Eurocopter EC120B N88HA accident near Goshen, Indiana

Probable cause

The failure of the Thomas coupling at the interface with the main gear box tail rotor output shaft, which resulted in the helicopter's loss of tail rotor control.

NTSB analysis

The flight instructor and helicopter-rated pilot were returning from an instructional flight in the helicopter. The flight instructor reported they heard a noise come from somewhere in the helicopter. They diverted to a nearby airport for a precautionary landing. As they approached the airport, the noise level increased. The instructor took the controls, and, about 15 ft above ground level and 25 knots, the helicopter began an uncommanded left yaw. The instructor responded with corresponding right pedal input; however, it did not stop the left yaw. The instructor immediately reduced engine power and initiated a hovering autorotation.

A postaccident examination revealed the tail rotor drive system's Thomas coupling interface with the main gear box tail rotor output shaft failed. The Thomas coupling was part of the installation of an after-market air conditioning system approved under a supplemental type certificate (STC). Laboratory examinations revealed that while the pulley within the tail rotor drive system was made to the STC holder's requirements, those requirements deviated significantly from the original equipment output flange. Changes were made to the spline profile, manufacturing method, and lubrication regimen. Any of these changes, either independently or collectively, likely contributed to the increased wear and failure of the Thomas coupling. After the accident, the manufacturer issued a service bulletin and the Federal Aviation Administration issued an emergency airworthiness directive that called for an immediate and repetitive inspection of the coupling every 25 flight hours or deactivating the air conditioning system by removing the STC's Thomas coupling and reinstalling the original equipment manufacturer's Thomas coupling half.

Text from the NTSB. Open the NTSB docket for CEN16LA047 for the full record.