Destin, FL, USA
N52LC
Larry J. Cook Lancair 320
Ten flight hours prior to the accident, the airplane's propeller was overhauled During cruise flight, a section of a blade separated. The pilot shut down the engine and attempted a forced landing at a nearby airport. While maneuvering to avoid a house on final approach the airplane stalled and landed hard just short of the runway. Metallurgical examination of the fractured blade surface revealed approximately 60% of the fractured surface was consistent with fatigue, which propagation from an area of corrosion on the blade’s cambered surface. Additionally, the propeller blades had been modified by shortening the blades. Hardware not approved by the propeller manufacturer was used for engine installation, and due to vibration concerns, the blade design was not approved by the propeller manufacturer for use on the accident airplane’s model engine.
On July 20, 2007, about 1120 central daylight time, an experimental, amateur-built, Lancair 320, N52LC, incurred substantial damage during a force landing at the Destin-Fort Walton Beach Airport (DTS), Destin, Florida. Visual meteorological conditions prevailed and a visual flight rule (VFR) flight plan was filed. The airplane was operated by a private individual, under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. The flight originated from the Pensacola Regional Airport (PNS), Pensacola, Florida, earlier that day, about 1100. The pilot stated his departure and climb to cruise was uneventful. He was cruising at 3,500 feet mean sea level (msl), about a quarter mile off the shoreline, and just had passed DTS. Without any warning, the airplane began to shake violently. He had a difficult time reaching the engine control levers, but managed to shut down the engine. He communicated to the air traffic controller, from which he was receiving flight following services, about the emergency. He elected to turn and land at DTS. He configured the airplane for best glide attitude. The glide to the airport was uneventful until the final approach. The airplane's altitude was low. He had to maneuver the airplane to fly over a house, and in doing so, the airplane lost airspeed and stalled. The airplane impacted the ground hard within the airport property and slid to the edge of runway 32, before coming to a stop. The pilot exited the airplane unharmed. Post accident examination showed that one of the propeller's blades was missing a 16 inch section, which was never recovered. A review by the Federal Aviation Administration (FAA) of the airplane's maintenance records revealed the separated blade had been straightened and the propellers assembly overhauled about 10 fight hours before the accident. After the accident the propeller assembly was sent to the Hartzell facility for a teardown examination, with FAA oversight. The examination revealed, the hub extension and mounting bolts were not approved by Hartzell, the propeller diameter had been reduced, and due to vibration stress concerns the blade design was not approved for the Lycoming model O-360 engine installed on the accident airplane. The separated blade was sent to the National Transportation Safety Board Materials Laboratory for metallurgical examination. The examination revealed the blade was fractured through the airfoil section 17.6 inches from the butt end. Optical examination of the fracture face revealed a flat shiny faceted region consistent with fatigue propagation through the aluminum alloys. The fatigue region covered approximately 60% of the fracture surface. The remaining fracture had a dull gray textured surface consistent with overstress separation. The fatigue region contained curved crack arrest markings. Magnified optical examinations uncovered a single fatigue origin site on the camber surface about 2.25 inches from the leading edge of the blade. The paint was intact on both sides of the blade and no damage or corrosion was optically apparent on the blade at the fatigue origin location. However, during scanning electron microscope a corrosion penetration was indicated by surface topography.
The separation of a portion of a propeller blade due to fatigue cracking which initiated in an area of corrosion that was not detected during propeller overhaul 10 flight hours before the accident.
Source: NTSB Aviation Accident Database
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