Branson, MO, USA
N6628K
Windler Tri Q2
The airplane was destroyed when it impacted trees and terrain during an uncontrolled descent, which resulted from an in-flight separation of the right elevator. A witness reported that the airplane appeared to be in a descent as if approaching to land, when something separated from the airframe. A second witness reported the airplane banked to the right, "winged over," and began a clockwise descending spiral. The right elevator was recovered approximately 1,250 feet from the accident site. Examination of the elevator revealed corrosion and fatigue failure of the mid span hinge pivot pin. The examination also noted surface corrosion on the mid span hinge pin in the area of the fatigue failure. Failure of the mid span hinge pin allowed the control surface to buckle, which subsequently fractured the integral torque tube running along the leading edge of the elevator. Flexing of the control surface permitted the outboard end of the torque tube to slide free of the outboard hinge support. This allowed the elevator to separate from the airframe. In addition, the torque tube cutout at the mid span hinge fitting appeared to be oversized from that shown in the aircraft construction plans. The cutout of the torque tube at the mid span hinge support extended around approximately 120 degrees at the forward side of the tube. According to the plans, the cutout for the hinge was shown to be approximately 80 degrees. The reduced cross section on the torque tube due to the oversized cutout resulted in increased stresses in the area of the cutout.
On September 22, 2006, about 0910 central daylight time, an amateur-built Windler Tri Q2, N6628K, piloted by a private pilot, was destroyed during an in-flight collision with trees and terrain near Branson, Missouri. The personal flight was being conducted under 14 CFR Part 91 without a flight plan. Visual meteorological conditions prevailed near the accident site. The pilot was fatally injured. The local flight departed the M. Graham Clark - Taney County Airport (PLK), Point Lookout, Missouri, about 0845. The intended destination was Kansas City International Airport (MCI), Kansas City, Missouri. A witness hiking in the area at the time of the accident reported that she saw the airplane about 1/4 mile from her position, approximately 100 feet above the trees. She stated the airplane appeared to be losing altitude as if "coming in to land." She reported that something appeared to separate and fall from the airplane. She subsequently lost sight of the airplane behind a tree line. Another witness reported that he noticed the airplane eastbound in level flight. However, as he watched, the airplane banked to the right, "winged over," and began a clockwise descending spiral. He lost sight of it behind trees prior to impact. The amateur-built Windler Tri Q2 airplane was a single-engine, two-place design. The Tri Q2 incorporated a fixed tricycle landing gear arrangement, instead of a conventional tailwheel arrangement as in the original Q2 design. The airplane featured a canard, in lieu of a conventional horizontal stabilizer. The main wing was placed aft of the cockpit. The accident site was located about 4 nautical miles northwest of PLK, in Henning State Park Conservation Area near Branson, Missouri. The site was located in a wooded area. Breaks in the tree limbs near the site were consistent with the airplane descending at a steep angle prior to impact. The airplane was fragmented. Intact portions of the wings exhibited leading edge crushing damage. The left and right ailerons, and the left elevator had separated from the airframe at the accident site. The rudder remained attached to the vertical stabilizer. However, the vertical stabilizer had separated from the airframe at the accident site. The right elevator was not located with the main wreckage. It was subsequently located approximately 1,250 feet northwest of the main wreckage. The elevator was retained by the NTSB for examination. The 85-horsepower Continental C85 engine was examined and partially disassembled. No anomalies consistent with a pre impact engine failure were observed. The right elevator was examined by the NTSB materials laboratory. The examination revealed the elevator had separated from the outboard hinge. The mid span hinge stud was fractured. In addition, the torque tube was fractured at the mid span hinge cutout and at the inboard end of the elevator. The mid span hinge stud was contained in the center of the pivot. A portion of the fracture surface at the upper aft side of the stud was relatively flat and discolored with a smooth boundary; features consistent with fatigue. The fracture surface exhibited ratchet marks, small steps in the surface formed when adjacent fatigue cracks originate on slightly offset planes, which is consistent with fatigue originating from multiple origins. The fatigue failure initiated from several locations at a thread root. The thread surfaces adjacent to the fracture surface exhibited pitting consistent with corrosion. The upper elevator skin was buckled at the mid span hinge. The torque tube was fractured at the mid span hinge, at the inboard edge of the hinge cutout in the torque tube. The upper side of the torque tube was displaced upward, particularly at the inboard end of the cutout adjacent to the fracture. Appearance of the fracture surfaces was consistent with an overstress failure. In addition, the cutout of the torque tube at the mid span hinge support extended around approximately 120 degrees at the forward side of the tube. According to the aircraft construction plans, the cutout for the hinge was shown to be approximately 80 degrees. The fracture of the torque tube at the inboard end of the elevator was nearly circumferential at the aft side of the tube. The fracture surface was on slant planes and exhibited deformation adjacent to the fracture consistent with overstress fracture.
Fatigue failure, originating in an area of surface corrosion, of the right elevator mid span hinge pin and the subsequent failure of the elevator torque tube. Additional causes are the separation of the elevator from the airframe due to the hinge pin and torque tube failures, and the pilot's inability to maintain control of the airplane after separation of the control surface.
Source: NTSB Aviation Accident Database
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