Incident Overview
On June 18, 2012, at 1045 central daylight time, a Piper PA-28R-200, registration N4350T, experienced a violent vibration during cruise flight. The private pilot executed a precautionary landing at the Marshall County Airport (C75) in Lacon, Illinois. The pilot was not injured. The airplane was registered to and operated by the Marshall County Flying Club under 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed, and no flight plan was filed. The airplane had departed C75 at 1008 for a local flight.
The pilot reported flying at 4,500 feet above mean sea level with 23 inches of manifold pressure and 2,350 to 2,400 rpm when abnormal vibrations began. He scanned the instruments and noted no other abnormal indications. He turned left toward C75, about 8 nautical miles away, and made a straight-in approach to runway 10, landing without incident.
Examination Findings
Inspection revealed that approximately 6 inches of a propeller blade tip had separated. The remaining blade was sent to the National Transportation Safety Board's Materials Laboratory. Examination showed the fracture surface was about 30¼ inches from the propeller butt. 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, indicating final overstress failure. The fatigue crack originated on the flat side of the blade, about 0.06 inches from the leading edge. A surface gouge at the origin measured about 0.02 inches long chordwise and 0.0016 inches deep. Scratch marks within the gouge suggested damage direction from leading to trailing edge, consistent with foreign object impact to a rotating propeller.
Maintenance and Tachometer Testing
Maintenance records indicated a new Hartzell propeller, model HC-C2YK-1BF, was installed on May 29, 2007. At the time of the incident, the propeller had 592.3 hours since new. The last annual inspection occurred on February 5, 2012, at 530.3 hours; the airplane had flown 62 hours since then.
The airplane's tachometer indicated a maximum rpm of 2,700 and a restricted propeller range from 2,100 to 2,350 rpm. The tachometer was tested with a digital device, yielding the following readings:
- Indicated 500 rpm: actual 500 rpm
- 1,000: 1,000
- 1,500: 1,500
- 2,000: 2,050
- 2,100: 2,150
- 2,200: 2,250
- 2,300: 2,350
- 2,400: 2,475
- 2,500: 2,575
- 2,700: 2,775
Thus, the tachometer over-read at higher rpm, potentially causing operation in a restricted range.
FAA Advisory
The Federal Aviation Administration issued Special Airworthiness Information Bulletin (SAIB) on May 14, 2008, alerting about propeller failures due to prolonged operation in restricted RPM ranges from inaccurate tachometers, missing markings, or placards. It noted that propeller manufacturers identify vibratory stress zones during certification, and if within operating range, operating restrictions may be imposed. Failure to comply could lead to propeller failure. Mechanical tachometer errors of 50 to 250 RPM have been reported. The most common failure is blade tip separation from fatigue cracks, aggravated by surface conditions like nicks or corrosion pits.