On June 18, 2012, at 1045 central daylight time, a Piper PA-28R-200, N4350T, experienced a violent vibration during cruise flight. The private pilot executed a precautionary landing at the Marshall County Airport (C75), Lacon, Illinois. The pilot was not injured. The airplane was registered to and operated by the Marshall County Flying Club under the provisions of the 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed at the time of the incident, and no flight plan was filed. The airplane departed C75 at 1008 on a local flight.
The pilot reported that he was flying at 4,500 feet above mean sea level with 23 inches of manifold pressure and 2,350 to 2,400 rpm when the airplane began to abnormally vibrate. He scanned the instruments and determined there were no other abnormal indications. He began a left turn direct to C75 which was about 8 nautical miles away. He made a straight in approach to runway 10 and landed without incident.
An inspection of the airplane revealed that about 6 inches of a propeller blade tip had separated from the propeller. The remaining propeller blade was sent to the National Transportation Safety Board's Materials laboratory for examination. The examination revealed that the fracture surface was about 30 ¼ inches from the butt of the propeller. About 70 percent of the fracture surface had a reflective luster and was relatively flat, indicative of a fatigue crack growth region. The remaining area had a rough matte surface, indicative of a final overstress failure. The origin of the fatigue crack was located on the flat side of the blade, about 0.06 inches from the leading edge of the blade. There was a surface gouge at the origin of the fatigue crack which was about 0.02 inches long chordwise, and about 0.0016 inches deep. The scratch marks within the gouge suggest that the direction of damage was from the leading to trailing edge, which is consistent with foreign object impact to a rotating propeller.
The airplane's maintenance records indicated that a new Hartzell propeller, model HC-C2YK-1BF, was installed on the airplane on May 29, 2007. At the time of the incident, the propeller had accumulated 592.3 hours since new. The last annual inspection of the propeller occurred on February 5, 2012, at a time of 530.3 hours. At the time of the incident, the airplane had flown 62 hours since the annual inspection.
The airplane's tachometer indicated that the maximum rpm was 2,700 rpm. The tachometer indicated that there was a restricted propeller range from 2,100 to 2,350 rpm.
The Federal Aviation Administration (FAA) issued a Special Airworthiness Information Bulletin (SAIB) concerning "Propeller RPM Restrictions and Placards" on May 14, 2008. The SAIB stated the following information:
"This Special Airworthiness Information Bulletin (SAIB) alerts you, owners, operators, pilots, mechanics, and certificated repair facilities of the potential for propeller failures on piston engine aircraft due to prolonged operation in a restricted RPM (revolutions per minute) range as a result of inaccurate tachometers, missing or improper tachometer markings, or missing or improper instrument panel placards…
Propeller manufacturers determine a propeller's operating vibratory stresses during certification. RPM zones of localized high vibratory stress are a relatively common finding during testing. If a zone of localized high vibratory stress occurs within the propeller operating range, then the propeller may be approved with the addition of an operating restriction, placard, or life limit...
Failure to comply with tachometer markings and instrument panel placards could result in prolonged operation within a restricted RPM range and increase the potential for a propeller failure.
Mechanical tachometers do not necessarily retain their accuracy for the life of the aircraft. Reports of tachometer errors of 50 RPM are common. Errors of 150 to 250 RPM have been reported. Using an inaccurate tachometer could result in a restricted RPM range or maximum RPM red line being unknowingly violated by the pilot, which could result in repetitive or prolonged exposure to damaging propeller stresses.
The most common failure associated with this condition is a blade tip separation that results from a fatigue crack, but failure of the propeller hub and/ or blade retention feature can also occur. Prolonged operation within a restricted RPM range, or above the maximum RPM, will be further aggravated by the presence of surface conditions such as nicks or corrosion pits."
The tachometer was sent to an avionics facility for testing. The results using a digital testing device indicated the following:
Tachometer Indication Instrument Reading
500 rpm 500 rpm
1,000 rpm 1,000 rpm
1,500 rpm 1,500 rpm
2,000 rpm 2,050 rpm
2,100 rpm 2,150 rpm
2,200 rpm 2,250 rpm
2,300 rpm 2,350 rpm
2,400 rpm 2,475 rpm
2,500 rpm 2,575 rpm
2,700 rpm 2,775 rpm