On December 12, 2000, at 1400 central standard time, a Cessna 172S, N582SP, experienced a separation of a portion of one of the propeller blades on takeoff. The takeoff was being made on runway 31 (5,200 feet by 100 feet) at the St. Cloud Regional Airport, St. Cloud, Minnesota. The separation occurred at an altitude of 400 to 500 feet above the runway and the certified flight instructor (CFI) was able to land the airplane on a parallel taxiway without incident. The CFI and the student pilot on board were not injured. The airplane received minor damage from vibration as a result of the propeller separation. The 14 CFR Part 91 instructional flight was operating in visual meteorological conditions and no flight plan was filed. The local flight was originating at the time of the incident.

A review of maintenance records and aircraft times revealed the propeller had 236 hours of total time prior to this incident.

The propeller was removed from the airplane and was shipped to the NTSB Metallurgical Laboratory in Washington D.C. for examination. The portion of the blade that separated was recovered at a later date and it too was shipped to the metallurgical laboratory. Inspection of the propeller revealed one blade had fractured about 14 inches from the centerline of the propeller hub.

The Material Laboratory Factual Report stated, "Examination revealed that about 60 percent of the fracture surface was on a flat chordwise plane, was faceted in appearance, and contained crack arrest positions, features typical of fatigue cracking in wrought aluminum alloys. The features in the fatigue region emanated from the flat face of the blade arm, near the trailing edge.... The fracture surface forward of the fatigue region was matte in appearance and on an irregular plane that was generally oriented at a 45 degree angle to the spanwise direction, typical of an overstress region stemming from the fatigue region."

A metallographic examination of the propeller blade indicated, "...the microstructure for the propeller was normal with the exception that the grain flow pattern followed the surface of the scallop region and not the surface of the blade." A metallurgical evaluation performed by Alcoa concluded "The presence of Cr [chromium] in the 'scalloped' region and not on the fatigued surface indicates that the chromic anodized solution penetrated the indication during the anodizing process. This observation, the grain flow below the 'scalloped' area...and the morphology of the 'scalloped' area indicate that the discontinuity was open to the surface prior to painting." See attached Material Laboratory Factual Report for more details.

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