Trona, CA, USA
N912TT
Goodlett Safari
The experimental helicopter impacted terrain after experiencing a failure of a flight control connecting rod. According to the pilot, he was maneuvering the helicopter about 50 feet above the ground when he heard a loud bang followed by a whirring or buzzing noise. The helicopter rolled to the left, which the pilot was unable to counter with control inputs, and impacted terrain on the left side. A post-accident examination of the helicopter by the pilot/builder revealed that a 27.875-inch aluminum control rod was fractured near the middle of the rod. The control rod was connected between the collective-cyclic mixing assembly and a walking beam that connected to the stationary swash plate of the left side (pilot side) of the helicopter. The break in the control rod was perpendicular to the longitudinal axis of the rod. Examination of the control rod by a metallurgical laboratory revealed the failure was a result of a fatigue crack that had propagated through nearly 75 percent of the control rod. A portion of the break was polished indicating that the crack had been present for quite some time. The pre-existing crack in the control rod reduced its strength to a point that allowed the flight control forces to fail the remaining material. According to the pilot/builder, a break in the control rod would result in flight characteristics consistent with that experienced by the pilot during the accident flight. The control system has been redesigned by the kit manufacturer as a result of this accident and now incorporates steel control rods as opposed to aluminum ones.
On September 3, 2005, about 0830 Pacific daylight time, a Goodlett Safari amateur-built helicopter, N912TT, impacted terrain near the Trona Pinnacles, Trona, California. The helicopter was registered to and operated by the pilot/builder. The private pilot, the sole occupant, sustained minor injuries. The airplane was substantially damaged. The pilot was operating the helicopter as a personal aerial videography flight under the provisions of 14 CFR Part 91. Visual meteorological conditions prevailed, and a flight plan had not been filed for the local flight that departed the pilot's personal helipad, located in Ridgecrest, California, at 0757. According to the pilot, he departed his helipad and flew about 20 miles east to Trona to videotape the pinnacles. He was maneuvering the helicopter about 50 feet above the ground when he heard a loud bang followed by a whirring or buzzing noise. The helicopter rolled to the left, which he tried to offset with full right cyclic, to no avail. The helicopter continued to roll left and impacted terrain on the left side. According to the pilot, the main rotor blades were destroyed and the tail boom was bent down. Post-accident examination of the helicopter by the pilot/builder revealed that a 27.875-inch aluminum control rod was fractured near the middle of the rod. The control rod was connected between the collective-cyclic mixing assembly and a walking beam that connected to the stationary swash plate of the left side (pilot side) of the helicopter. The break in the control rod was perpendicular to the longitudinal axis of the rod. A portion of the break was "very clean (looking as if it was cut by a saw)" and the other portion was rough as if "it were broken by force." The builder believed that a pre-existing crack or stress-induced flaw in the control rod reduced its strength to a point that allowed the flight control forces to fail the remaining material. According to the pilot/builder, the helicopter underwent its last conditional inspection on September 12, 2004. The helicopter accumulated a total of 522.7 hours of operation at the time of the accident and accrued 59 hours since its last inspection. The pilot/builder sent the fractured control rod to the kit manufacturer (Canadian Home Rotors), and they, in turn, sent the fractured control rod to TestLabs International LTD., Winnipeg, Canada. The kit manufacturer requested that TestLabs perform a metallurgical failure analysis of the anodized aluminum flight control rod. The following outlines pertinent excerpts from their report and a description of photographs taken of the rod's fracture surfaces: The rod was a hollow tube measuring 0.5 inches at the outside diameter, 0.325 inches at the inside diameter, and 27.875 inches in length. The fracture surface was relatively flat with 25 percent displaying a flat polished region, 50 percent of the fracture exhibiting numerous parallel curved arches consistent with "progressive crack growth," and the remaining 25 percent displaying fracture features consistent with "final rapid fracture." The area including the parallel curved arches also displayed a black discoloration. Spot chemical analyses were conducted on the black discolored regions of the fracture surface and on the outer anodized surface of the control rod. The chemical composition of the black discolored regions of the fracture surface indicated significantly lower oxygen levels as compared to the outer anodized surface. In addition the anodized surface exhibited a substantial quantity of sulfur, whereas sulfur was not detected within the discoloration of the fracture surfaces (it was noted that sulfuric acid electrolyte is one of the common electrolytes used to anodize aluminum). According to the pilot/builder, a break in the control rod would result in flight characteristics consistent with that experienced by the pilot during the accident flight. According to the kit manufacturer, they have since switched to steel control tubes and have eliminated the walking beam that was originally placed between the collective-cyclic mixing assembly and the stationary swash plate. The reason provided by the manufacturer was that removing the walking beam lessoned the vibration that some of their kit builders had experienced. However, since the control rods were continuous from the mixing assembly to the stationary swash plate, and therefore longer, the manufacturer switched from the aluminum control rods to steel. The kit manufacturer indicated that many of their customers have already incorporated the changes.
the in-flight fatigue failure of a control system connecting rod, which resulted in a loss of helicopter control.
Source: NTSB Aviation Accident Database
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