Accident Details
On June 14, 2008, at approximately 1215 eastern daylight time, a twin-engine Piper PA-34 airplane, registration N11HR, experienced a hard landing during which substantial damage occurred. The event took place at Ocala International Airport-Jim Taylor Field in Ocala, Florida. The aircraft was being operated by the pilot as a personal cross-country flight under instrument flight rules (IFR) and in accordance with 14 CFR Part 91. The private certificated pilot and three passengers were not injured. Visual meteorological conditions prevailed on final approach, and an instrument flight plan had been filed. The flight originated from Houston, Texas, around 0630.
Pilot Statement
During a telephone conversation with the National Transportation Safety Board investigator-in-charge on June 18, the pilot reported that when the airplane was approximately 6 to 10 feet above the runway during landing, the left engine's propeller moved to the feathered position without command. The airplane subsequently dropped hard onto the runway, causing the nose landing gear to collapse. The pilot stated that over the previous several months, the left propeller had uncommonly feathered while taxiing after landing. On the advice of his mechanic, the propeller was overhauled, which did not resolve the issue, and the engine was replaced. The pilot and mechanic determined that the propeller would likely not feather uncommanded in flight. For the accident flight, the pilot planned to land with extra power but found himself high and long on final approach. When he reduced engine rpm to idle near 6 to 10 feet above the runway, the left propeller feathered. The airplane struck the runway hard, driving the nose landing gear through the top of the airplane's nose and breaking the windshield.
Investigation
On June 20, the propeller and propeller governor were removed and taken to an FAA certificated propeller repair station. Examination revealed no evidence of mechanical failure or anomaly.
Additional Information
A propeller shop owner in Tucson, Arizona, noted that uncommanded propeller feathering on light twin-engine airplanes was a common problem in his area, particularly during summer months. He cited a similar issue at a local college aeronautical training center involving Beech 76 Duchess aircraft. That investigation, conducted with the FAA, attributed the problem to engine oil viscosity. Factors contributing to the issue, according to the shop owner, include ambient and engine operating temperatures, oil viscosity and grade (e.g., 15W50 vs. 5W40), oil cooler size and capacity, engine wear and tolerances, propeller governor volume, and propeller cylinder spring pressure. Several remedies were suggested, such as using a higher viscosity oil, installing a larger oil cooler, rebuilding or replacing the engine, changing the governor to a higher volume unit, or reducing the propeller cylinder spring pressure. In the college case, reducing spring pressure from 41 psi to 22 psi resolved the problem. The propeller manufacturer maintains a list of approved aircraft for this modification. A canvas of propeller shops on the East and West Coasts revealed similar complaints during summer, attributed to low oil viscosity, and pilots were advised to shut down at lower rpm to engage mechanical stops before oil pressure bleeds off. The shops reported that mechanics were often unaware of these oil viscosity issues and sometimes sent airworthy propellers and governors for unnecessary repairs.