Enstrom TH-180 N180TH accident near Menominee, Michigan

Probable cause

The failure of the torque meter mounting bolts, which resulted in a loss of engine power to the main drive belt and rotors.

NTSB analysis

The commercial pilot of the helicopter departed to conduct a flight test of an onboard data acquisition system. About 18 minutes later, the pilot decided to return to the airport because of weather moving into the area. On final approach, the pilot felt a "jolt in the airframe," and the helicopter yawed left and began to descend. The pilot added engine power, but the main rotor rpm continued to decay until the low rotor rpm horn sounded. The pilot entered an autorotation to a city street in a residential neighborhood. During the landing, the main rotor struck an electric utility pole and the helicopter slid 300 ft before coming to a stop.

A postaccident examination of the helicopter revealed that the eight mounting bolts for the engine torque meter had sheared, separating the engine from the main belt drive system. The torque meter was test equipment installed between the engine prop flange and a modified lower drive pulley for the purpose of gathering engine data during the flight tests. No other preaccident anomalies were found.

Text from the NTSB. Open the NTSB docket for CEN16LA105 for the full record.