Introduction
On September 3, 2009, at approximately 1300 Pacific daylight time, a Robinson Helicopter R44, registration N8361X, was substantially damaged during a practice autorotation at Zamperini Field Airport (TOA) in Torrance, California. The helicopter was registered to Business Financial Services Corp., Wilmington, Delaware, and operated by Universal Air Academy, La Verne, California, under Title 14 Code of Federal Regulations Part 91. The commercial pilot, the sole occupant, was not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the personal cross-country flight, which originated from La Verne, California, at 1215 with an intended destination of Torrance.
Accident Sequence
According to a written statement from the pilot, 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 began recovery by “slowly rolling on throttle.” He reported noticing the rotor RPM “increase quickly” and, due to the close proximity to the ground, performed “a landing in the safest manner I could on the helipad.” After landing, the pilot shut the helicopter down normally.
Examination Findings
Examination by a Federal Aviation Administration (FAA) inspector revealed structural damage to the tail boom and separation of the tail rotor drive shaft at the forward attach point. The helicopter was transported to a secure location for further examination.
Subsequent examination of the airframe and engine by Textron Lycoming Engines and Robinson Helicopters, conducted at Robinson Helicopter facilities in Torrance under FAA supervision, found that flight control continuity was established throughout the cyclic, collective, and tail rotor systems. The governor controller was tested functionally and operated within service limits; after reinstallation, a signal generator test confirmed proper RPM response. The governor clutch functioned normally.
The drive belts were intact and remained in their grooves with no evidence of rolling or jumping. The sprag clutch locked and unlocked normally when rotated by hand. The tail rotor drive shaft was bent and separated in multiple locations, exhibiting scoring around the circumference at regularly spaced intervals. The damper assembly was heavily damaged and rotated freely when removed. The intermediate flex coupling assembly was damaged, while the forward and aft flex couplings were intact. No mechanical anomalies were noted with the airframe that would have precluded normal operation.
The engine was intact and undamaged. Rotational continuity was established, and thumb compression was obtained on all six cylinders. After replacing a defective starter, the engine started successfully and ran at various power settings; a magneto check showed a 10 percent RPM drop in both left and right positions. No mechanical anomalies were observed with the engine that would have precluded normal operation.
Additional Information
In a written statement to the NTSB investigator-in-charge, a Robinson Helicopter representative reported that Technical Report 430 (RTR 430) describes critical shafting speeds for the R44, identifying four different “whirl modes” with associated critical speeds. 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 indicate that whirl modes are controlled within the normal operating speed range, but an additional whirl mode frequency exists at approximately 134% RPM, which is outside the normal operating speed range and is not damped by the damper bearing assembly. This situation is similar for the R22 model helicopter. The representative noted two prior occurrences where engine and rotor RPM reached excessively high speeds, resulting in damage to multiple tailcone bulkheads, the tail rotor drive shaft (including scoring at spacing correlating with the bulkheads), the damper assembly, the tail rotor push-pull tube, wiring, and the intermediate flex coupling.