What happened
On April 29, 2014, a Hughes 369A helicopter, registration N37929, was performing a mission to locate a radio-tagged Bighorn sheep near Bullfrog, Utah. While the aircraft was in a 50-foot hover over steeply sloped terrain, it experienced an uncommanded left yaw and a simultaneous drop in rotor RPM.
The pilot responded by lowering the collective and executing a hovering autorotation to a nearby rock ledge. Upon touching down on the ledge, the helicopter tipped over and tumbled approximately 60 feet downslope. The impact caused the tail boom to sever from the main fuselage. The pilot and a passenger, both of whom sustained 2 serious injuries, were able to exit the aircraft and request assistance using a cell phone and a personal locator beacon.
The investigation
Investigators examined the wreckage in a desert canyon and performed a detailed analysis of the engine and rotor systems. The examination of the Rolls Royce 250-C20B engine, the rotor drive system, and the fuel control unit revealed no mechanical anomalies that would have prevented normal operation. While the engine initially experienced hot starts during testing, it was eventually able to start and produce full power through a complete acceptance test program.
Technical representatives from the manufacturer found no condition in the fuel control unit that would cause a sudden reduction in fuel flow. Additionally, the aircraft's performance review indicated there was sufficient power available for the mission's weight and atmospheric conditions. The damage to the main rotor blades was determined to be consistent with low rotor energy at the time of impact.