly, which allowed the nut to lose its torque, ultimately resulting in thermal damage and disintegration of the No. 8 bearing. This resulted in the compressor turbine shaft losing its retention and concentric location. The No. 8 bearing spanner nut had a partial crimp that exhibited a tool pry mark consistent with its removal during the last disassembly; however, it had not been blended away, nor had a new crimp been applied, consistent with the procedures described by the engine manufacturer’s overhaul manual. Additionally, the inside of the bearing’s sump cover showed rotational scoring consistent with contact with the No. 8 bearing spanner nut during engine operation, and rub damage on the power turbine support and 2nd stage turbine wheel, which was indicative of a loss of axial and radial retention of the gas producer rotor. This condition ultimately resulted in the total loss of engine power.

Text from the NTSB. Open the NTSB docket for ERA24LA076 for the full record.