CEN16LA105

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.

On February 12, 2016, about 1123 central standard time, an Enstrom TH-180 helicopter, N180TH, was substantially damaged after it impacted obstructions and terrain during an off-airport emergency landing near the Menominee-Marinette Twin County Airport (MNM), Menominee, Michigan. The pilot was not injured. The helicopter was registered to and operated by Enstrom Helicopter Corporation, as a 14 Code of Federal Regulations Part 91 local test flight. Visual meteorological conditions (VMC) prevailed and a flight plan had not been filed. The helicopter had departed MNM, about 1057.The purpose of the test flight was to ensure the data acquisition system was operating properly. The pilot departed to the north, but because of snow squalls moving into the area, decided to return to the airport. The pilot turned off the data acquisition computer and because of the westerly wind, set up for a right base to runway 32. On final approach, he felt a "jolt in the airframe"; the helicopter yawed left and began to descend. The pilot added throttle, which increased engine rpm but had no effect on the rotor rpm. The low rotor rpm horn sounded, and the pilot entered an autorotation to a city street in a residential neighborhood. During touchdown the main rotor blades struck an electric utility pole as the helicopter slid about 300 feet down the street and came to rest upright. The impacts resulted in the collapse of the skid landing gear and substantial damage to the tail cone, the pylon center section, and the lower section of the cockpit structure. Adequate fuel was found onboard, but there was no fuel spill, and no postimpact fire. A postaccident examination of the helicopter revealed the 8 mounting bolts for the engine torque meter had sheared separating the engine from the main belt drive system. The torque meter was special test equipment installed between the engine prop flange and a modified lower drive pulley. It was installed to gather engine data during the flight tests. No other preaccident anomalies were found. An onboard GoPro Hero point-of-view digital camera was center-mounted in the rear of the cockpit and positioned such to provide an over-the-shoulder view of the helicopter flight controls and instrument panel. The micro secure digital (SD) card was removed from the camera and sent to the National Transportation Safety Board Recorders Laboratory for read out. The micro SD card contained several still image files that appeared to be captured in a time-lapse photo mode. Metadata showed the still images were captured every 10 seconds. For additional details and information, see the NTSB Onboard Image Recorder Specialist's Factual Report. At 1115 the automated surface observation system at MNM, recorded wind from 330 degrees at 13 knots with gusts to 24 knots, visibility 10 miles, broken clouds at 4,300 feet, temperature minus 10 degrees Celsius (C), dew point minus 19 degrees C, with an altimeter setting of 30.06 inches of mercury.