Accident Details
On February 17, 2011, at 1530 eastern standard time, an amateur-built experimental Flaglor Kolb MK III airplane, registration N213, experienced a violent vibration during cruise flight at 1,500 feet above ground level, followed by a total loss of engine power. The pilot performed a forced landing to trees and the ground in the vicinity of Stetson, Maine. The personal flight was operating under 14 CFR Part 91 without a flight plan. Visual meteorological conditions prevailed. The airplane sustained structural damage to the right wing and forward fuselage. The sport pilot reported minor injuries. The flight originated from Old Town, Maine, at 1330 and was en route to Dexter, Maine.
Aircraft Information
The aircraft was an amateur-built experimental Flaglor Kolb MK III, registered to and operated by a private owner. The pilot reported that about 30 flight hours before the accident, the airplane experienced a severe vibration in flight. He shut down the engine and made a successful forced landing to a nearby airport. Examination then revealed a cracked triangular aluminum plate on the rear of the propeller hub. The front and rear propeller hubs were replaced by Warp propellers. He installed the propeller hub and same propeller blades on July 10, 2010, and resumed flying without vibrations. The total hours on the propeller blades are unknown.
Accident Sequence
Examination of the crash site revealed the airplane impacted trees and came to rest upright in 3 to 4 feet of snow, on a heading of 360 degrees magnetic. The forward nose section of the cabin area was bent upward and to the right. The forward windscreen, instrument panel, and cabin roof were partially separated. The cabin seats remained intact and the pilot's seat belts and shoulder harness were in use. The fuel tank was not ruptured and was more than half full. Continuity of the flight control system was confirmed from the cabin area aft to all flight control surfaces. The right main landing gear remained attached to the fuselage and was bent rearward.
Post-Accident Examination
The right wing remained attached to the wing root. The leading edge of the right wing was damaged, extending about 2 feet outboard of the wing root. The remainder of the leading edge sustained minor damage. The wing strut remained attached to the wing and the fuselage. The right flap and aileron remained attached at all attachment points. The left wing remained attached to the wing root and was not damaged. The wing strut remained attached to the wing and the fuselage. The tail boom was bent upward. The vertical fin, rudder assembly, horizontal stabilizers, and both elevators were not damaged.
The engine assembly remained attached to the engine mounts aft of the cabin area. Both carburetors had separated from the engine assembly and were hanging by their fuel lines and throttle cables. One composite propeller blade was delaminated, and the remaining two propeller blades were not damaged. Examination of the remaining engine assembly and accessories revealed no anomalies.
Propeller Blade Analysis
The propeller blades (labeled Nos. 1, 2, and 3) and both propeller hubs were sent to the NTSB Materials Laboratory for analysis. The fracture and separation of a portion of the after body of blade No. 1 initiated as a transverse fracture, originating near the trailing edge. The transverse fracture was progressive, with the crack propagating in some combination of fatigue and/or stress rupture under continually applied loads. Blade No. 2 exhibited a pattern of parallel transverse cracks on its aft side, with no evidence of delamination. Blade No. 3 was visibly damaged, with a significant delamination or disbonding of the woven carbon-fiber composite skin from the forward side, along with a delamination at the trailing edge. Blade No. 3 also exhibited a pattern of parallel transverse cracks on its aft side. This damage was consistent with bending of the tip of the blade forward under air loading.
Hub Fracture Analysis
The hub that was replaced in July 2010 was fractured as a result of a fatigue crack that extended 5.7 inches, with over stress regions at either end that were 0.09 inch and 0.23 inch long. The fatigue crack originated from a stress concentration caused by the sharp indentation of the edge of a faceplate bolted to the aft outer surface of the hub. The Rotax model "E" gear box was forwarded to an authorized repair station and placed in a bonded area until an examination could be conducted by the manufacturer under the supervision of a Federal Aviation Administration inspector. Examination of the "E" gear box revealed no internal worn parts or signs of vibration producing damage.