ERA12IA459

During departure, as the airplane reached an altitude of about 800 feet above ground level, the tip of one propeller blade separated. The flight instructor returned the airplane to the runway and landed without incident. The separated blade tip was not located; however, the remaining section of propeller blade was examined, and a fracture analysis was performed. The results of the analysis revealed fatigue crack propagation, followed by a final overstress fracture. The fatigue crack origin could not be accurately determined because the pertinent portion of blade’s leading edge suffered from postfracture damage, which likely occurred during removal or shipment. This postfracture damage also precluded determination of fatigue crack growthe timing or propagation. However, the fatigue and overstress fractures were consistent with other blade tip fractures that have occurred as a result of excessive vibrations within the propeller blade, typically as a result of a propeller ground strike. The incident airplane’s propeller blades were manufactured 32 years before the incident, and the manufacturer imposed no life limits on the blades as long as they passed the required overhaul inspections. The incident propeller blades were overhauled twice in the 3 years before the incident. The first overhaul was performed as a result of a propeller strike, and the reason for the second overhaul was not documented. At the time of the incident, the blades had accumulated 852 hours since the most recent overhaul, which occurred 18 months before the incident.

HISTORY OF FLIGHT On July 15, 2012, at 1400 eastern daylight time, a Socata TB 10, N189TB, experienced a partial propeller blade separation after takeoff from North Central State Airport (SFZ), Pawtucket, Rhode Island. The airplane was not damaged, and the private pilot and certified flight instructor (CFI) were not injured. Visual meteorological conditions prevailed, and no flight plan was filed for the local, instructional flight. The flight was operated under the provisions of Title 14 Code of Federal Regulations Part 91. According to the CFI, the private pilot was at the controls as the airplane departed from runway 23. As it reached an altitude of 800 feet, the pilots heard what sounded like a "shotgun blast," followed by the airplane vibrating "violently." The CFI took over the flight controls, reduced the power, and initiated a 180-degree turn, to land on runway 33. He pulled the mixture to reduce power on the engine and landed without incident, on runway 33. The CFI noted that during the event, he observed no oil on the windscreen, and no abnormal engine indications from the cockpit gauges. He reported that the airplane continued to vibrate until it touched down on the runway and the propeller stopped windmilling. Examination of the airplane revealed the tip of one propeller blade had separated with approximately 3.5 inches missing. The remaining section of propeller blade was retained and sent to the NTSB Materials Laboratory for examination. According to the examination, visual inspection revealed 80 percent of the fracture surface was relatively flat with a reflective luster, indicative of fatigue propagation. The remaining 20 percent had an uneven rough matte surface, indicative of an overstress fracture region. Microscopic inspection of the fatigue regions revealed an exact origin could not be identified. Additionally, the blade's cross-sectional profile appeared different than the expected blade cross-sectional profile, in the region near the origin's general location. Further scanning electron microscope imaging of this surface revealed markings indicative of mechanical abrasion. X-ray examination revealed the blade's material composition was consistent with type AA2025 aluminum alloy. Additionally, hardness and conductivity testing determined measurements consistent with the specified material for the blade. According to the propeller logbook, a major overhaul was performed on the propeller blades on February 18, 2009, as a result of a propeller strike. The propeller was reinstalled on the airplane on March 3, 2009, at a tachometer time of 5,007 hours. Another entry was noted in the propeller logbook on April 27, 2011, at a tachometer time of 5,449 hours. The entry stated "reinstalled propeller assembly after repair by Sensenich Propeller." The reason for the repair was unidentified in the logbook, and there was no associated entry in the airframe logbook. The most recent annual inspection was completed on the propeller onJanuary 18, 2012 at a tachometer time of 5,677 hours. The time since propeller overhaul was 669 hours, as of this date. The tachometer time noted at the time of the incident was 5,860 hours. Both propeller blades were manufactured by Hartzell Propellers on March 14, 1980. According to Hartzell, as long as the propeller blades pass the required overhaul inspections, there is no life limit imposed.