Primary finding
Probable cause
A total loss of engine power during a practice autorotation due to the improper engine idle setting and rigging of the carburetor heat control which resulted in a hard landing.
Investigator assessment
Analysis narrative
The pilot reported that he was conducting a practice autorotation during a proficiency flight about 400 ft above ground level (agl). As he reduced the engine speed to idle, the engine lost total power. The pilot entered an autorotation and completed a forced landing to a field. The helicopter landed hard and the tailboom was severed by the main rotor blades. Examination of the engine revealed that the idle speed was set below 50 percent, which was below the manufacturer’s recommendation of 60 to 65 percent. Additionally, the carburetor heat control cable was rigged improperly, and therefore, was jammed in the full heat position. The combination of these factors would have likely made the engine more susceptible to losing power, especially with an abrupt power reduction initiated by the pilot during the simulated autorotation. After reviewing the available maintenance records, it is possible the engine idle speed was improperly set, and the carburetor heat control was improperly rigged when the engine was overhauled and installed in the helicopter; however, investigators were unable to establish exactly when the misrigging took place or why it was not noticed during subsequent maintenance.
Source record
Factual narrative
On October 1, 2023, about 1230 central daylight time, a Robinson Helicopter Company R44, N379SH, was substantially damaged when it was involved in an accident near Hochatown, Oklahoma. The pilot was not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot, who was also a flight instructor, reported that he was conducting a practice autorotation during a proficiency flight about 400 ft agl. As he reduced the engine speed to idle to simulate a loss of engine power, the engine lost total power. The pilot entered an autorotation and completed a forced landing to a field. The helicopter landed hard and the tailboom was severed by the main rotor blades. The pilot exited the helicopter unassisted. The helicopter sustained substantial damage to the tailboom and the main rotor blades. A postaccident examination revealed that the carburetor heat control knob was jammed in the full heat position. Further examination revealed that the carburetor heat control cable was rigged so that the cable was pulled beyond the end of the cable conduit sleeve (figure 1). When the carburetor heat knob was actuated, the control cable would bind with the conduit sleeve. Figure 1: Carburetor heat control cable as found. The cable is free from the conduit sleeve (circled in yellow). Figure 2 shows a normal engagement of the carburetor heat control cable in the conduit sleeve. Figure 2. The normal engagement of the carburetor heat cable and conduit. Cable is inserted into the conduit sleeve (circled in yellow). No other anomalies were found within the rigging or flight controls that would have precluded normal operation. Single contact marks on the fore and aft faces of the upper sheave and adjacent components are 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. The idle was set to below 50 percent (the typical idle speed without V-belts is 60-65 percent). Robinson Helicopter Company Safety Notices 27 and 38 advise on how to slowly reduce throttle while practicing autorotations. RHC Safey Notice 27 states that during simulated power failures, “throttle should be rolled off smoothly, never ‘chopped.’” A review of the helicopter maintenance records revealed that on August 25, 2022, the engine was overhauled and installed, and an annual inspection of the helicopter was completed. The last 100-hour inspection was completed on August 16, 2023. The helicopter had accumulated 44.2 hours since the 100-hour inspection. Investigators were unable to determine the last time the carburetor control cable was rigged or when the engine idle was set.