CAMDEN, AL, USA
N9086T
Continental Copters TOMCAT MK5A
The pilot had completed a second pass over the field he was spraying when he lost cyclic control of the helicopter. He reduced power and the helicopter impacted the ground, coming to rest on its right side. The helicopter's cyclic control rod was metallurgically examined. One end of the cyclic control rod had separated at the thread relief radius and the failure appeared consistent with an overstress separation. Hardness readings taken during an ultrasonic inspection revealed the tensile strength was below the original rod design strength.
On July 14, 1999, at 1930 eastern daylight time, a Continental Copters Inc. Tomcat MK5A helicopter, N9086T, collided with the ground following a loss of cyclic control during a low altitude aerial application maneuver in Camden, Alabama. The helicopter was operated by Farm Flying Service, Inc. under the provisions of Title 14 CFR Part 137, and visual flight rules. Visual meteorological conditions prevailed, and no flight plan was filed for the local agricultural flight. The commercial pilot received minor injuries, and the helicopter sustained substantial damage. The flight originated from Camden, Alabama, at an unknown time. According to the pilot, he had just completed a second pass over the field he was spraying when he lost cyclic control of the helicopter. He stated that he reduced power and the helicopter impacted the ground, coming to rest on its right side. The helicopter's cyclic control rod (P/N 074-140-843-11) was sent to the NTSB's Materials Laboratory for analysis. One end of the cyclic control rod had separated at the thread relief radius, and the other end was bent at the thread relief radius. The fractured end of the control rod was examined. The fractured face appeared perpendicular to the control rod axis for approximately 50 percent of the diameter. The fractured face then reversed direction, leaving a lip "typical of compressive bending." According to the report, the "surface features on the fracture face appeared to be consistent with an overstress separation." The separated end of the control rod was removed and ultrasonically inspected. The surface of the fracture face revealed "ductile dimples indicative of an overstress situation." Hardness readings taken from the flats of the separated end ranged from a tensile strength of 101,000 to 116,000 psi (pounds per square inch), to that of 99,000 to 117,000 psi on a different scale. According to the control rod drawing (074-140-843-9) supplied by the Texas Helicopter Company, both readings were below the original rod strength of 125,000 to 145,000 psi.
The failure of the cyclic control rod due to an overstress separation resulting from a material defect (inadequate quality of material) by the manufacturer.
Source: NTSB Aviation Accident Database
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