Torrance, CA, USA
N8361X
ROBINSON HELICOPTER R44
The pilot reported that he initiated a practice autorotation to a paved helipad. As the helicopter descended through about 100 feet agl, the pilot initiated a recovery by “slowly rolling on throttle.” The pilot stated that as the helicopter descended, he noticed the rotor RPM “increase quickly.” Subsequently,the helicopter landed on the helipad uneventfully. Post accident examination of the helicopter revealed that the tail boom of the helicopter was structurally damaged, and the tail rotor drive shaft was bent and separated in multiple locations. The drive shaft exhibited scoring around the circumference at regularly spaced intervals on the shaft. The damper assembly was heavily damaged, and the intermediate flex coupling assembly was damaged. According to the helicopter’s manufacturer, this damage to the tail rotor drive shaft is indicative of a rotor over speed to at least 134%. No pre-accident mechanical anomalies were noted with the engine or airframe that would have precluded normal operation.
On September 3, 2009, about 1300 Pacific daylight time, a Robinson Helicopter R44, N8361X, was substantially damaged during a practice autorotation at the Zamperini Field Airport (TOA), Torrance, California. The helicopter was registered to Business Financial Services Corp., Wilmington, Delaware, and operated by Universal Air Academy, La Verne, California, under the provisions of Title 14 Code of Federal Regulations Part 91. The commercial pilot, sole occupant of the helicopter, was not injured. Visual meteorological conditions prevailed and no flight plan was filed for the personal flight. The cross-country flight originated from La Verne, California at 1215 with an intended destination of Torrance. In a written statement, the pilot reported that he initiated a practice autorotation to the northern helicopter pad from an altitude of 800 feet mean sea level (msl) at an airspeed of 65 knots and rotor RPM of 100 percent. As the helicopter descended through about 100 feet above ground level (agl), the pilot initiated a recovery by “slowly rolling on throttle.” The pilot stated that as the helicopter descended, he noticed the rotor RPM “increase quickly” and that due to the close proximity to the ground, he performed “a landing in the safest manner I could on the helipad.” After landing, the pilot shut the helicopter down normally. Examination of the helicopter by a Federal Aviation Administration (FAA) inspector revealed that the tail boom of the helicopter was structurally damaged. The tail rotor drive shaft was separated at the forward attach point. The helicopter was transported to a secure location for further examination. Examination of the airframe and engine by Textron Lycoming Engines and Robinson Helicopters was conducted at the facilities of Robinson Helicopter, Torrance, California under the supervision of the Federal Aviation Administration. Examination of the airframe revealed that flight control continuity was established throughout the cyclic, collective, and tail rotor systems. The governor controller was removed from the airframe and placed on a test bench. A functional test was performed and the governor controller operated within service limits. The governor controller was reinstalled in the airframe. Using a signal generator in place of tachometer points, the governor controller was functionally tested. The RPM on the engine tachometer matched the input from the signal generator. When the signal generator input a low RPM signal, the governor motor advanced the throttle. When the signal generator input a high RPM signal, the governor motor reduced the throttle. The governor clutch functioned normally. The drive belts were intact and remained within their grooves. No evidence of rolling or jumping was observed. The sprag clutch locked and unlocked normally when rotated by hand. The tail rotor drive shaft was bent and separated in multiple locations. The driveshaft exhibited scoring around the circumference at regularly spaced intervals on the shaft. The damper assembly was heavily damaged and rotated freely when removed from the aircraft. The intermediate flex coupling assembly was damaged. The forward and aft flex couplings were intact. No mechanical anomalies were noted with the airframe that would have precluded normal operation. Examination of the engine revealed that the engine was intact and undamaged. The top and bottom spark plugs were removed. The crankshaft was rotated by hand using the engine-cooling fan. Rotational continuity was established throughout and thumb compression was obtained on all six cylinders. The spark plugs were reinstalled within their respective positions. Utilizing the helicopter’s fuel source, an attempt to start the engine was unsuccessful. Examination of the engine starter revealed that the starter was defective and emitted a grinding sound. The starter was replaced with a serviceable unit. Using standard starting procedures, the engine was started successfully and run at various power settings. A magneto check was performed and produced a 10 percent drop in rpm in the left and right positions. No mechanical anomalies were observed with the engine that would have precluded normal operation. In a written statement to the NTSB investigator-in-charge, Robinson Helicopter representative reported that within the Robinson Technical Report (RTR) 430, critical shafting speeds for the R44 are described. Four different "whirl modes" are identified along with the critical speeds that are associated with them. Robinson Helicopter stated that the damper bearing assembly is designed to control these vibration modes within the "normal speed operating range" of the tail rotor driveshaft. Testing and analysis within RTR 430 show that the whirl modes are controlled within the normal operating speed range of the R44 and that there is an additional whirl mode frequency that is not damped by the damper bearing assembly. Robinson Helicopter further stated that this whirl mode occurs at approximately 134% rpm and outside of the normal operating speed range of the R44. They also noted that this situation is similar for the R22 model helicopter. Robinson Helicopter reported that they have seen within two prior occurrences where engine and rotor rpm has been allowed to reach excessively high speeds, damage was observed to multiple bulkheads inside the tailcone, tail rotor drive shaft (including scoring at spacing that correlates with the tailcone bulkheads), damper assembly, tail rotor push pull tube, wiring, and intermediate flex coupling.
A main rotor overspeed due to the pilot’s failure to maintain rotor RPM within allowable limits during a practice autorotation.
Source: NTSB Aviation Accident Database
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