Accident Overview
On October 1, 2023, at approximately 1230 central daylight time, a Robinson Helicopter Company R44, registration N379SH, was substantially damaged in an accident near Hochatown, Oklahoma. The pilot, who also held a flight instructor certificate, was not injured. The helicopter was being operated as a Part 91 personal flight.
Events During Flight
The pilot reported conducting a practice autorotation during a proficiency flight at an altitude of about 400 feet above ground level. Intending to simulate a loss of engine power, he reduced the engine speed to idle. At that moment, the engine lost total power. The pilot initiated an autorotation and executed a forced landing to a field. The landing was hard, resulting in the tailboom being severed by the main rotor blades. The pilot exited the helicopter without assistance.
Postaccident Examination
The helicopter sustained substantial damage to the tailboom and main rotor blades. A postaccident examination revealed that the carburetor heat control knob was jammed in the full heat position. Further inspection showed that the carburetor heat control cable was rigged such that the cable was pulled beyond the end of the cable conduit sleeve. When the carburetor heat knob was actuated, the control cable would bind with the conduit sleeve. No other anomalies were found within the rigging or flight controls that would have precluded normal operation.
Engine and System Analysis
Single contact marks on the fore and aft faces of the upper sheave and adjacent components were consistent with the engine not running at the time of impact. The V-belts and tail boom were cut and removed to facilitate an engine run. The engine started and ran at various power settings, but the idle was set to below 50 percent. The typical idle speed without V-belts is 60 to 65 percent. Robinson Helicopter Company Safety Notices 27 and 38 advise pilots on how to slowly reduce throttle while practicing autorotations. Notice 27 states, "throttle should be rolled off smoothly, never 'chopped.'"
Maintenance Records
A review of the helicopter maintenance records indicated that on August 25, 2022, the engine was overhauled and installed, and an annual inspection was completed. The last 100-hour inspection was completed on August 16, 2023. The helicopter had accumulated 44.2 hours since that inspection. Investigators were unable to determine the last time the carburetor control cable was rigged or when the engine idle was set.