Star, ID, USA
N49SJ
de Havilland DHC-6-300
The pilot reported that during the landing roll, the aircraft began to veer to the left. When aileron and rudder control were not effective to re-align the aircraft's direction of travel, the pilot transferred over to the nose wheel steering control. The pilot applied corrective input all the way to the right when he heard a "clink" sound. The aircraft exited the side of the runway and collided with the terrain. Post-accident inspection of the aircraft found that the nose wheel steering cable was broken. The cable was sent to the National Transportation Safety Board Materials Laboratory for examination. The specialist reported that the end of many wires exhibited fractures on a flat plane that was perpendicular to the longitudinal axis, typical of a fatigue crack. About 90 percent of the flat areas extended through the fracture face. The metallurgist reported that approximately 50 percent of the wires from the cable contained fatigue cracking.
HISTORY OF FLIGHT On May 1, 2001, approximately 1030 mountain daylight time, a deHavilland DHC-6-300, N49SJ, registered to Department of Interior, Office of Aircraft Services (OAS) and operated by the Bureau of Land Management as a public use flight, was substantially damaged when the aircraft veered off the runway at Snake River Skydiving airport, Star, Idaho, and collided with the terrain. Visual meteorological conditions prevailed and no flight plan was filed. Neither the commercial pilot nor the passenger were injured. The flight originated from Boise, Idaho, approximately 0933. Personnel from OAS reported that the purpose of the flight was for smoke jumper training. After the smoke jumpers (12 jumpers) exited, the pilot landed the aircraft at the Snake River Skydiving airport. The pilot reported during an interview, that prior to landing, he confirmed that the nose wheel steering control was centered, and that the runway was clear of cows. He also noted that the windsock at the end of the runway was indicating a crosswind. The aircraft touched down on the grass overrun surface and rolled about 300 feet to the start of the pavement surface without incident. The pilot reported that he noted that the airspeed was about 70 knots at the beginning the of pavement surface and that the aircraft was tracking straight. The pilot applied reverse power at about 60 knots and the aircraft began to veer to the left. The pilot reported that up to this point he was using rudder and aileron inputs, however now they were not effective and the aircraft continued to arc to the left. The pilot then transferred over to the nose wheel steering tiller control and applied pressure to the right. The pilot continued to increase pressure to the nose wheel steering tiller all the way to the right when he heard a "planking sound," or a "clink." The aircraft then exited the side of the runway and collided with the terrain. DAMAGE TO AIRCRAFT The nose gear sheared off and the left engine propeller contacted the ground. A cow located off to the side of the runway was also struck at some point along the ground track. Maintenance personnel inspecting the aircraft reported that the nose wheel steering tiller control cable was broken. WRECKAGE AND IMPACT INFORMATION Measurements were taken of the ground signatures that remained visible on the grass overrun surface and the skid marks found on the runway pavement. Ground disturbance on the grass indicated that the aircraft touched down about 330 feet prior to the beginning of the pavement surface. About 270 feet after the beginning of the pavement surface, rubber marks were identified down the center of the runway. About 200 feet further, the left arc begins and travels another 200 feet before the aircraft exits the side of the runway. The aircraft traveled through the dirt along the side of the runway for about 150 feet until the nose wheel separated and the aircraft slid to a stop. TESTS AND RESEARCH The upper and lower torque arms, the entire length of nose wheel steering control cable and steering collar were sent to the National Transportation Safety Board Materials Laboratory in Washington DC for examination. The upper and lower torque arms separated at the hinge portion of the lower torque link. The metallurgist reported that the fracture face revealed ductile dimple features typical of overstress separation. The collar was not cracked. The steering cable was examined with a scanning electron microscope (SEM). The examination revealed that the end of many wires exhibited fractures on a flat plane that was perpendicular to the longitudinal axis, typical of a fatigue crack. About 90 percent of the flat areas extended through the fracture face. The metallurgist reported that approximately 50 percent of the wires from the cable contained fatigue cracking. ADDITIONAL INFORMATION The flight manual for the DHC-6 approach procedures indicate that the nose wheel steering lever is to be centered and aligned with the index marks. For normal landing procedures, the manual indicates that after touch down on the main wheels, brakes are to be applied after the nose wheel makes contact. Reverse power as required and nose wheel steering as required. Crosswind landing procedures indicate that: "The preferred technique requires that the upwind wing be lowered during the approach with sufficient opposite rudder applied to align the aircraft with the runway. As airspeed decreases during the flare and rollout, both of these control applications must be increased. The nose wheel should be held on the ground during the ground roll, along with "into wind" aileron. Directional control should be maintained with rudder. Nose wheel steering and brakes should be used only at taxi speeds." The aircraft wreckage was released to the Air Safety Investigator participant from the Office of Aircraft Services on June 14, 2001. The upper and lower torque arms, nose wheel steering cable, collar and steering actuator assembly were released to the same investigator on September 6, 2001.
Fatigue failure of the nose wheel steering cable during the landing roll. Rough/uneven terrain was a factor.
Source: NTSB Aviation Accident Database
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