ERA12IA169

According to the pilot, he conducted a thorough preflight inspection of the airplane with no anomalies noted. Then, when the airplane was at an altitude of 7,000 feet mean sea level, an extreme vibration began. The pilot reduced engine power, verified the airplane’s flight controls were still operational, and declared an emergency with the air traffic controller that he was in contact with. As the vibration continued, the pilot elected to shut down the engine and, as the propeller was windmilling, the vibration continued. The pilot reduced the airspeed in order to stop the propeller rotation, then immediately obtained best glide airspeed once the propeller stopped. As soon as the propeller stopped windmilling, the vibration ceased. The pilot landed the airplane without incident and it was towed to the fixed base operator. A postincident engine teardown revealed that there were no anomalies that would have precluded normal operation. Examination of a fracture surface on one of the propeller blades revealed that the blade tip failed as a result of fatigue cracking that initiated at the leading edge of the blade. The propeller blade was inspected about 72 operating hours before the incident, as part of the airplane’s 100-hour annual inspection, and no discrepancies were noted at that time.

On January 28, 2012, about 1705, central standard time, a Cessna 172S, N427CP, made a forced landing at Jackson-Evers International Airport (JAN), Jackson, Mississippi. The pilot, the sole occupant on board, was not injured and the airplane incurred minor damage. The airplane was registered to and operated by the Civil Air Patrol, Maxwell AFB, Alabama, under the provisions of 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed at that time and an instrument flight rule flight plan had been filed. a company flight plan had been filed. The flight originated from Hawkins Field Airport (HKS), Jackson, Mississippi, about 1645, earlier that day. According to the pilot, he conducted a thorough preflight inspection of the airplane with no anomalies noted. After departure, when the airplane was about 8 miles south west of JAN, at an altitude of 7,000 feet mean sea level, an extreme vibration began. The cabin environment experienced heavy turbulence. The pilot reduced engine power, verified the airplane’s flight controls were still operational, and declared an emergency with the air traffic controller. He then reduced the airspeed to best glide airspeed and maneuvered the airplane for JAN. The vibration continued, he elected to shutdown the engine, and as the propeller was windmilling the vibration continued. Again the pilot reduced the airspeed in order to stop the propeller rotation, and then immediately obtain best glide airspeed again once the propeller stopped. As soon as the propeller stopped windmilling, the vibration ceased. The pilot landed the airplane at JAN without incident and it was towed to the fixed base operator. An inspection of the airplane revealed that about a three inch section from one of the fixed pitched propeller blades had fractured and separated during flight. According to Federal Aviation Administrator records, the airplane was manufactured in 2001 and was equipped with a Lycoming IO-360-L2A, 180 horsepower engine with a McCauley model 1A170E/JHA7660 fixed pitch propeller. The most recent 100-hour/annual inspection was performed on July 8, 2011, and at that time the airframe, engine, and propeller had accrued 1735.2 total hours of time in service. According to the engine maintenance log, the most recent oil change was performed on July 11, 2011, at 1778.6 hours of time in service. On the day of the accident, the tachometer indicated 1807 total hours of time in service. In addition, the propeller maintenance log indicated that on March 16, 2010, the propeller blades were cleaned and “dressed.” Then, during the most recent inspection, the propeller was inspected in accordance with a 100-hour inspection and found to be in airworthy condition. A postincident engine teardown revealed that there were no visible anomalies and the engine was well within the static balance standards. The crankshaft inspection revealed that all of the internal rotating masses were within limits as set by the manufacturer.The crankshaft dynamic balance check indicated that the crankshaft flange was within limits. In addition, the engine model was not equipped with counterweights or harmonic dampeners. The propeller was retained by the National Transportation Safety Board and sent to the NTSB Materials Laboratory, Washington, D.C., for metallurgical examination. The examination of the fracture surface revealed that fracture markings were indicative of fatigue progression in aluminum alloys that initiated near the leading edge and propagated chordwise through about 65 percent of the blade. The remaining fracture was consistent with tensile overstress separation. For more information, the Materials Laboratory Report is located in the docket for this accident.