CEN11LA646

While maneuvering the helicopter at an altitude of about 400 feet above ground level, the pilot heard the low rotor aural warning in his headset and observed that the rotor speed gauge was reading about 80 percent and decreasing. He lowered the collective, initiated a left turn, and began an autorotative descent to hilly terrain that contained trees and power lines. During the autorotation, the pilot attempted to avoid the power lines, and the helicopter impacted terrain. Although the pilot reported that he believed the engine lost power, a postaccident examination of the helicopter revealed no evidence of any preimpact mechanical failures or malfunctions. Additionally, the helicopter’s engine was run in a test cell for 1 hour 50 minutes with no anomalies detected.

On September 15, 2011, at 0915 central daylight time, a Bell 206B single-engine helicopter, N3181J, sustained substantial damage when it impacted terrain while maneuvering near Spicewood, Texas. The private pilot, who was the registered owner, and passenger were not injured. Visual meteorological conditions prevailed, and a flight plan was not filed for the 14 Code of Federal Regulations Part 91 personal flight. The local flight departed approximately 0815 from Spicewood Airport (88R), Spicewood, Texas. According to the pilot, he and the passenger departed 88R on a local pleasure flight with approximately 70 gallons of fuel. Approximately 1 hour into the flight and 4 miles southwest of 88R, the pilot reported hearing the low rotor aural warning in his headset and observed the rotor speed gauge to be reading approximately 80 percent and decreasing. At an altitude of approximately 400 feet above ground level, the pilot lowered the collective, initiated a left turn and began an autorotative decent to hilly terrain that contained trees and power lines. During the autorotation, the pilot attempted to avoid the power lines, and the helicopter impacted terrain. Subsequently, the helicopter rolled over and came to rest on its right side. The pilot reported that after the impact, the engine was still operating at an unknown power level, and he used the fuel cutoff to stop the engine. The pilot stated he believed the helicopter experienced an uncommanded reduction in engine power. The pilot reported the wind was calm and the temperature was 86 degrees Fahrenheit. Postaccident examination of the helicopter showed the tail boom had separated and the main rotor blades were bent and twisted. No visual external damage was noted on the Allison 250-C20B engine. The engine was covered in fire suppressant material, which was used by fire rescue for precautionary purposes due to the dry vegetation in the surrounding area. The engine fuel, oil, and pneumatic lines exhibited no visual damage and all connections were secure. The engine was removed and sent to Rolls-Royce Corporation for further examination. A review of the maintenance records revealed the helicopter underwent its most recent annual inspection on February 22, 2011. At the time of accident, the engine had accumulated 14,459 total hours and 107 hours since compressor and turbine section overhaul. On November 8, 2011, the engine was examined by the NTSB and Rolls-Royce Corporation. The N1 drive train remained rotationally free and N2 remained locked. The turbine area was flushed with water and approved alcohol solution at which time fire suppressant material was cleared and N2 became rotationally free. The engine was placed into a test cell for testing in accordance to Rolls-Royce production test specifications. The engine was test run for approximately 1 hour and 50 minutes with no anomalies noted. After the engine run, a Pc system check was performed and no leaks were detected.